[2] K. W. E. Cheng, M. Liu, and Y. L. Ho, "Experimental confirmation of frequency correlation for bifurcation in current-mode controlled buck-boost converters," IEEE Power Electronics Letters, vol. 1, no. 4, pp. 101-103, 2003.

Frequency scaling in pulse-width modulated buck-boost power converters can be used to predict the first bifurcation point at various frequency operations. This letter first derives novel frequency correlation for the power converter. This is then examined by experimental results at high-frequency operation.

[3] O. Kukrer and H. Komurcugil, "Variable sampling frequency pwm waveforms," IEEE Power Electronics Letters, vol. 1, no. 1, pp. 14-16, 2003.

A method for online generation of single-phase near optimal pulse width modulated (PWM) waveforms is described. This method is based on using variable sampling periods in sampled PWM. Simulation results are presented for a single-phase full-bridge inverter feeding a resistive-inductive (R-L) load. Total harmonic distortions of current waveforms are found to be less, compared to PWM with constant sampling period.

[4] P. G. Maranesi and M. Riva, "Automatic modeling of pwm dc-dc converters," IEEE Power Electronics Letters, vol. 1, no. 4, pp. 97-100, 2003.

Different small-signal models are available for describing the dynamics of pulse width modulators (PWM) dc-to-dc converters. Discrete time models reach the best compliance in real life situations. Some converter control modes, in particular phase shifting, provide advantages of modeling in discrete time. On the other hand, the burden of getting such exact models is often tedious. Currently, automatic tools are available that are able to provide the state-space model in discrete time of any PWM dc-dc converter switching at constant frequency. The authors claim that the use of these tools is advisable for the analysis and control optimization of switching power supplies. To support their opinion, this letter refers to a new, very accurate small-signal model of a phase-shifting converter which has been published recently. Its excellent performance, as well as the difficulties of the derivation procedure, are compared with the precise forecasts and the short processing time of an automatic modeling tool. The paper concludes in favor of automatic modeling.

[5] T. Tepsa and T. Suntio, "Adjustable shunt regulator based control systems," IEEE Power Electronics Letters, vol. 1, no. 4, pp. 93-96, 2003.

Adjustable shunt regulators known as TL431 are extensively used to build the control systems in switched-mode converter applications. The resulting control system is typically a two-loop system, where one loop defines the low frequency behavior, and the other loop the higher frequency behavior, respectively. The shunt regulator is regarded to possess characteristics similar to an operational amplifier, even if it is basically a transconductance amplifier having current as an output signal instead of voltage as well as positive feedback instead of negative feedback. The small-signal response of a shunt regulator shows that it may be modeled as a voltage-controlled current source with a small transconductance gain and an equivalent capacitor as output impedance, which may be extracted from the open-loop frequency response by inspection. The variance in the parameters may be high depending on the manufacture, and operating point, i.e., cathode current, which is normally chosen to be close to 1 mA due to the specifications. The analysis shows, however, that the cathode current should be at least 5 mA for stable equivalent circuit parameters. If low cathode current is used, the control system may not be better than a Zener diode replacing the shunt regulator.

[6] M. A. Perales, M. M. Prats, R. Portillo, J. L. Mora, J. I. Leon, and L. G. Franquelo, "Three-dimensional space vector modulation in abc coordinates for four-leg voltage source converters," IEEE Power Electronics Letters, vol. 1, no. 4, pp. 104-109, 2003.

Four-leg inverters have been selected as one of the preferred power converter topologies for applications that require a precise control of neutral current, like active filters. The main advantage of this topology lies in an extended range for the zero sequence voltages and currents. However, the addition of a fourth leg extends the space vectors from two to three dimensions, making the selection of the modulation vectors more complex. Most of the algorithms that deal with this problem require an αβγ transformation. This paper presents a new space vector modulation algorithm using abc coordinates (the phase voltages) avoiding the αβγ transformation. Thanks to the use of abc coordinates, the algorithm is much simpler and more intuitive than in αβγ representation, drastically reducing the complexity of modulation algorithm and the computational load associated to it.

[7] M. A. M. Prats, L. G. Franquelo, R. Portillo, J. I. Leon, E. Galvan, and J. M. Carrasco, "A 3-d space vector modulation generalized algorithm for multilevel converters," IEEE Power Electronics Letters, vol. 1, no. 4, pp. 110-114, 2003.

A three-dimensional (3-D) space vector algorithm of multilevel converters for compensating harmonics and zero sequence in three-phase four-wire systems with neutral is presented. The low computational cost of the proposed method is always the same and it is independent of the number of levels of the converter. The conventional two-dimensional (2-D) space vector algorithms are particular cases of the proposed generalized modulation algorithm. In general, the presented algorithm is useful in systems with or without neutral, unbalanced load, triple harmonics and for generating 3-D control vectors.

[8] B. Diong, K. Corzine, S. Basireddy, and S. Lu, "Multilevel inverter-based dual-frequency power supply," IEEE Power Electronics Letters, vol. 1, no. 4, pp. 115-119, 2003.

Most existing power supplies for induction heating equipment produce voltage at a single (adjustable) frequency. Recently, however, induction heating power supplies that produce voltage at two (adjustable) frequencies simultaneously have been introduced and commercialized. These represent a significant development particularly for heat-treating workpieces with uneven geometries, such as gears. Still, the existing approaches to dual-frequency voltage generation could be improved upon to achieve better control, higher efficiency, and reduced electromagnetic interference. This letter proposes the use of multilevel inverters for providing power at two frequencies simultaneously. It describes how the stepping angles for the desired output from such inverters can be determined. Furthermore, experimental results are presented as verification of the concept and to demonstrate the achievement of improved harmonic level control and reduced device switching frequency.

[9] M. A. M. Prats, G. Escobar, E. Galvan, J. M. Carrasco, and R. Portillo, "A switching control strategy based on output regulation subspaces for the control of induction motors using a three-level inverter," IEEE Power Electronics Letters, vol. 1, no. 2, pp. 29-32, 2003.

In this work we present the design of a new switching control strategy for a three-level converter aimed to improve the performance of a direct torque control (DTC) for induction machines, special attention is given to the reduction of torque ripple. As in classical DTC, the proposed technique is aimed to directly regulate the outputs: torque and flux amplitude. We show that classical DTC can be considered as a particular case of the proposed control strategy. The proposed criterion to design the switching control sequence is based on a quadratic criterion of the output errors plus a prediction in one-step ahead, and an interesting partition of the input state space generated by the output regulation subspaces (ORS) to facilitate the selection of the control vector for reducing the computational load. As a result, a control vector is selected without the requirement of an auxiliary space vector or any other modulation technique. Simulations results using the proposed control technique confirm the validity of this approach.

[10] F. Z. Peng, X. Yuan, X. Fang, and Z. Qian, "Z-source inverter for adjustable speed drives," IEEE Power Electronics Letters, vol. 1, no. 2, pp. 33-35, 2003.

This paper presents a Z-source inverter system and control for adjustable speed drives (ASD). The Z-source inverter employs a unique LC network to couple the inverter main circuit to the diode front end. By controlling the shoot-through duty cycle, the Z-source can produce any desired output ac voltage, even greater than the line voltage. As results, the new Z-source inverter system provides ride-through capability under voltage sags, reduces line harmonics, and extends output voltage range. Simulation results will be presented to demonstrate the new features.

[11] B. A. Welchko and J. M. Nagashima, "The influence of topology selection on the design of ev/hev propulsion systems," IEEE Power Electronics Letters, vol. 1, no. 2, pp. 36-40, 2003.

The adoption of the 42 V Powernet standard has focused substantial research effort into the design of electric machines for hybrid vehicles. This letter investigates the potential performance benefits afforded by adopting a cascaded inverter topology on the overall system and motor performance. As a particular design example, this letter shows that a cascaded inverter driving an open winding motor can increase the high-speed power density of an induction motor by 73%. For an interior permanent magnet motor, the cascaded topology can increase low-speed torque by 9% and high-speed power by up to 300%. In all cases, the power increase is achieved without increasing the phase current over a more traditional system.

[12] K. El Khamlichi Drissi, P. C. K. Luk, B. Wang, and J. Fontaine, "Effects of symmetric distribution laws on spectral power density in randomized pwm," IEEE Power Electronics Letters, vol. 1, no. 2, pp. 41-44, 2003.

The effects on the spectral power density (SPD) of randomized pulse width modulation (RPWM) waveforms by the choice of different probability distribution laws are investigated. The distribution laws under studied are symmetric uniform, normal, laplace, and parabolic. A comparison method, based on a normalized deviation with respect to the uniform distribution law, is devised. It is shown, by way of analytical derivation and simulations, that uniform distribution produces power spectrum comparable to those generated by other relatively more complex distributions under studied. Significantly, it means that uniform distribution, which is the simplest to implement, is the preferred probability law for randomization in a RPWM scheme.

[13] Y. Xu, L. M. Tolbert, F. Z. Peng, J. N. Chiasson, and J. Chen, "Compensation-based non-active power definition," IEEE Power Electronics Letters, vol. 1, no. 2, pp. 45-50, 2003.

This paper presents a new definition of nonactive current from which the definitions of instantaneous active and nonactive power are also derived. The definitions are consistent with the traditional power definitions and valid for single-phase and polyphase systems, as well as periodic and nonperiodic waveforms. The definitions are applied to a shunt compensation system. The paper elaborates on the compensation of three different cases of nonperiodic current: single-phase disturbance, three-phase sub-harmonics, and three-phase stochastic current. Simulation results give credibility to the applicability of the definition for a diversity of load currents. According to different compensation cases and the goals to be achieved, different averaging time intervals for the compensator are chosen which will determine the compensator's energy storage requirement and the extent of residual distortion in the source current.

[14] J. T. Boys, G. A. Covic, and Y. Xu, "Dc analysis technique for inductive power transfer pick-ups," IEEE Power Electronics Letters, vol. 1, no. 2, pp. 51-53, 2003.

Inductively coupled power transfer (ICPT) systems are now being used in applications where their efficiency and stability are critically important. In this paper, an ICPT pick-up is analyzed under conditions where the pick-up tuning is assumed to be essentially perfect, and all diodes are assumed to have continuous current flow in them. The analysis proceeds by replacing the existing parallel resonant tank and rectifier circuit with a dc source and transformer equivalent, thereby reducing the complexity of the circuit, and eliminating the diodes. The dc equivalent is shown to have the same energy storage and power capability as the original circuit and is accurate under all loadings for both transient and steady-state conditions. With this simplification, analytic transfer functions for the pick-up with respect to its excitation current and its control switch duty cycle are developed. The ICPT system model is shown to have multiple poles, with complex zeroes on the right half plane. The model allows a sophisticated controller to be designed to give the ICPT system the required stability at high efficiency.

[15] B. Ozpineci and L. M. Tolbert, "Characterization of sic schottky diodes at different temperatures," IEEE Power Electronics Letters, vol. 1, no. 2, pp. 54-57, 2003.

The emergence of silicon carbide- (SiC-) based power semiconductor switches, with their superior features compared with silicon- (Si-) based switches, has resulted in substantial improvement in the performance of power electronics converter systems. These systems with SiC power devices have the qualities of being more compact, lighter, and more efficient; thus, they are ideal for high-voltage power electronics applications. In this study, commercial Si pn and SiC Schottky diodes are tested and characterized, their behavioral static and loss models are derived at different temperatures, and they are compared with respect to each other.

[16] K. A. Corzine and X. Kou, "Capacitor voltage balancing in full binary combination schema flying capacitor multilevel inverters," IEEE Power Electronics Letters, vol. 1, no. 1, pp. 2-5, 2003.

Recently, the full binary combination schema (FBCS) method has been introduced to control the flying capacitor multilevel inverter. This method has the primary advantage that the number of voltage levels can be increased for a given number of semiconductor devices when compared to the conventional control methods. However, due to the difficulty of balancing the capacitors, the new schema requires fixed floating sources to provide the dc voltages. This paper reveals an approach onf balancing the capacitors, thus expanding the application fields of FBCS inverters to the family of the flying capacitor multilevel inverters under the condition of choosing a suitable modulation index. Simulation results demonstrate the proposed voltage balancing control.

[17] F. Z. Peng, L. Chen, and F. Zhang, "Simple topologies of pwm ac-ac converters," IEEE Power Electronics Letters, vol. 1, no. 1, pp. 10-13, 2003.

This letter proposes a new family of simple topologies of PWM ac-ac converters with minimal switches. With extension from the basic dc-dc converters, a series of ac-ac converters such as buck, boost, buck-boost, Cuk, and isolated converters are obtained. By PWM duty ratio control, they become a "solid-state transformer" with a continuously variable turns ratio. All the proposed ac-ac converters in this paper employ only two switches. Compared to the existing circuits that use six switches or more, they can reduce cost and improve reliability. The operating principle and control method of the proposed topologies are presented. Analysis and simulation results are given using the Cuk ac-ac converter as an example. The analysis can be easily extended to other converters of the proposed family.

[18] K. D. T. Ngo and A. Gangupomu, "Improved method to extract the short-circuit parameters of the becm," IEEE Power Electronics Letters, vol. 1, no. 1, pp. 17-18, 2003.

The current method for extracting the short-circuit broadband extended cantilever model (BECM) parameters requires measurement of short-circuit currents, and is problematic over a wide frequency range, prohibiting each short-circuit parameter to be directly measured. The improved method presented herein eliminates the measurements of short-circuit currents. Each short-circuit BECM parameter is directly found from two measurements, an open-circuit voltage gain and a short-circuit admittance. The method is demonstrated for a published five-winding BECM.

[19] J. R. Stack, T. G. Habetler, and R. G. Harley, "Effects of machine speed on the development and detection of rolling element bearing faults," IEEE Power Electronics Letters, vol. 1, no. 1, pp. 19-21, 2003.

This research investigates the effects of a variable machine speed on machine vibration and the implications for bearing fault detection. These effects are important to understand because when ignored they can significantly hinder the ability to detect bearing faults. Experimental results verify that a variable machine speed can directly and nonlinearly alter the level of machine vibration. This is due to differences in mechanical damping and resonance at various machine speeds. While this effect is difficult to notice in healthy machines, it can become significant as bearing health degrades. An additional effect that speed can exert is on the rate of development of a bearing fault. Variations in speed can actually retard or temporarily mask the increase in machine vibration due to a bearing fault. This phenomenon is observed in experimental trials as the bearing fault enters an advanced and more deteriorated stage. This can inadvertently make a machine appear healthy even though a bearing failure is imminent. However, by understanding these effects, a more skillful application of the available condition monitoring tools in variable speed applications is achieved.

[20] A. J. Forsyth and G. A. Ward, "Ultralow-inductance, multielement transformer-rectifier," IEEE Power Electronics Letters, vol. 1, no. 4, pp. 120-122, 2003.

A coaxially-connected, multielement transformer-rectifier is described for high-frequency power supply applications. Each elemental transformer has an optimally interleaved, single-layer winding, and the coaxial interconnection enables increased output voltage and higher power throughput to be achieved, whilst minimizing the leakage inductance. A leakage inductance of 0.55 μH is achieved in a 2.0 kW, 500 kHz transformer-rectifier.

[21] B. G. Dobbs and P. L. Chapman, "A multiple-input dc-dc converter topology," IEEE Power Electronics Letters, vol. 1, no. 1, pp. 6-9, 2003.

A new topology for multiple energy source conversion is presented. The topology is capable of interfacing sources of different voltage-current characteristics to a common load, while achieving a low part count. A fixed frequency switching strategy is investigated and the resulting operating modes are analyzed. The analysis is verified by experimentation. The results show that the converter is an enabling technology for power diversification and optimization.

[22] P. Beccue, J. Neely, S. Pekarek, and D. Stutts, "Utilization of a piezoelectric polymer to sense harmonics of electromagnetic torque," IEEE Power Electronics Letters, vol. 1, no. 3, pp. 69-73, 2003.

In this paper, the use of a piezoelectric polymer material to measure the harmonics of electromagnetic torque produced by a permanent magnet synchronous machine is described. The advantages of the polymer include low cost, durability, and flexibility. In addition, wide-bandwidth sensors are relatively easy to design and couple to drive system hardware for harmonic evaluation or to use in feedback-based control. To illustrate the use of the polymer, the electrical and mechanical properties of three sensors are described. The results of time-domain simulation and hardware experiments are used to validate that the voltage obtained from the sensors is linearly related to the torque ripple produced by the machine.

[23] A. Kwasinski, P. T. Krein, and P. L. Chapman, "Time domain comparison of pulse-width modulation schemes," IEEE Power Electronics Letters, vol. 1, no. 3, pp. 64-68, 2003.

This paper shows that classical space vector modulation (SVM) is functionally identical to double-sided uniform-sampled pulse width modulation (UPWM). Consequently, direct conclusions about harmonic distortion, losses, dc bus utilization, and ease of implementation are made that clarify some existing misconceptions about SVM. Since UPWM is conceptually simple and involves few steps, it is possible that computation may be reduced in practice. UPWM algorithms can avoid the sector and switch sequence tracking tasks in an SVM algorithm. The linear modulation range extension "inherent" to SVM is associated with triplen harmonic injection, and does not provide advantages over conventional third-harmonic injection techniques. The equivalence of SVM and UPWM means that SVM has spectral distortion in base-band as is known to occur in UPWM. This has implications for naturally-sampled sine-triangle PWM (NPWM) - which is known not to generate baseband distortion.

[24] D. D.-C. Lu, D. K.-W. Cheng, and Y.-S. Lee, "A single-switch ac/dc converter with voltage regulated storage capacitor," IEEE Power Electronics Letters, vol. 1, no. 3, pp. 78-82, 2003.

A single-stage power-factor-corrected ac/dc converter (SSPFC) usually causes high voltage stress on the intermediate storage capacitor, due to the lack of control of this voltage. The storage capacitor voltage varies largely with line voltage, and load current and is usually higher than the peak line voltage. This paper presents a new single-switch SSPFC based on a flyback topology for which the storage capacitor voltage is loosely regulated by the output voltage. Without using extra power switches to increase the control dimension, the proposed converter uses a flyback converter with dual-output transformer to achieve the control purpose. The range of storage capacitor voltage change against the change of input voltage and load current is significantly reduced. Moreover, the maximum storage capacitor voltage can stay below the peak line voltage at high line condition. Experimental results verifying the operation of the proposed SSPFC are also reported.

[25] X. Wang, R. Yao, and F. Rao, "Three-step impedance criterion for small-signal stability analysis in two-stage dc distributed power systems," IEEE Power Electronics Letters, vol. 1, no. 3, pp. 83-87, 2003.

Small-signal stability analysis methods based on an impedance criterion originate from the minor loop gain method and are gradually utilized in two-stage dc distributed power systems. In this paper, we conclude that the impedance criterion directly dependent on output impedance Z_o (s) of the source subsystem and input impedance Z_i (s) of the load subsystem is possible but gives an incorrect stability analysis for systems with a regulated source subsystem. Through introducing a mapped pure impedance of the load subsystems and the preliminary system, we develop a general three-step impedance criterion, with which a correct small-signal stability analysis can be guaranteed, regardless of the type of source subsystem. Furthermore, we introduce the application of the three-step impedance criterion in two small-signal stability analysis cases and utilize it in an example system to predict the stability shift process arising from the variation on the load resistance and input voltage value.

[26] C.-H. Tso and J.-C. Wu, "A ripple control buck regulator with fixed output frequency," IEEE Power Electronics Letters, vol. 1, no. 3, pp. 61-63, 2003.

This paper presents a ripple control buck regulator for stepping down high-voltage adaptor or battery supplies to low-voltage CPU core supply in notebook computers. Methods and circuits are proposed to achieve fixed switching frequency operation. The control loop adjusts delay of the controller and thus controls the output frequency. This is done by using a phase-frequency detector (PFD) to lock the frequency of switching signal with the input clock. The delay value is determined automatically according to operating conditions. Simulation results showed fast load and line transient responses and fixed frequency operations in steady states. In addition, stability of the regulator is shown under a very low output-to-input voltage ratio condition that is difficult for conventional PWM controllers.

[27] S.-K. Han, G.-W. Moon, and M.-J. Youn, "A voltage-balanced phase-shifted three-level dc/dc converter operating from high-input voltage," IEEE Power Electronics Letters, vol. 1, no. 3, pp. 74-77, 2003.

A voltage-balanced phase-shifted three-level dc/dc converter is proposed. Its switch voltage stress is ensured to be only one-half of the input voltage and its four-step operation can reduce considerably the output inductor current ripple. Moreover, it features a small filter, no voltage unbalance problem, static/dynamic sharing of the switch voltage, high-efficiency, and high-power density. It is very suitable for high power converters operating from a high-input voltage.

[28] S. Busquets-Monge, J. Bordonau, D. Boroyevich, A. Gilabert, and J. Salaet, "Erratum: Output voltage distortion characterization in multilevel pwm converters (ieee power electronics letters 2004 (2): 3)," IEEE Power Electronics Letters, vol. 2, no. 3, pp. 108, 2004.

[29] B. M. T. Ho, H. S. H. Chung, and W. L. Lo, "Use of system oscillation to locate the mpp of pv panels," IEEE Power Electronics Letters, vol. 2, no. 1, pp. 1-5, 2004.

This letter proposes the use of system oscillation in a perturbation-based maximum power point (MPP) tracker to locate the MPP of photovoltaic (PV) panels. Instead of using an explicit perturbation source, the tracker controller is designed to make the overall system self-oscillate, so that the duty cycle of the main switch in the power conversion stage (PCS) is inherently modulated with a small-amplitude variation at a predefined frequency around the required steady-state value. The tracking mechanism is based on comparing the ac component (due to the variation of the duty cycle) and the average value of the input voltage of the PCS to determine the quiescent duty cycle. The proposed technique does not approximate the panel characteristics and can globally locate the MPP under wide insolation conditions. The tracking capability has been verified experimentally with a 10-W PV panel in a controlled setup. Performances at the steady state and during the large-signal change of the insolation level have been studied.

[30] A. Tarkiainen, R. Pollanen, M. Niemela, and J. Pyrhonen, "Identification of grid impedance for purposes of voltage feedback active filtering," IEEE Power Electronics Letters, vol. 2, no. 1, pp. 6-10, 2004.

A voltage feedback active filter is vulnerable to unknown grid impedance. To overcome this problem we propose an identification method, which uses the control system of a frequency selective active filter to measure the grid impedance at selected frequencies. The usefulness of the method is experimentally demonstrated with a 19 kVA active rectifier with a voltage feedback active filtering function. The voltage feedback active filtering is performed in a case in which the active filter control is not stable before the impedance is identified with the method proposed. It is shown that the use of the measured grid impedance in the control system greatly enhances the dynamic stability of the system. Also, the grid impedance measurements are provided in two cases.

[31] S. Busquets-Monge, J. Bordonau, D. Boroyevich, and S. Somavilla, "The nearest three virtual space vector pwm - a modulation for the comprehensive neutral-point balancing in the three-level npc inverter," IEEE Power Electronics Letters, vol. 2, no. 1, pp. 11-15, 2004.

This letter presents a new modulation approach for the complete control of the neutral-point voltage in the three-level three-phase neutral-point-clamped voltage source inverter. The new modulation approach, based on the virtual space vector concept, guarantees the balancing of the neutral-point voltage for any load (linear or nonlinear) over the full range of converter output voltage and for all load power factors, the only requirement being that the addition of the output three-phase currents equals zero. The implementation of the proposed modulation is simple according to the phase duty-ratio expressions presented. These expressions are only dependent on the modulation index and reference vector angle. The performance of this modulation approach and its benefits over other previously proposed solutions are verified experimentally.

[32] S. Jain and V. Agarwal, "A new algorithm for rapid tracking of approximate maximum power point in photovoltaic systems," IEEE Power Electronics Letters, vol. 2, no. 1, pp. 16-19, 2004.

This paper presents a new algorithm for tracking maximum power point in photovoltaic systems. This is a fast tracking algorithm, where an initial approximation of maximum power point is (MPP) quickly achieved using a variable step-size. Subsequently, the exact maximum power point can be targeted using any conventional method like the hill-climbing or incremental conductance method. Thus, the drawback of a fixed small step-size over the entire tracking range is removed, resulting in reduced number of iterations and much faster tracking compared to conventional methods. The strength of the algorithm comes from the fact that instead of tracking power, which does not have a one-to-one relationship with duty cycle, it tracks an intermediate variable &beta, which has a monotonically increasing, one-to-one relationship. The algorithm has been verified on a photovoltaic system modeled in Matlab-Simulink software. The algorithm significantly improves the efficiency during the tracking phase as compared to a conventional algorithm. It is especially suitable for fast changing environmental conditions. The proposed algorithm can be implemented on any fast controller such as the digital signal processor. All the details of this study are presented.

[33] M. Ferdowsi and A. Emadi, "Estimative current mode control technique for dc-dc converters operating in discontinuous conduction mode," IEEE Power Electronics Letters, vol. 2, no. 1, pp. 20-23, 2004.

A new control technique for dc-dc converters is introduced and applied to a boost converter operating in discontinuous conduction mode (DCM). In contrast to conventional control methods, the principal idea of the proposed control scheme is to obtain samples of the required signals and estimate the required switch-on time. The proposed technique is applicable to any converter operating in DCM, including power factor correctors (PFC), however, this letter mainly focuses on boost topology. In this letter, the main mathematical concept of a new control algorithm is introduced, as well as the robustness investigation of the proposed method with simulation and experimental results.

[34] S. Busquets-Monge, J. Bordonau, D. Boroyevich, A. Gilabert, and J. Salaet, "Output voltage distortion characterization in multilevel pwm converters," IEEE Power Electronics Letters, vol. 2, no. 1, pp. 24-28, 2004.

One of the main features to consider in the development of new pulsewidth modulations (PWM) for multilevel converters is the high-frequency output-voltage distortion. In this letter, a novel per-switching-cycle figure, the harmonic distortion of order n for switching cycle k (HD_n,k), is introduced to quantitatively characterize the output three-phase voltage harmonic distortion of multilevel converters around all the integer multiples of the switching frequency. This figure allows for the decomposing of the modulation design problem within an output voltage fundamental cycle into an independent set of smaller problems for every switching cycle. The expression of HD_n,k as a function of the switching states' duty-ratio is presented for the three-level three-phase neutral-point-clamped voltage source inverter and it can be easily obtained for any other multilevel converter. From the evaluation of HD_n,k over 1/6th of the output-voltage fundamental-period the value of HD_n is obtained, providing a measure of the output voltage distortion in a fundamental period. This information is obtained at a lower computational cost than conventional fast Fourier transform (FFT) analysis. The accuracy of the HD_n distortion predictions is verified by comparing it to FFT-based results obtained from simulation and experiments. The expression to compute the total harmonic distortion (THD) as a function of HD_n is also derived.

[35] X. Gao and R. Ayyanar, "A high-performance, integrated magnetics scheme for buck-cascaded push-pull converter," IEEE Power Electronics Letters, vol. 2, no. 1, pp. 29-33, 2004.

This letter proposes a new integrated magnetics scheme for the buck cascaded current-fed push-pull converter. While integrating the transformer and inductor into a common structure, the proposed scheme also alters the operating characteristics of the basic converter. This leads to several advantages such as, continuous output current, zero voltage switching (ZVS), and zero current switching (ZCS) for the switches of the push-pull stage, and reduced conduction losses. The principle of operation of the proposed scheme is analyzed using the gyrator-capacitor model. Experimental results from a 12 V/5 A proof-of-concept prototype are presented demonstrating the advantages of the proposed scheme.

[36] J. C. A. Floriani, "Generalized analysis of current ripple in a pulsewidth modulation h-bridge converter with unipolar-bipolar switching," IEEE Power Electronics Letters, vol. 2, no. 3, pp. 83-86, 2004.

The aim of this letter is to describe a generalized analysis of the current ripple occurring in an H-bridge with pulsewidth modulation (PWM) technique. The general equation of the current ripple is obtained. By means of this equation, it is shown that the two strategies commonly used for the bridge control (unipolar and bipolar) are two limit cases of a general method. The control condition with minimum ripple is found. Finally, experimental results of a typical example are shown (applied to a dc drive) in different conditions, thus confirming the theoretical results.

[37] A. V. Timbus, R. Teodorescu, F. Blaabjerg, and U. Borup, "Online grid measurement and ens detection for pv inverter running on highly inductive grid," IEEE Power Electronics Letters, vol. 2, no. 3, pp. 77-82, 2004.

Photovoltaic (PV) and other sources of renewable energy are being used increasingly in grid-connected systems, for which stronger power quality requirements are being issued. Continuous grid monitoring should be considered so as to provide safe connections and disconnections from the grid. This letter gives an overview of the methods used for online grid measurement with PV inverters. Emphasis is placed on a method based on the injection of a noncharacteristic harmonic in the grid. Since this injection is regarded as a disturbance for the grid, different issues, i.e., the influence on total harmonic distortion (THD), the accuracy of line impedance measurement and the ENS (German abbreviation of Main Monitoring units with allocated Switching Devices) detection are studied. Laboratory results conducted on an existing PV inverter are presented to demonstrate the behavior of the PV inverter under different grid conditions. Some of the injection parameters are tuned in order to get an accurate measurement of line impedance.

[38] Y. Zhu, L. Yan, and B. Lehman, "A lossless active clamping circuit for current doubler topologies," IEEE Power Electronics Letters, vol. 2, no. 3, pp. 92-95, 2004.

This letter presents a new lossless clamping circuit on the secondary side for dc-dc converters with a current-doubler structure. This circuit reduces ringing on secondary-side rectifiers due to leakage inductance. The ringing loss is recovered to the load. As a result, efficiency is improved, and the voltage stress on secondary-side rectifiers is reduced significantly. These improvements allow dc-dc topologies with a current doubler to operate efficiently at high switching frequencies. The operating principle of the clamping circuit is detailed. Simulations and experimental results validate the proposed technique.

[39] S. S. M. Chan, H. S. H. Chung, and R. S. Y. Hui, "Self-oscillating dimmable electronic ballast for fluorescent lamps," IEEE Power Electronics Letters, vol. 2, no. 3, pp. 87-91, 2004.

This letter presents a low-cost solution for converting the popularly adopted nondimmable electronic ballast circuit for fluorescent lamps with self-oscillating series resonant inverter into a dimmable one. The dimming function is achieved by increasing the switching frequency of the inverter from the natural frequency of the resonant tank, so that less energy is coupled to the lamp. Control of the switching frequency is based on deriving an adjustable dc current source from the inductor in the resonant tank to control the operating point of the saturable chokes for driving the switches in the inverter. The overall circuit does not require any integrated circuit. A 17-W prototype has been built and tested. Theoretical predictions have been verified with experimental results. The lamp can be dimmed to 10% of the full brightness.

[40] J. W. Kimball, T. L. Flowers, and P. L. Chapman, "Low-input-voltage, low-power boost converter design issues," IEEE Power Electronics Letters, vol. 2, no. 3, pp. 96-99, 2004.

Issues associated with boost converter design and performance are investigated when a low input voltage is used. Low-input-voltage sources include single fuel cells, single solar cells, and thermoelectric devices. The primary context is interfacing single micro fuel cells to portable electronic loads, such as mobile phones. Efficiency and circuit startup are the two most difficult issues for a low-cost design. It is shown in theory and experiment that the boost converter has a voltage collapse point. A simple startup technique is proposed that is appropriate for some applications.

[41] Y. Zhang, R. Zane, A. Prodic, R. Erickson, and D. Maksimovic, "Online calibration of mosfet on-state resistance for precise current sensing," IEEE Power Electronics Letters, vol. 2, no. 3, pp. 100-103, 2004.

An approach for online current sensing calibration is presented where an auxiliary switch and a precision sense resistor are connected in parallel with a main power switch to achieve accuracy comparable to the sense resistor method, together with the advantage of essentially no additional power loss. The proposed current-sensing circuit and the calibration methods are particularly well suited for digital controller implementations where the required control and calibration functions can be easily accomplished. Experimental results with a digitally controlled 1.5-V 15-A synchronous buck converter demonstrate functionality of the online calibration approach, showing a significant improvement in accuracy over voltage sensing across the power MOSFET on-resistance.

[42] C. Buttay, T. B. Salah, D. Bergogne, B. Allard, H. Morel, and J.-P. Chante, "Avalanche behavior of low-voltage power mosfets," IEEE Power Electronics Letters, vol. 2, no. 3, pp. 104-107, 2004.

This letter addresses the behavior of low voltage power MOSFETs under avalanche, with a paralleling point of view. It is shown that during avalanche, up-to-date technology MOSFET transistors exhibit a resistance far in excess of their on-state resistance (R_DSon). A novel test setup is proposed to measure "avalanche" resistance. A simple model of breakdown voltage is then proposed. It becomes possible to perform fast simulations using this model to study current balance between paralleled transistors under avalanche operation. It is shown that considering avalanche resistance reduces the influence of breakdown voltage mismatches and allows for better current sharing.

[43] P. Athalye, D. Maksimovic, and R. Erickson, "Variable-frequency predictive digital current mode control," IEEE Power Electronics Letters, vol. 2, no. 4, pp. 113-116, 2004.

A variable-frequency predictive digital control method for the inductor current in switched-mode converters is introduced in this letter. This method is predictive in nature because the transistor off-time required for achieving the target current is calculated ahead of time. The transistor on-time is kept constant, which results in variable switching frequency. The control laws for continuous and discontinuous modes of operation of the three basic converters are derived. Stability and robustness criteria are presented. The variable-frequency predictive control obviates the need for current-loop compensation and has the advantages of single-cycle response and relatively simple implementation. The control method is demonstrated in a digital signal processor (DSP) for a boost power factor corrector (PFC), which shows excellent current tracking and a very low harmonic distortion of the line current.

[44] M. M. Islam, D. R. Allee, S. Konasani, and A. A. Rodriguez, "A low-cost digital controller for a switching dc converter with improved voltage regulation," IEEE Power Electronics Letters, vol. 2, no. 4, pp. 121-124, 2004.

A new control algorithm with improved regulation is presented for a switching dc buck converter. The controller is realized with hardware description language (HDL) and can be implemented in any process. The controller uses four decision levels, combines pulse frequency modulation (PFM), pulse width modulation (PWM), and uses dithering for improved regulation. The controller is prototyped on a field programmable gate array (FPGA) and experimental results show good performance over input and load disturbances. Importantly, the controller does not require high resolution analog-to-digital conversion for signal processing, and also does not require fast digital clocking. This controller has significant potential to be widely used in industrial applications where cost and design time are of great concern.

[45] J.-W. Park and R. G. Harley, "Secondary control for a series reactive compensator based on a voltage-source pwm inverter," IEEE Power Electronics Letters, vol. 2, no. 4, pp. 117-120, 2004.

This letter describes a new secondary control scheme of a series reactive compensator for a power system based on a single voltage-source pulse-width-modulated (PWM) inverter. The controllable capacitive reactance can be used as a supplementary control variable for the secondary (external) controller (SC) of a series capacitive reactance compensator to improve the dynamic transient and damping performances of the power system. From the viewpoint of agent-based global dynamic optimization of a system, the selection and use of suitable input signals for the SC are investigated. Detailed simulation results show that the SC with local feedback loop (LFL) has a powerful control performance; however, it requires the controllable compensation for a reference change due to different operating conditions. On the other hand, the SC with global feedback loop (GFL) avoids the need of reference compensation; moreover, its dynamic control performance is improved when the dual inputs (frequency and active power signal) are used, compared to when only the frequency is used as an input signal.

[46] P. C. Loh, D. M. Vilathgamuwa, S. K. Tang, and H. L. Long, "Multilevel dynamic voltage restorer," IEEE Power Electronics Letters, vol. 2, no. 4, pp. 125-130, 2004.

This letter presents the implementation and control of a high voltage dynamic voltage restorer (HVDVR) for use in power distribution network to compensate for sags in utility voltages. The proposed HVDVR is implemented using a multilevel inverter topology with isolated dc energy storages, allowing the direct connection of the HVDVR to the distribution network without using a bulky and costly series injection transformer. A control algorithm, incorporating P+ resonant and Posicast compensators, is also presented for controlling the HVDVR with perfect reference voltage tracking and effective damping of transient voltage oscillations at the instant of sag compensation. Finally, simulation results are presented to verify the performance of the proposed multilevel HVDVR.

[47] P. Cantillon-Murphy, T. C. Neugebauer, C. Brasca, and D. J. Perreault, "An active ripple filtering technique for improving common-mode inductor performance," IEEE Power Electronics Letters, vol. 2, no. 2, pp. 45-50, 2004.

Active ripple filtering is the replacement of large passive components in power filter circuits with smaller passive components and active control circuitry. This letter focuses on common-mode filters, where a large common-mode inductor (choke) is replaced by two smaller chokes and active op-amp control. The technique is appropriate when improved attenuation is required at relatively low frequencies and the high-frequency filtering requirements are easily met. Smaller chokes save significantly in material and winding costs. The technique is more advantageous if wire-wound chokes can be replaced by planar printed circuit board chokes. The use of the technique in an automotive electromagnetic interference (EMI) filter application is explored in detail.

[48] P. Midya, K. Haddad, and M. Miller, "Buck or boost tracking power converter," IEEE Power Electronics Letters, vol. 2, no. 4, pp. 131-134, 2004.

A cascade of buck and boost converter is presented here. The control operates in a manner that the converter is either in buck or boost (BOB) mode on a cycle by cycle basis. It transitions between the modes seamlessly to provide a tracking power conversion function for modulating the power supply of a variable envelope radio frequency (RF) power amplifier. The control algorithm and its implementation using switched capacitor circuits is described. Simulation and measured experimental results including converter efficiency, tracking accuracy, and spectrum at the output of the RF power amplifier are provided. This control technique allows seamless transition between the buck and boost modes while tracking RF envelopes with bandwidth greater than 100 kHz, and maintaining extreme accuracy and extremely low ripple. The efficiency of this converter operating at 1.68 MHz is close to 90% over a wide range of conversion ratios. The area of the power converter is extremely small allowing this to be integrated into a cellular telephone. The controller was integrated as part of a larger power management IC as well as a discrete IC.

[49] A. P. Hu and S. Hussmann, "Improved power flow control for contactless moving sensor applications," IEEE Power Electronics Letters, vol. 2, no. 4, pp. 135-138, 2004.

An improved power flow control method for contactless moving sensor applications is proposed. The method allows the design of a system where sensors with different power ratings or a wide range of load variations can be implemented. A phase-controlled variable inductor is used to tune the resonant circuit of the power pickups of an inductively coupled power transfer (ICPT) system according to the actual power requirements of the sensors, thereby, helping to reduce the power losses without affecting the maximum power transfer capacity. Soft switching is achieved in the variable inductor control, and the effect of the equivalent tuning parameters on the power flow is analyzed theoretically. Simulation results show that a significant improvement of the existing controllers is achieved at no load or very lightly loaded conditions.

[50] B. A. Welchko, T. M. Jahns, and T. A. Lipo, "Fault interrupting methods and topologies for interior pm machine drives," IEEE Power Electronics Letters, vol. 2, no. 4, pp. 139-143, 2004.

This letter investigates methods to interrupt the phase currents induced when interior permanent magnet (IPM) machine drives suffer short-circuit or uncontrolled generator mode faults. A fault-tolerant silicon switch is proposed in which the antiparallel diode in a reverse-blocking inverter switch is replaced by a thyristor. A reduced-parts-count fault-interrupting topology is also proposed which consists of delta-connected thyristors inserted at the center star point of wye-connected IPM machine stator windings. Control of the proposed reduced-parts-count fault-tolerant drive is discussed and simulation results are presented to verify operation of the proposed topology.

[51] V. G. Agelidis, A. Balouktsis, and I. Balouktsis, "On applying a minimization technique to the harmonic elimination pwm control: The bipolar waveform," IEEE Power Electronics Letters, vol. 2, no. 2, pp. 41-44, 2004.

The well-known harmonic elimination pulse-width modulation (HE-PWM) method for inverter control is revisited. The HE-PWM waveform presents many challenges. It has multiple solutions that not only need to be found as easily and as fast as possible, but must also be evaluated in order to identify the best technique when overall harmonic performance is concerned. Algorithms presented so far rely on starting values that are close to the exact solutions to ensure convergence. A new method based on resultant theory promises limited success since it can only work when a small number of harmonics is to be eliminated. In this paper, it is shown that a minimization technique in combination with a random search results in a relatively simple approach that finds all possible sets of solutions. It is confirmed that numerous independent sets of solutions exist and the ones that offer better harmonic performance are identified. Three cases are reported in detail, including when two, four and six nontriplen odd harmonics are to be eliminated.

[52] Y. Yin and R. Zane, "Digital phase control for resonant inverters," IEEE Power Electronics Letters, vol. 2, no. 2, pp. 51-53, 2004.

This letter presents an approach for direct digital phase control of resonant inverters that is based on inductor current or voltage sensing. Compared to frequency control, phase control provides the advantages of self-tuning to the tank resonant frequency, reduced sensitivity for improved control near resonance, and inherent protection against operation below resonance to avoid hard switching. The digital control algorithm suitable for implementation using standard CMOS logic is derived. The design details of an experimental test platform based on a Xilinx field programmable gate array (FPGA) and experimental results for a typical resonant inverter are provided.

[53] A. M. El-Refaie, D. W. Novotny, and T. M. Jahns, "A simple model for flux weakening in surface pm synchronous machines using back-to-back thyristors," IEEE Power Electronics Letters, vol. 2, no. 2, pp. 54-57, 2004.

Flux weakening in surface permanent magnet (PM) synchronous machines is revisited in this letter. The condition for achieving infinite constant power speed ratio (CPSR) is explained from the machine equivalent circuit and phasor diagram point of view. Back-to-back thyristors, or triac, switches feeding the three phases of a surface PM synchronous machine will be shown to be equivalent to a simple series reactance with respect to fundamental component behavior. Using such switches is equivalent to adding a series inductance to the machine. This additional inductance helps extend the CPSR of surface PM synchronous machines. This is significant because extending the CPSR of surface PM machines is usually a challenging task due to the presence of low-permeability surface magnets and the resulting low machine inductance.

[54] J. Calvente, L. Martinez-Salamero, H. Valderrama, and E. Vidal-Idiarte, "Using magnetic coupling to eliminate right half-plane zeros in boost converters," IEEE Power Electronics Letters, vol. 2, no. 2, pp. 58-62, 2004.

A dynamic analysis of the boost converter with an output filter reveals that magnetic coupling between inductors allows transfer of the zeros to the left half-plane of the control-to-output transfer function. Similar results requiring smaller magnetic components are obtained by combining magnetic coupling with damping of the output filter. The analysis is based on the application of the Routh-Hurtwitz's criterion to the numerator of the transfer function in order to derive the design conditions for the converter parameters. A design example illustrates the procedure, and experimental results verify the theoretical predictions. The application of these techniques will allow the design of high efficiency voltage boost-based regulators with dynamic behavior similar to buck-derived structures.

[55] K. K. S. Leung and H. S. H. Chung, "Derivation of a second-order switching surface in the boundary control of buck converters," IEEE Power Electronics Letters, vol. 2, no. 2, pp. 63-67, 2004.

A second-order switching surface in the boundary control of buck converters is derived in this letter. The formulated switching surface can make the overall converter exhibit better steady-state and transient behaviors than the one with a first-order switching surface. The switching surface is derived by estimating the state trajectory movement after a switching action, resulting in a high state trajectory velocity along the switching surface. This phenomenon accelerates the trajectory moving toward the target operating point. The proposed control scheme has been successfully applied to a 120-W buck converter. The large-signal performance and a comparison with the first-order switching surface have been studied.

[56] P. Midya, B. Roeckner, and S. Bergstedt, "Digital correction of pwm switching amplifiers," IEEE Power Electronics Letters, vol. 2, no. 2, pp. 68-72, 2004.

Pulsewidth modulated (PWM) signals for driving a switching audio amplifier can be synthesized in the digital domain with extremely high linearity and precision. However, nonidealities associated with the power stage degrade output performance. A method to digitally correct for these nonidealities, resulting in very low total harmonic distortion (THD) and high signal-to-noise ratio (SNR) performance, is presented. This method also provides excellent rejection of power supply noise which is otherwise absent in digital PWM amplifiers. To meet noise requirements for hi-fi audio, the feedback structure is a fourth-order structure which shapes the noise beyond the audio band. The method has been implemented on a bread board, and state-of-the-art performance was achieved. Total harmonic distortion of 85 dB and dynamic range of 100 dB was measured using Audio Precision test equipment.

[57] P. Midya, K. Haddad, and M. Miller, "Erratum: Buck or boost tracking power converter (ieee power electronics letters (dec. 2004) 2:4 (131-134))," IEEE Power Electronics Letters, vol. 3, no. 4, pp. 152, 2005.

[58] P. Athalye, D. Maksimovic, and R. Erickson, "Erratum: Variable-frequency predictive digital current mode control (ieee power electronics letters (2004) 2 (113-16))," IEEE Power Electronics Letters, vol. 3, no. 1, pp. 40, 2005.

[59] K.-B. Lee and F. Blaabjerg, "Erratum: A nonlinearity compensation method for a matrix converter drive (ieee power electronics letters (mar. 2005) 1:1 (19-23))," IEEE Power Electronics Letters, vol. 3, no. 2, pp. 81, 2005.

[60] C. L. Toh, N. R. N. Idris, and A. H. M. Yatim, "Constant and high switching frequency torque controller for dtc drives," IEEE Power Electronics Letters, vol. 3, no. 2, pp. 76-80, 2005.

The letter presents a new method of increasing the switching frequency of a direct torque control (DTC) of induction machines. The method simply replaces the hysteresis comparator of the conventional DTC drives with a triangular waveform-based constant switching frequency controller. By synchronizing the digital signal processor (DSP) sampling with a triangular waveform and with an appropriate systematic controller design, a high switching frequency DTC drive is possible without requiring a high-frequency space-vector modulator. The implementation of the controller is simple and operates based on waveform comparisons; in this letter it is implemented using a combination of a DSP and a field programmable gate array device. Simulation and experimental results indicate that the controller both reduces the torque ripple and maintains a constant switching frequency. © 2005 IEEE.

[61] N.-Y. Dai, M.-C. Wong, Y.-H. Chen, and Y.-D. Han, "A 3-d generalized direct pwm algorithm for multilevel converters," IEEE Power Electronics Letters, vol. 3, no. 3, pp. 85-88, 2005.

A three-dimensional (3-D) generalized direct pulse width modulation (PWM) algorithm is proposed for multilevel converters in a three-phase, four-wire system. It is proved to be equivalent to the newly proposed generalized 3-D space vector modulation (SVM). However, the direct PWM greatly simplifies the calculation process and is much easier to implement in digital controllers. The direct PWM can be used in all applications needing a 3-D control vector, such as active filters, uninterruptible power supplies, etc. Simulation and experimental results are given to show the validity of the proposed control strategy. © 2005 IEEE.

[62] P. C. Loh and G. H. H. Pang, "Flux-based two-degrees-of-freedom algorithm for three-phase electronic converter control," IEEE Power Electronics Letters, vol. 3, no. 3, pp. 89-91, 2005.

This letter integrates the tracking robustness of two-degrees-of-freedom control and fast dynamic response of flux-based, pulse-width modulation to develop a new current controller for high performance, three-phase electronic converter control. Theoretical analysis shows that the controller can simultaneously achieve good steady-state, transient and harmonic performance, which are challenges not previously met by existing current controllers reported in the literature. Experimental results are presented to verify the performance and practicality of the proposed controller. © 2005 IEEE.

[63] B. Ozpineci, L. M. Tolbert, and J. N. Chiasson, "Harmonic optimization of multilevel converters using genetic algorithms," IEEE Power Electronics Letters, vol. 3, no. 3, pp. 92-95, 2005.

In this letter, a genetic algorithm (GA) optimization technique is applied to determine the switching angles for a cascaded multilevel inverter which eliminates specified higher order harmonics while maintaining the required fundamental voltage. This technique can be applied to multilevel inverters with any number of levels. As an example, in this paper a seven-level inverter is considered, and the optimum switching angles are calculated offline to eliminate the fifth and seventh harmonics. These angles are then used in an experimental setup to validate the results. © 2005 IEEE.

[64] P. Pejovic, P. Bozovic, and D. Shmilovitz, "Low-harmonic, three-phase rectifier that applies current injection and a passive resistance emulator," IEEE Power Electronics Letters, vol. 3, no. 3, pp. 96-100, 2005.

A new three-phase diode bridge rectifier that provides low harmonic distortion of the input currents applying current injection technique is proposed in this paper. The rectifier applies a novel passive resistance emulator consisting of four diodes and a transformer with the volt-ampere rating of 3.57% of the rectifier-rated power. Optimization of the transformer turns ratio is performed in order to minimize the input current total harmonic distortion (THD). It is shown that with the optimal turns ratio the input current THD equals 3.72%. Dependence of the input current THD on the load current is analyzed, and it is shown that at low load currents the rectifier operates in the discontinuous conduction mode with the THD of 7.79%. The analytical results are experimentally verified on a 2 kW rectifier, indicating that the input current THD lower than 8% is provided within a wide range of the load current variations applying simple circuitry. © 2005 IEEE.

[65] R. S. Balog and P. T. Krein, "Commutation technique for high-frequency link cycloconverter based on state-machine control," IEEE Power Electronics Letters, vol. 3, no. 3, pp. 101-104, 2005.

A technique for commutating the load current of a high-frequency link, pulse-width modulated cycloconverter is presented. As the load current makes a sign transition, switch operation becomes critical to avoid commutation failure. Previous techniques required either dead time or the use of large inductors to limit shoot-through current, thereby compromising performance and efficiency. The technique presented here identifies the switching sequence that is necessary to allow load current to commutate naturally without distortion. The switching sequence is managed with a state machine. © 2005 IEEE.

[66] S. Musunuri and P. L. Chapman, "Improvement of light-load efficiency using width-switching scheme for cmos transistors," IEEE Power Electronics Letters, vol. 3, no. 3, pp. 105-110, 2005.

This paper presents a derivation of the optimum width of transistors to minimize losses in monolithic CMOS buck converters. The high optimal width requires a tapered inverter chain gate driver. A technique called "width switching" is presented. It can be integrated with the inverter chain to maintain maximum converter efficiency over a wide power range, particularly at light load. Experimental results are presented from a chip containing CMOS transistors optimized for power levels between 50 mW and 200 mW. Challenges in implementing the width-switching scheme and other applications are also discussed. © 2005 IEEE.

[67] C. J. Gajanayake, D. M. Vilathgamuwa, and P. C. Loh, "Small-signal and signal-flow-graph modeling of switched z-source impedance network," IEEE Power Electronics Letters, vol. 3, no. 3, pp. 111-116, 2005.

The Z-source inverter is a recently proposed converter topology that uses a unique X-shaped impedance network on its dc side for achieving both voltage-buck and boost capabilities. In the process of designing control schemes for the Z-source inverter, knowledge of the transfer function representing its unique dc impedance network is essential. Toward this end, this letter presents dynamic small-signal modeling of the Z-source impedance network using perturbed mathematical analysis and a signal-flow graph with parasitic components taken into consideration. In particular, the developed average control-to-output model reveals the presence of a right-hand-plane zero in the network transfer function, whose trajectories with variations in network parameters can be studied using classical root-locus analyses. Using the graphical signal-flow modeling approach, various disturbance-to-output transfer functions can also be derived with their parameter sensitivity similarly studied. Lastly, simulation and experimental results are presented for verifying the dynamic phenomena identified in this letter. © 2005 IEEE.

[68] H. Deng, R. Oruganti, and D. Srinivasan, "Pwm methods to handle time delay in digital control of a ups inverter," IEEE Power Electronics Letters, vol. 3, no. 1, pp. 1-6, 2005.

With the popularity of micro-processors, digital controllers are widely used in uninterruptible power supply (UPS) inverters. These digital control systems of UPS inverters require a time interval for sampling and computation, which sometimes affects the performance of inverters. In this paper, the problem of time delay in digital control of a UPS inverter is discussed. Then two novel pulsewidth modulation (PWM) methods, the two-polarity PWM method and the asymmetric PWM method, are proposed to handle the time-delay problem. Both of these PWM methods can achieve a wide range of duty ratio between 0.05-0.95, which is independent of inverter model. Furthermore, they are easy to implement using a digital micro-controller. Experimental results are presented in the paper to verify feasibility of the proposed PWM methods. © 2005 IEEE.

[69] A. Garrigos, J. A. Carrasco, J. M. Blanes, and E. Sanchis, "Modeling the sequential switching shunt series regulator," IEEE Power Electronics Letters, vol. 3, no. 1, pp. 7-13, 2005.

This letter characterizes, in terms of the bandwidth and limit cycle frequency of its constituent subsystems, the sequential switching shunt series regulator - S⁴R, a high-efficiency, low-mass and volume power cell devised to power the next generation of regulated power buses in telecommunication spacecrafts. Transconductance power source modeling is used to obtain linear and nonlinear models. These are used to establish a design control strategy which involves the dynamic response in large load requirements or at the end of the satellite life. Simulations and experimental results are also given to demonstrate the validity of the model. © 2005 IEEE.

[70] K. Jin, X. Ruan, and F. Liu, "Improved voltage clamping scheme for zvs pwm three-level converter," IEEE Power Electronics Letters, vol. 3, no. 1, pp. 14-18, 2005.

This paper proposes an improved zero-voltage-switching pulsewidth- modulation (ZVS PWM) three-level converter, which is improved from the original ZVS PWM three-level converter by merely exchanging the position of the resonant inductance and the transformer, such that the transformer is connected with the lagging switches. The improved converter has several advantages over the original, e.g., the clamping diodes conduct only once in a switching period, and the resonant inductance current is smaller in zero state, leading to a higher efficiency and reduced duty-cycle loss. A blocking capacitor is usually introduced to the primary side to prevent the transformer from saturating. This paper analyzes the effects of the blocking capacitor in different positions, and a best scheme is determined. A 2.5-kW prototype converter verifies the effectiveness of the improved converter and the best scheme for the blocking capacitor. © 2005 IEEE.

[71] K.-B. Lee and F. Blaabjerg, "A nonlinearity compensation method for a matrix converter drive," IEEE Power Electronics Letters, vol. 3, no. 1, pp. 19-23, 2005.

This paper presents a new method to compensate the nonlinearities for matrix converter drives. The nonlinearities of matrix converter drives such as commutation delay, turn-on and turn-off time of the switching devices, and on-state switching device voltage drop is corrected by a new matrix converter model using the direction of current. The proposed method does not need any additional hardware or complicated software and it is easy to realize by applying the algorithm to the conventional vector control. The proposed compensation method is applied for high-performance induction motor drives using a 3-kW matrix converter system without a speed sensor. Experimental results show the proposed method provides good compensating characteristics. © 2005 IEEE.

[72] M. D. Mulligan, B. Broach, and T. H. Lee, "A constant-frequency method for improving light-load efficiency in synchronous buck converters," IEEE Power Electronics Letters, vol. 3, no. 1, pp. 24-29, 2005.

The low-voltage synchronous rectifier buck topology suffers from low efficiency at light loads due to dissipation that does not scale with load current. In this paper we present a method for improving light-load efficiency in synchronous buck converters by reducing gate drive losses. We propose a new gate drive technique whereby the gate voltage swing dynamically scales with load current such that gate drive loss is traded for conduction loss. Since conduction losses scale with the square of load current, an optimal gate swing exists that, at light loads, is shown to be less than the supply voltage. Using this method we obtain a 6.25% increase in converter efficiency at a load current of 10 mA and operating at a constant switching frequency of 2 MHz. © 2005 IEEE.

[73] S. Iqbal, "A three-phase symmetrical multistage voltage multiplier," IEEE Power Electronics Letters, vol. 3, no. 1, pp. 30-33, 2005.

A three-phase symmetrical Cockcroft-Walton (CW) multistage voltage multiplier (VM) is proposed. It consists of one smoothing column and six oscillating columns. The oscillating columns are connected to three-phase power through center-tap transformers. The capacitors of the smoothing column are charged six times per cycle by six oscillating columns and are discharged six times through the load, unlike the conventional symmetrical VM in which they are charged and discharged twice per cycle. The three-phase symmetrical structure completely eliminates the first five harmonic components of load-generated voltage ripple. Theoretical analysis indicates that the proposed three-phase symmetrical CW-VM has one-third the voltage ripple and voltage drop of the conventional single-phase symmetrical CW-VM. Experimental and simulation results of the proposed three-phase symmetrical CW-VM as well as of the conventional single-phase symmetrical CW-VM are presented. A comparison shows that the three-phase symmetrical CW-VM has significantly less voltage ripple, half the voltage drop, and a fourfold increase in output power over the conventional single-phase symmetrical CW-VM. © 2005 IEEE.

[74] X. P. Fang, Z. M. Qian, and F. Z. Peng, "Single-phase z-source pwm ac-ac converters," IEEE Power Electronics Letters, vol. 3, no. 4, pp. 121-124, 2005.

The letter proposes a new family of simple topologies of single-phase PWM ac-ac converters with a minimal number of switches: voltage-fed Z-source converter and current-fed Z-source converter. By PWM duty-ratio control, they become "solid-state transformers" with a continuously variable turns ratio. All the proposed ac-ac converters in this paper employ only two switches. Compared to the existing PWM ac-ac converter circuits, they have unique features: providing a larger range of output ac voltage with buck-boost, reversing or maintaining phase angle, reducing in-rush and harmonic current, and improving reliability. The operating principle and control method of the proposed topologies are presented. Analysis, simulation, and experimental results are given using the voltage-fed Z-source ac-ac converter as an example. The analysis can be easily extended to other converters of the proposed family. The proposed converters could be used in voltage regulation, power regulation, and so on. © 2005 IEEE.

[75] E. C. W. De Jong, J. A. Ferreira, and P. Bauer, "Thermal design based on surface temperature mapping," IEEE Power Electronics Letters, vol. 3, no. 4, pp. 125-129, 2005.

A method of extracting a conservative thermal model from an existing PCB assembled converter is investigated. This improves upon thermal management by increasing the thermal management contribution of the PCB itself. A thermal calibration loop is proposed in which a given converter is analyzed and data extracted, in order to create a thermal map of the surface temperature from which the component layout and thermal profiles can be estimated. Thermal figures of merit are vital to quantify the thermal adjustments, recorded in this thermal map, which are required during thermal calibration. The thermal figures of merit are also flexible enough to allow for specific optimization objectives such as high power density, or overall reliability. Two graphical means to predict temperature profiles required in the thermal calibration loop have been investigated: a thermal resistor network method with a purely analytical approach, suitable for relatively small systems where the geometry and loss analysis are simple (fewer than ten components), or a more elaborate method using a finite difference method algorithm, implemented in a spreadsheet environment. Both provide flexible means for PCB thermal layout and provide straightforward graphical visualization. A case study illustrates the complete design method. © 2005 IEEE.

[76] S.-C. Tan, Y. M. Lai, and C. K. Tse, "Implementation of pulse-width-modulation based sliding mode controller for boost converters," IEEE Power Electronics Letters, vol. 3, no. 4, pp. 130-135, 2005.

This letter addresses the various issues concerning the implementation of a pulse-width modulation (PWM) based sliding mode (SM) controller for boost converters. The methods of modeling the system and translation of the SM control equations for the PWM implementation are illustrated. It is shown that the control technique is easily realized with simple analog circuitries. Various experiments are conducted to test the static and dynamic performances of the system. © 2006 IEEE.

[77] J. W. Kimball, P. T. Krein, and K. R. Cahill, "Modeling of capacitor impedance in switching converters," IEEE Power Electronics Letters, vol. 3, no. 4, pp. 136-140, 2005.

Switched capacitor (SC) converters are gaining acceptance as alternatives to traditional, inductor-based switching power converters. Proper design of SC converters requires an understanding of all loss sources and their impacts on circuit operation. In the present work, an equivalent resistance method is developed for analysis, and equivalent resistance formulae are presented for various modes of operation. Quasiresonant converters are explored and compared to standard SC converters. Comparisons to inductor-based switching power converters are made. A number of capacitor technologies are evaluated and compared for applications to both SC converters and inductor-based converters. The resulting model can be used to accurately predict and optimize converter performance in the design phase. © 2005 IEEE.

[78] S. S. M. Chan, H. S. H. Chung, and S. Y. R. Hui, "A lamp power control scheme for dimmable electronic ballasts to minimize the temperature effect on the lamp brightness," IEEE Power Electronics Letters, vol. 3, no. 1, pp. 34-39, 2005.

A hybrid lamp power control scheme for dimmable electronic ballasts is proposed. Apart from adjusting the brightness of the lamp, the proposed method can also minimize the ambient temperature effect on light output. Instead of regulating the lamp current, the proposed method is based on regulating the power supplying the lamp. First, the dimming operation is achieved by regulating the dc voltage at the ballast inverter input. Second, variation of the lamp power (due to the temperature effect) is regulated by sensing the inverter average input current to adjust the switching frequency of the inverter. An 18 W prototype has been built and tested. The lamp power and luminous output of the prototype at the ambient temperature from 5°C to 35° C have been studied. © 2005 IEEE.

[79] E. Gubia, P. Sanchis, A. Ursua, J. Lopez, and L. Marroyo, "Frequency domain model of conducted emi in electrical drives," IEEE Power Electronics Letters, vol. 3, no. 2, pp. 45-49, 2005.

A harmful aspect of adjustable speed drives is the presence of large high-frequency stray currents. The most important, from the view of electromagnetic compatibility, are common-mode currents at the output and input sides of the converter. Currents at the output flow through the installation ground while those at the input flow through the grid ground. These common-mode currents can cause disturbances in other units that are connected within the same power section or placed close to the drive. This paper proposes a simulation model for a complete drive system based on the frequency domain. The model accurately reproduces the behavior of common-mode currents at any point of the system and allows the user to understand the influence of each system element on the currents. Thus, the model is useful both for designing filter structures and placing them at the proper position inside the adjustable speed drive. The model is validated by means of experimental results on a 5 kVA adjustable speed drive. © 2005 IEEE.

[80] J. Ahola, E. M. Vartiainen, and T. Lindh, "Phase retrieval from impedance amplitude measurement," IEEE Power Electronics Letters, vol. 3, no. 2, pp. 50-52, 2005.

In this letter, we propose a numerical method for computing the phase spectrum of impedance. The developed method is based on the maximum entropy model (MEM) and enables one to retrieve the phase spectrum directly from an impedance amplitude spectrum. The validity of the method is tested with an input impedance measurement of a distribution transformer in the frequency band 100 kHz-30 MHz, where both the amplitude and the phase spectrum of the impedance are measured. According to the tests carried out, the method is useful and it may be applied, for example, in the research of active niters, power line communication, and electromagnetic interference. © 2005 IEEE.

[81] J. S. Lawler, J. Bailey, and J. McKeever, "Minimum current magnitude control of surface pm synchronous machines during constant power operation," IEEE Power Electronics Letters, vol. 3, no. 2, pp. 53-56, 2005.

The dual-mode inverter control (DMIC) was initially developed to provide broad constant power speed range (CPSR) operation for a surface mounted permanent magnet machine (PMSM) having low inductance. The DMIC interfaces the output of a common voltage source inverter (VSI) to the PMSM through an ac voltage controller. The ac voltage controller consists of three pairs of anti-parallel silicon controlled rectifiers (SCRs), one anti-parallel SCR pair in series with each winding of the motor. In a recent paper a fundamental frequency model of DMIC type controllers was developed using an equivalent reactance interpretation of the in-line SCRs. In this work, the same fundamental frequency model is used to show that the DMIC may have considerable loss reduction benefits even if the motor winding inductance is large. Specifically, it is shown that the SCRs enable maximum watts per rms amp control during constant power operation. The rms motor current can be minimized for any given power level and sufficiently large speed with DMIC. A fixed winding inductance and a conventional inverter can only be optimized for a single speed and power level. The performance predicted by the fundamental frequency model of the DMIC is compared to that of a conventional PMSM drive where the motor has sufficiently large inductance to achieve an infinite CPSR. It is shown that the SCRs can reduce motor current by a factor of 0.7071 at high speed and rated power. This would reduce the motor copper losses by 50% and reduce the conduction losses in the VSI by 29.3%. At less than rated power the percentage of motor/VSI loss reduction enabled by the SCRs is seen to be even larger. © 2005 IEEE.

[82] P. Sanchis, J. Lopez, A. Ursua, and L. Marroyo, "Electronic controlled device for the analysis and design of photovoltaic systems," IEEE Power Electronics Letters, vol. 3, no. 2, pp. 57-62, 2005.

The characterization and design of photovoltaic systems is a difficult issue due to the variable operation atmospheric conditions. With this aim, simulators and measurement equipments have been proposed. However, most of them do not deal with real atmospheric conditions. This letter proposes an electronic device that first measures the real evolution of the I-V characteristic curves of photovoltaic modules and generators, and then physically emulates in real time these curves to test photovoltaic inverters. The device consists of a dc-dc converter, a microcontroller and a data storage unit. The two operation modes (emulation and measurement) are digitally driven by the microcontroller. The converter current is controlled by means of a variable-hysteresis control loop, whose reference is provided by the microcontroller. In addition, a digital voltage control loop is designed to find out the complete characteristic curves of the photovoltaic generators. A 15-kW prototype is designed and built that can measure three times per second the characteristic curves of up to seven generators and then emulate their electrical behavior to test photovoltaic inverters. With the proposed device, the optimal configuration and performance of photovoltaic modules and generators, as well as the operation of photovoltaic inverters can be thoroughly analyzed under real atmospheric conditions. © 2005 IEEE.

[83] S. Lineykin and S. Ben-Yaakov, "Analysis of thermoelectric coolers by a spice-compatible equivalent-circuit model," IEEE Power Electronics Letters, vol. 3, no. 2, pp. 63-66, 2005.

The objective of this work was to develop a PSPICE-compatible equivalent circuit of a thermoelectric cooler (TEC). Equivalent circuits are convenient tools for power electronics engineers since they help in presenting a problem in electronic circuit terms and can assist in the design of power stages and the control circuitry and algorithms. A methodology is developed for extracting the parameters of the proposed model from manufacturers' data of TECs. The present model is compatible with PSPICE or other electronic circuit simulators. An important feature of the model is its ability to generate small-signal transfer functions that can be used to design feedback networks for temperature-control applications. Several examples of successful utilization of the model are presented. Data of many different manufacturers were examined and the model parameters were extracted. In all cases, the model was found to reproduce accurately the performance of commercial TECs. The accuracy of the model was also verified by experiments. © 2005 IEEE.

[84] Q. Li and P. Wolfs, "A leakage-inductance-based zvs two-inductor boost converter with integrated magnetics," IEEE Power Electronics Letters, vol. 3, no. 2, pp. 67-71, 2005.

A two-inductor boost converter topology has conduction loss and transformer utilization advantages in converting low-voltage higher current inputs to high output voltages. In this letter, a new zero-voltage switching (ZVS) two-inductor boost converter with integrated magnetics is proposed. In the new topology, the two current source inductors, a resonant inductor and a two-winding transformer, are integrated into one single magnetic core with three windings. Two windings simultaneously perform the functions of the current source inductors and the transformer primary. The transformer leakage inductance forms the resonant inductance. This leads to a much more compact converter design with a significant reduction in the number of core and winding components. A theoretical analysis establishes the operating point of the ZVS converter. Both of the theoretical and experimental waveforms, including flux waveforms for the legs of the integrated core structure, are presented at the end of the letter. © 2005 IEEE.

[85] A. Cichowski and J. Nieznanski, "Self-tuning dead-time compensation method for voltage-source inverters," IEEE Power Electronics Letters, vol. 3, no. 2, pp. 72-75, 2005.

A new method of dead-time compensation is proposed, capable of self-tuning to the inverter and load properties. The tuning, performed real-time, relies on parameter selection of a generic compensation characteristic. During the parameter search, the quality of compensation is being continuously assessed against the harmonic distortion of output currents. The method enjoys very effective compensation of both harmonic and linear distortion caused by dead time and other parasitic agents. © 2005 IEEE.

[86] M. Borage, S. Tiwari, and S. Kotaiah, "A passive auxiliary circuit achieves zero-voltage-switching in full-bridge converter over entire conversion range," IEEE Power Electronics Letters, vol. 3, no. 4, pp. 141-143, 2005.

A passive auxiliary circuit is proposed to achieve zero-voltage-switching (ZVS) over the entire conversion range in a full-bridge (FB) pulse-width modulated (PWM) converter (FBZVS converter) with minimum conduction loss penalty. The stored energy in the auxiliary circuit is minimal under the full-load condition. It increases progressively as the load current decreases. The proposed auxiliary circuit is passive, simple and can be viewed as an add-on to the conventional FBZVS converter. The principle of operation is described and the performance is demonstrated on a 100 kHz, 500 W prototype. © 2005 IEEE.

[87] J. Y. Zhu, "Interpreting small signal behavior of the synchronous buck converter at light load," IEEE Power Electronics Letters, vol. 3, no. 4, pp. 144-147, 2005.

The pulse-width-modulation (PWM) buck converter with synchronous rectifiers operating at light load is usually modeled by its continuous conduction mode (CCM) model. However, the actual power-stage small-signal control-to-output response shows a different behavior from what the traditional CCM model predicts, specifically, more damping around the double-pole frequency, instead of more resonance. This paper presents a modified small-signal light-load model for a synchronous buck converter. The developed model accurately predicts the actual small-signal behavior of a PWM converter at light load. The derived averaged switch model for light load can also be used for the small-signal models of the other basic PWM converters operating in CCM at light load. © 2005 IEEE.

[88] K. D. T. Ngo, S. K. Mishra, and M. Walters, "Synthetic-ripple modulator for synchronous buck converter," IEEE Power Electronics Letters, vol. 3, no. 4, pp. 148-151, 2005.

Comprising a hysteretic comparator and a ripple synthesizer, the synthetic-ripple modulator (SRM) allows voltage-hysteretic modulation to be realized in low-voltage buck converters where the natural voltage ripple is too small for reliable hysteretic operation. Circuit implementation, steady-state operation, and design equations are described for an SRM controlling a buck dc-dc converter. The basics are verified experimentally by a buck converter switched at 420 kHz and delivering 10 A at 1.8 V. © 2005 IEEE.