Martin H. L. Chow

Single-stage single-switch isolated PFC regulator with unity power factor, fast transient response and low voltage stress

In this paper, a simple control method is presented for a single-stage single-switch isolated power-factor-correction (PFC) regulator that can simultaneously achieve unity power factor and fast output voltage regulation while keeping the voltage stress of the storage capacitor low. The converter topology comprises essentially a cascade combination of a discontinuous-mode boost converter and a continuous-mode forward converter. The proposed control utilizes variation of both duty cycle and frequency. The role of varying the duty cycle is mainly to regulate the output voltage. Changing the frequency, moreover, can achieve unity power factor as well as low-voltage stress. Basically, the switching frequency is controlled such that it has a time periodic component superposed on top of a static value. While the time periodic component removes the harmonic contents of the input current, the static value is adjusted according to the load condition so as to maintain a sufficiently low-voltage stress across the storage capacitor. The theory is first presented which shows the possibility of meeting all three requirements using a combined duty cycle and frequency control. An experimental prototype circuit is presented to verify the controllers functions.

A family of PFC voltage regulator configurations with reduced redundant power processing

This paper discusses a systematic method for deriving basic converter configurations that achieve power factor correction (PFC) and voltage regulation. The discussion begins with a general three-port representation of power supplies that provide PFC and voltage regulation. Based on this representation and a power flow consideration, a systematic procedure is derived to generate all possible minimal configurations. Among these configurations, only a few have been known previously and used in practice. It is found that the efficiency of PFC voltage regulators can be improved by reducing the amount of redundant power to be processed by the constituent converters. A systematic circuit synthesis procedure is proposed for creating PFC voltage regulators with reduced redundant power processing. Experimental measurements verify the improved efficiency.

Interleaved three-phase forward converter using integrated transformer

An interleaved three-phase forward converter using an integrated transformer is proposed in this paper. This type of converter has the attractive features of flexible voltage conversion ratio, high output current (due to the parallel connection of outputs), near-zero output-current ripple (due to the output-current-ripple cancellation), fast transient response (due to the small effective output-filtering inductance), and is particularly suitable for high-output-current and low-output-voltage applications such as telecommunication and computer systems. The integrated transformer of the proposed converter consists of three step-down transformers on a single magnetic core. The z-parameter (gyrator) model and the equivalent-circuit model of the integrated transformer are derived. Based on the equivalent-circuit model, the principle of operation of the proposed converter is explained. The analysis and design criteria of the basic circuit, the operation of the regenerative LC snubber circuit, the simulation, and experimental verification are also described.

New single-stage PFC regulator using the Sheppard-Taylor topology

This paper describes a new usage of the DC/DC converter developed by D.I. Sheppard and B.E. Taylor in 1983 for achieving high power factor and output regulation. This converter may be viewed as a cascade of a modified boost stage and a buck stage, with the two stages sharing the same active switch. Two possible operation regimes are described. In the first regime, the converters input part, which is a modified boost converter, operates in discontinuous mode, and the output part, which is a buck converter, operates in continuous mode. In this regime, high power factor is naturally achieved, and the output voltage is regulated by duty-cycle modulation via a simple output feedback. In the second regime, the input part operates in continuous mode, and the output part operates in discontinuous mode, with duty-cycle modulation maintaining a high power factor and frequency modulation regulating the output. Some comparisons between the Sheppard-Taylor converter and conventional boost and buck cascade are given in the paper.

A novel method for elimination of line-current harmonics in single-stage PFC switching regulators

This paper studies a particular single-stage power-factor-correction (PFC) switching regulator employing a discontinuous-conduction-mode (DCM) boost-input cell and a continuous-current-mode (CCM) forward output cell. Although this single-stage PFC regulator can provide a reasonably high power factor when its PFC stage is operating in discontinuous mode, substantial reduction in line-current harmonics is possible by applying a suitable frequency-modulation scheme. This paper derives a frequency-modulation scheme and proposes a practical implementation using a simple translinear analog circuit. A quantitative analysis on the total harmonic distortion (THD) of the line current when the circuit is subject to a limited range of frequency variations is presented along with some considerations for practical design. Experimental data obtained from a prototype confirms the effectiveness of the proposed frequency-modulation scheme. The proposed analog translinear circuit allows custom integrated circuit implementation, making it a viable low-cost solution to the elimination of line-current harmonics in switching regulators.

Analogue implementation of a neural network controller for UPS inverter applications

An analogue neural-network controller for UPS inverter applications is presented. The proposed neural-network controller is trained off-line using patterns obtained from a simulated controller, which had an idealized load-current-reference. Simulation results show that the proposed neural-network controller can achieve low total harmonic distortion under nonlinear loading condition and good dynamic responses under transient loading condition. To verify the performance of the proposed NN controller, a hardware inverter with an analogue neural network (NN) controller (using mainly operational amplifiers and resistors) is built. Additionally, for comparison purposes, a PI controller with optimized parameters is built. Experimental results confirm the simulation results and show the superior performance of the NN controller especially under rectifier-type loading condition. Implementing the analogue neural-network controller using programmable integrated circuits is also discussed.

Practical Design and Evaluation of a 1 kW PFC Power Supply Based on Reduced Redundant Power Processing Principle

Using the reduced redundant power processing (R2P2) principle, a single-phase power-factor correction (PFC) power supply can achieve a higher overall efficiency as a result of the use of a noncascading structure that involves less repeated processing of the input power. This paper investigates a single-phase noncascading PFC power supply based on the R2P2 principle. The circuit employs a current-fed full-bridge converter as the PFC preregulator, and a buck-boost converter as the voltage regulator. This paper addresses the design of this noncascading PFC power supply and in particular the relationships between the gained efficiency, the transient response and the size of the energy storage. Experimental results obtained from a 1 kW laboratory prototype are presented.

Single stage high power factor converter using the Sheppard-Taylor topology

This paper describes a new usage of the DC/DC converter developed by D.I. Sheppard and B.E. Taylor in 1983, for achieving high power factor and output regulation. This converter may be viewed as a cascade of a modified boost stage and a buck stage, with the two stages sharing the same active switch. Two possible operation regimes are described. In the first regime, the converters input part, which is a modified boost converter, operates in discontinuous mode, and the output part, which is a buck converter, operates in continuous mode. In this regime, high power factor is naturally achieved and the output voltage is regulated by duty cycle modulation via a simple output feedback. In the second regime, the input part operates in continuous mode and the output part operates in discontinuous mode, with duty cycle modulation maintaining a high power factor and frequency modulation regulating the output. Compared to the usual boost-buck cascade operating in the first regime, the proposed converter has a wider operating range. When operating in the second regime, the modified boost stage has the ability of producing a harmonic free input current, unlike the standard boost PFC whose current always suffers a cusp distortion.

Control of bifurcation in current-programmed DC/DC converters: an alternative viewpoint of ramp compensation

This paper reexamines the conventional current-mode control strategy as applied to dc/dc converters in the light of “avoiding bifurcation.” This alternative viewpoint permits convenient selection of parameter values to guarantee stable operation. The traditional slope compensation is viewed as a means to keep the system sufficiently remote from the first bifurcation point. It is shown that excessive bifurcation clearance is accompanied by undesirably slow dynamical response. A variable ramp compensation is proposed to dynamically adjust the slope magnitude so that the system is kept clear of bifurcation, yet responds sufficiently fast during transients. The results have been confirmed by experimental measurements.

Design and Performance Considerations of PFC Switching Regulators Based on Noncascading Structures

This paper systematically explores the performances of power-factor-correction (PFC) switching regulators based on noncascading structures in terms of efficiency, input-current harmonic distortion, and load voltage regulation. The investigation begins with simplified power-flow diagrams, which represent the noncascading PFC switching regulators and describe the essential features of the noncascading PFC switching regulators to achieve PFC and voltage regulation. Based on these diagrams, the noncascading PFC switching regulators can be classified into three categories, each offering a different set of performances. The first category permits a tradeoff between the efficiency and the input-current harmonic distortion, the second permits a tradeoff between the efficiency and the size of the storage element for the load voltage regulation, and the third allows a tradeoff among all the performances. With detailed analyses through analytical approaches, simulation results illustrate the performances of these three categories of noncascading PFC switching regulators. An experimental prototype of the third category has been built to validate the analyses.