CONSTANT FREQUENCY VARIABLE POWER FACTOR CONTROL OF SINGLE PHASE BOOST AC-DC CONVERTER

Abstract

A constant switching frequency method for controlling a four-switch single phase ac-dc boost type converter, to draw near-sinusoidal input currents at any desired power factor is presented. The method, named Predicted (on-time) Equal Charge Criterion (PECC) method, controls the on-time of the switching devices of the converter such that the average of the input current equals the average of the sinusoidal reference template over the switching interval, for each switching interval. The method has the advantages of constant switching frequency, variable power factor operation capability, fast current loop response, bidirectional power flow capacity and capability for regenerative operation.

An initial method (Method A) that required very few components for control was simulated using SABER simulation tool. The method was unstable, with the input current reaching runaway values. This necessitated a modification, which resulted in a computationally more complex method (Method B), which was studied using analysis based simulation. Of the two possible ways of implementing Method B (termed mode sequences}, each one resulted in stable operation in one half of the line voltage waveform. A combination of the two mode sequences, one for each half cycle, gave stable operation, but considerable amount of transients were observed near zero crossings. Stable and transient-free operation was achieved by introducing a transition interval between mode sequences while changing over from one to the other. Further simulation studies showed that variable power factor operation and regenerative operation are possible by suitably phase shifting the reference current with respect to the input voltage.

The PECC method was implemented on an IGBT converter, switching at 10 kHz, using an Intel 80386 microprocessor-based system. Due to requirement of time for Analog-to-Digital conversion of power quantities and computation of on-time, in the implementation, during each switching period, the on-time of the appropriate devices of the converter is computed for the subsequent switching period. The performance was observed and the waveforms plotted with the PECC method. Operation was observed to be satisfactory at different ranges of input and output voltage, power ratings and power factors. It was seen that the input current waveforms were near-sinusoidal without any serious distortions. Slight deviations from the simulation results were observed. These were studied and measures for minimising these deviations taken. The spectrum of the input current waveforms showed components centred around integral multiples of the switching frequency, clearly establishing that the operation was at constant frequency. Variable power factor operation was also observed to be satisfactory. The response of the system to step changes in input current reference was seen to be very fast, with the system responding to the changes within 3-4 switching intervals.

The simulation and experimental results for the PECC methods show the method offers advantages of constant frequency, variable power factor, bidirectional power flow capability, and excellent dynamic current response. These make the method attractive in applications like front end converters in UPS systems, traction and low power ac drives.