Analysis and design of a three-phase series-parallel resonant converter
Date
1995
Authors
Zheng, (Raymond) Lu
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Abstract
This thesis proposes a three-phase de-de series-parallel resonant converter (SPRC) with high frequency transformer isolation for medium to high power applications. The analysis, design, simulation and experimental results of the three-phase SPRC are presented in detail in this thesis.
The variable frequency operation of three-phase SPRC in continuous current and lagging PF mode has been presented for both 180 degree and 120 degree wide gating pulse control schemes. Detailed operation of the converter has been explained using the operating waveforms and the equivalent circuit diagrams during different intervals of operation. It has been shown that all the switches turn-on with ZVS and the converter is operating in the lagging PF mode under the 180 degree wide gating scheme control. It has also been shown that the operation with ZVS under 120 degree wide gating scheme control is not practical because this type of operation results in large peak currents through the switches since the converter is operating much above resonance. A single-phase equivalent circuit model has been obtained for analysis purpose. Based on this model, complex ac circuit analysis of three-phase SPRC has been presented. The closed form solutions have been obtained for component ratings of the converter. Design curves have been obtained and a design example has been presented to illustrate the design procedure. The SPICE simulation results for the designed converter have been presented in detail for various load conditions. The theoretical and simulation results have been compared to verify the theory.
Based on the equivalent circuit model and superposition principle, a complete Fourier series analysis in the frequency domain using constant current model for variable frequency control operation of the converter has been presented. The expressions for various voltages and currents have been derived. The various design curves have been obtained and a design of 1 kW converter has been presented. For the designed converter, waveforms obtained from the theory and SPICE simulation have been presented for various load conditions. The theoretical results have been also compared with the SPICE simulation results. It has been observed that the results obtained from the Fourier series analysis method are much closer to simulation results compared to the complex ac circuit approximate analysis approach. Further, a prototype of redesigned 500 W variable frequency controlled converter has been built and tested in the laboratory with an open loop control circuit. The results from theory, simulation and experiments have been compared and shown to be in a good agreement.
The fixed frequency control operation of the three-phase SPRC using variable pulse width gating control scheme has been discussed. Three-phase SPRCs for different switching frequency ratios and two capacitor ratios have been designed under worst case conditions (maximum load, minimum input voltage). These designed converters have been simulated to investigate their performance using SPICE program for variable load conditions while operating under variable pulse width fixed frequency pulse-width modulation (PWM) control. The operating principle with fixed frequency PWM control has been explained for the converter while operating at half load condition. It has been shown that the region of the converter operating with ZVS under fixed frequency PWM control is very narrow. It is clear that fixed frequency PWM operation of the three-phase SPRC has switching losses and results in lower efficiency compared to variable frequency operation.