Single-stage soft-switched high-frequency transformer isolated AC-to-DC bridge converter and extension to multiphase converter

This thesis presents the operation, analysis, simulation and experimental results of a single-phase single-stage soft-switched high frequency (HF) transformer isolated ac-to-dc bridge converter with low total harmonic distortion (THD) and its extension to ac-to-dc multiphase converter. A single-phase single-stage soft-switched ac-to-dc bridge converter cell has been proposed based on a new gating scheme. Due to the discontinuous current mode (DCM) operation of the boost inductor, natural power factor correction and low THD are ensured. The single-stage ac-to-dc multiphase converter is realized based on this bridge converter cell to reduce HF harmonic components in the line current.; The steady-state operation of the single-stage bridge converter is explained for all operating modes. Intervals of operation in these modes are identified and analyzed. The steady-state solutions are presented. Based on these solutions, design curves are obtained. Design example of a 1.7 kW converter is presented to illustrate the design procedure. Operational characteristics are obtained for different line and load conditions. PSPICE simulation results for the designed converter are presented. An experimental prototype is built to verify the operation and performance of the converter. All four switches in the fixed frequency bridge converter undergo soft switching (common switch requires an auxiliary circuit) for a wide line and load range.; A single-stage HF transformer isolated ac-to-dc multiphase converter is proposed. The analyses of the single-stage bridge converter cell are extended to the multiphase converter. It is shown that N = 3 is near the optimum number of cells to reduce the input current HF harmonic components. A design example of a 166 to 260 V (rms) input, 420 V output, 5 kW converter switching at 50 kHz is presented. PSPICE simulation results are obtained for the designed converter to study its performance for varying load and line conditions. A 3-cell 1.5 kW experimental prototype is built and experimental results are obtained. All the results show that HF harmonics in the line current are reduced and output voltage ripple frequency is increased. Each cell handles equal power and all bridge-switches are soft switched. As a result, uniform thermal distribution is obtained.; Small-signal analysis of the single-stage ac-to-dc bridge converter cell is presented for all operating modes using state-space averaging technique. Based on this analysis, small signal transfer functions are obtained. Frequency response of the transfer functions are plotted using MATLAB program and verified by PSPICE simulation results. A closed loop control system is designed and frequency response of the overall loop gain is presented. Large-signal transient behavior of the converter cell is studied with open loop using PSPICE simulation program for step change in line and load conditions. The simulation results show that the closed loop system is required to improve the converter performance during step increase in line voltage.