Push-Pull Forward Three-Level Converter with Reduced Rectifier Voltage Stress

This paper proposes a push-pull forward (PPF) three-level (TL) converter, which is suitable for low and wide-range input voltage applications, such as fuel cell power system. Half of the switches sustain half of the input voltage, and the others sustain one and one-half input voltage. Input ripple current and output filter inductor ripple current can be reduced with the TL waveform of the secondary rectified voltage. However, voltage stress of the rectifier diode is high with the conventional TL control strategy in a high output-voltage situation, considering voltage spike across the rectifier diode caused by the leakage inductance. When 1/2 level of the secondary rectified voltage shows first, and I level of that appears later, the voltage stress of the rectifier diode can be reduced by paralleling with external capacitor. Operating principle, control strategy, and selection of the external capacitor are analyzed. Finally, experimental results verify the theoretical analysis.