A Resonant Push-Pull DC-DC Converter

This paper presents a novel topology of a resonant push-pull DC-DC converter. The primary side of the transformer applies a traditional two-transistor push-pull circuit structure, while the secondary side of the transformer includes a circuit of duplex windings and a LC resonant voltage-double structure. The average output voltage of the converter is four times of the voltage-double resonant capacitors. The LC resonance of the voltage-double capacitors and the secondary leakage inductors of the transformer transfers the energy from the primary side to the secondary side. The circuit adopts the strategy of fixing turn-on time and frequency modulation method. All switches and diodes can achieve ZCS (zero current switching). The paper analyses every operation mode in detail. Because of symmetrical working of the secondary upper and lower windings of the transformer, it can be transformed into two-port network to analyze the AC equivalent circuit through fundamental analysis method. And on this basis, the paper deduces the gain characteristic expression of the AC fundamental wave, plots the curve of voltage gain with different frequency ratio m, excitation inductance/leakage inductance ratio h and quality factor Q by MATLAB, and utilizes the voltage gain curve to design the circuit experimental parameters. At last, a sample converter of 20-28 V input/360 V output/rated power 400 W is built and tested. The experimental waveforms verify the correctness of the circuit and validity of the gain model. The experimental conversion efficiency indicates that the switch achieves ZCS to reduce the switching loss of existing push-pull circuit effectively and improve the conversion efficiency. The measured efficiency was 93.5% at rated load.