This paper proposes an asymmetric forward-flyback dc-dc converter that has high power-conversion efficiency ηe over a wide output power range. To solve the problem of ringing in the voltage of the rectifier diodes and the problem of duty loss in the conventional asymmetric half-bridge (AHB) converter, the proposed converter uses a voltage doubler structure with a forward inductor Lf in the second stage, instead of using the transformer leakage inductance, to control output current. Lf resonates with the capacitors in the voltage doubler to achieve a zero-voltage turn-on of switches and a zero-current turn-off of diodes for a wide output power range. The proposed converter could operate at a wider input voltage range than the other AHB converters. ηe was measured as 95.9% at output power PO = 100 W and as 90% at PO = 10 W, when the converter was operated at input voltage 390 V, output voltage 142 V, and switching frequency 100 kHz.
- DC-DC power conversion
- Transformer windings
Fig. 1. Circuit structure of the proposed converter.
EXPECTED SIMULATION RESULTS
Fig. 2. Voltage and current waveforms of switches of the proposed converter
at (a) PO = 100 W and (b) PO = 10 W.
Fig. 3. Voltage and current waveforms of D1 and D2 at PO = 100 W: (a) the proposed converter, (b) the conventional AHB converter, and (c) the converter of .
The proposed asymmetric forward-flyback dc-dc converter had high power conversion efficiency ηe for a wide range of output power PO. The problems of voltage ringing and duty loss in the conventional AHB converter was solved by adopting a forward inductor Lf in the voltage doubler circuit of the secondary stage. The proposed converter used an unbalanced secondary turns of transformer which allowed it to operate for a much wider range of input voltage than the other converter  that uses a voltage doubler structure in the secondary stage. The proposed converter also reduced the voltage stress on switches and the current stress on diodes significantly compared to the dual resonant converter (the converter of ). The proposed converter had ηe ≥ 90% for 10 ≤ PO ≤ 100 W at VIN = 390 V, VO = 142 V, and fS = 100 kHz (the highest ηe = 95.9%, at PO = 100 W), and could operate at 330 ≤ VIN ≤ 440 V. The proposed asymmetric forward-flyback dc-dc converter is a good candidate for developing a step-down dc-dc converter for applications that require high power-conversion efficiency over wide ranges of input voltage and output power.
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