High-power density design of a soft-switching high-power bidirectional DC-DC converter

A typical non-isolated bi-directional dc-dc converter technology is to combine a buck converter and a boost converter in a half-bridge configuration. In order to have high-power density, the converter can be designed to operate in discontinuous conducting mode (DCM) such that the passive inductor can be minimized. The DCM associated current ripple can be alleviated by multiphase interleaved operation. However DCM operation tends to increase turn-off loss because of a high peak current and its associated parasitic ringing due to the oscillation between the inductor and the device output capacitance. Thus the efficiency is sufferred with the conventional DCM operation. Although to reduce the turn-off loss, a lossless capacitor snubber can be added across the switch, the energy stored in the capacitor needs to be discharged before device is turned on in order to realize zero-voltage switching. This paper adopts a gate signal complimentary control scheme to turn on the non-active switch and divert the current into the anti-paralleled diode of the active switch so that the main switch can turn on under zero-voltage condition. Thus both soft switching turn-on and turn-off are achieved. This diverted current also eliminates the parasitic ringing in inductor current. For capacitor value selection, there is a trade-off between turn-on and turn-off losses. This paper suggests the optimization of capacitance selection through a series of hardware experiments to ensure the overall power loss minimization under complimentary DCM operating condition. A 100kW hardware prototype is constructed and tested. The experimental results are provided to verify the proposed design approach.