Analysis on the Effect of Secondary Side Devices for the Operation of GaN Based LLC Resonant Converter

This paper focuses on the LLC resonant converter working in the zero-voltage-switching (ZVS) region where the switching frequency (f
s
) is larger than the resonant frequency (f
r
). The secondary side devices critically affect both the normal operation and deadtime operation of the LLC converter. The effect of diode junction capacitance (C
j
) and reverse recovery on the ZVS condition during deadtime has been discussed in the literature, but their effect on the waveform during normal operation has not. In this paper, a detailed description and analysis of deadtime operation with Silicon and Silicon Carbide (SiC) diodes are provided firstly and an accurate model for their effect on the following normal operation is derived based on the deadtime analysis. From the derived model, an improved ZVS condition for f
s
> f
r
region is presented, which can also provide the guideline for the selection of secondary side devices. To verify the proposed analysis and model, a 694 kHz/100 W GaN based LLC converter prototype with different secondary side devices is built and tested.