Analysis of Diode Reverse Recovery Effect on ZVS Condition for GaN-Based LLC Resonant Converter

LLC resonant converter can achieve zero voltage switching (ZVS) for primary-side devices and zero current switching (ZCS) for secondary-side rectifiers. However, the reverse recovery and junction capacitance (C-j) of secondary-side diode critically affect the ZVS condition of primary-side switches. The effect of C-j has been discussed in literature, but not the reverse recovery. In this paper, the reverse recovery charge (Q(rr)) is converted to an equivalent capacitance (C-rr_(eq)) for the study of primary-side ZVS performance. An accurate model during deadtime is derived and further applied to characterize ZVS performance with different reverse recovery charges in different regions. The concept of establishing parameter C-total to consider both C-j and C-rr_eq is proposed to evaluate the effect of the secondary-side rectifiers. This concept provides the guideline for diode and synchronous rectification MOSFET selection to ensure ZVS condition for LLC converters. To verify the concept and the derived model, a 200/400 V 400 W LLC resonant converter prototype operating from 200 to 700 kHz is built and its ZVS performances with different diodes are compared. Two issues caused by Q(rr) effect, including V-ds reverse charging and asymmetrical waveform during deadtime, are explained thoroughly as well.