Analysis and Design of SR Driver Circuit for LLC DC-DC Converter Under High Load Current Application

In low output voltage high load current LLC dc-dc converters, synchronous rectifiers are usually used to reduce the secondary loss. The proper conduction time of synchronous rectifiers (SRs) is key to improve efficiency and make the converter run more reliably and stably. In high load current LLC dc-dc converters, voltage ringing across SRs makes SRs turn-on early when the current flowing through SRs decreases to zero, which causes the circuit to work abnormally. This paper presents analysis and design of SR driver circuit for LLC dc-dc converter under high load current application and eliminates the effect of voltage ringing across SRs by a RC filter. The proposed filter is simple and lossless, which is easy to utilize in the circuit. The voltage ringing across SRs is analyzed and the design parameters of the filter are given in this paper. The experimental results show that 1.26kW LLC dc-dc converter with the proposed filter works reliably at high load current with high efficiency and high-power density.

I. Introduction

High efficiency and high power-density switching power supplies are increasingly required. For this reason, resonant dc-dc converters, such as LLC dc-dc converter, which have become more popular and is used widely in power supply application as this circuit benefits from simplicity, low cost, high efficiency and soft-switching. However, due to the forward voltage drop of rectifier diodes, there is significant loss on rectifier diodes in some low output voltage high load current applications. Therefore, synchronous rectifiers (SRs) are always utilized for high load current LLC dc-dc converters to reduce the secondary loss [1].