AN-9067 — Analysis of MOSFET Failure Modes in LLC Resonant Converter

The trend in power converters is towards increasing power densities. To achieve this goal, it is necessary to reduce power losses, overall system size, and weight by increasing the switching frequency. High reliability is also very important for today’s switched-mode power supplies (SMPS). The zero-voltage-switching (ZVS) or zero-currentswitching (ZCS) topologies that allow for high-frequency switching while minimizing the switching losses are of interest. The ZVS topology operating at high frequency can improve the efficiency as well as reduce the size of the application. It also reduces the stress on power switches and therefore improves the reliability. LLC resonant half-bridge converters are becoming a popular topology because they can provide these benefits. It has become widely accepted for applications from high-end servers to flat-panel display power supplies, but the ZVS bridge topologies including LLC resonant half bridge require a MOSFET with fast reverse-recovery body diode for better reliability. This application note discusses potential failure mode and mechanism in LLC resonant converters and provides a simple and cost-effective solution to prevent failures.