Active clamp LLC resonant converter for point-of-load applications

This paper describes an active clamp LLC resonant converter intended for point-of-load (POL) power supplies. The active clamp LLC converter inherits advantages of the standard LLC resonant converter including zero voltage switching (ZVS) of primary side MOSFETs, zero current switching (ZCS) of synchronous rectifier MOSFETs and 50% duty cycle operation of all switching devices. The active clamp addresses the voltage oscillation across the rectifier devices caused by transformer secondary-side leakage inductances and MOSFET output capacitors by clamping voltage to twice the output voltage while reducing the output capacitor current ripple. The output voltage can be regulated by simple on/off control, and the converter is well suited for a multiple-parallel-module configuration. The converter module operation, modeling, and design approach are verified on two 1 MHz, 24 V-to-3.3 V, 6 W experimental prototypes.