Off-Time Control of LLC Resonant Half-Bridge Converter to Prevent Audible Noise Generation Under a Light-Load Condition

This paper presents a control method that can prevent audible noise generation in an LLC resonant half-bridge dc-dc converter under a light-load condition, while achieving the same input voltage range and power-conversion efficiency ηe as the burst-control method that generates an audible noise. The proposed method reduces switching and conduction losses at light load by skipping several pairs of switch-control pulses, while varying the switching frequency less than the normal-control method does. This skip-control method enables the LLC resonant converter to have a large magnetizing inductance, and to have high η
_e
over a wide range of load variation. At an input voltage of 385 V, an output voltage of 24 V, and a resonant frequency of 180 kHz, the proposed method achieved η e ≥ 85.46% for an output power range of 7.2-360 W; the highest η
_e
was 96.08% at P
_o
= 336 W.