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 <italic>LLC</italic> resonant half-bridge dc–dc converter under a light-load condition, while achieving the same input voltage range and power-conversion efficiency <italic>η_e</italic> 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 <italic>LLC</italic> resonant converter to have a large magnetizing inductance, and to have high <italic>η_e</italic> 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 <italic>η _e</italic> ≥ 85.46% for an output power range of 7.2–360 W; the highest <italic>η_e</italic> was 96.08% at <inline-formula><tex-math notation="LaTeX">$P_o$</tex-math></inline-formula> = 336 W.