Design of A High Efficiency Half-Bridge LLC Resonant Converter for Low Power Application

Abstract

The half-bridge (HB) LLC resonant converter is widely
used in high and medium power applications, because of
several advantages such as zero-voltage-switching (ZVS)
capability over a wide input range and low component
count. Most LLC resonant converters employ frequency
modulation scheme to regulate the output power.
However, in wide input voltage applications with widely
varying hold-up time requirement, large switching losses
are incurred during low power applications, which results
in a low overall efficiency. A variable magnetizing
inductance has been proposed for certain
applications. However, this is not easy to implement as the
relationship is not linear. This paper proposes a high
efficiency, high power density half-bridge LLC resonant
converter at low power application. High efficiency is
reached at all power levels by introducing an operating
mode: Low-power mode. Here, the output power is
regulated by adjusting the voltage across the primary
resonant capacitor. This provides for accurate mode
control and linear power control. This method provides
flexibility and ease of design for applications with largely
varying power level requirement.