A More-Efficient Half-Bridge LLC Resonant Converter: Four Methods For Controlling The MOSFET

by Gordon Wang and Alex Lin, Fairchild Semiconductor, Taipei, Taiwan, How2Power Today, Sep 28 2012

Focus:
Using a half-bridge LLC resonant converter in a power application can deliver higher efficiency and a more-compact design. The primary MOSFET of the LLC resonant converter can easily reach zero voltage switching (ZVS) and zero current switching (ZCS), and that saves energy. Also, because the circuitry of the LLC resonant converter works without a storage inductor on the secondary side, the PCB is smaller. LLC resonant converters typically use a secondary-side synchronous rectifier (SR) that operates in boundary conduction mode (BCM) or discontinuous conduction mode (DCM). But, regardless of the operating mode, current in the secondary-side SR can introduce power losses and reduce the overall efficiency of the design. One way to reduce these losses is to use detection signals to control the turn-on and turn-off of the MOSFETs. This article describes four different methods for doing so: detecting the secondary-side currents, detecting the voltage of the secondary-side SR, detecting the turn-on period of the secondary-side SR, and synchronizing with the LLC signal. The tradeoffs of each method are discussed.

What you’ll learn:

• How to improve efficiency of a half-bridge LLC resonant converter by reducing losses in the secondary-side synchronous rectifier

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