Power Tips: How to Implement Synchronous Rectifiers in a Resonant LLC Half-Bridge

Other Parts Discussed in Post: PMP8920

Resonant LLC half bridge converters are great for offline, high power applications (200-800W) because the primary FETs benefit from zero voltage switching (ZVS).  The LLC converter needs a fairly narrow input range, so it is usually accompanied by a PFC front-end.  At these power levels, losses in the output rectifying diodes can become a big problem with lower output voltages.  Replacing the diodes with synchronous FETs seems like an obvious decision to mitigate these losses, but how do you control the synchronous FETs?

Luckily, the secondary current and voltage waveforms in the LLC half-bridge are well suited for using a GREEN rectifier™ controller device.  The GREEN rectifier controller is typically used in flyback power supplies, as shown in Figure 1, where it monitors the drain-to-source voltage on the synchronous FET.  

GREEN rectifier controller used in a flyback converter

Figure 1. GREEN Rectifier controller used in a flyback converter.

When the body diode starts to conduct, the GREEN rectifier controller senses the forward voltage drop and turns-on the FET.  While the FET is on, the controller continues to sense the drain-to-source voltage, and turns the FET off when the current has decayed close to 0 Amps.  The synchronous FET is held off until the next switching cycle.  By controlling the synchronous FET in this way, the FET is essentially operated as an ideal diode.  In fact these types of controllers are sometimes referred to as “near ideal diode emulators.”

When a GREEN rectifier controller is used in a resonant half-bridge topology, the secondary of the transformer is usually configured as a current-doubler, as shown below in Figure 2.  Each secondary winding has its own FET (or bank of paralleled FETs), and requires its own rectifier controller.  The synchronous FETs will see a voltage stress equal to twice the output voltage.  By combining the ZVS features of the LLC with synchronous rectification, very high efficiencies can be obtained.  PMP5967, for example, generates a 12V/460W output from a 380VDC input with over 94% efficiency.

GREEN Rectifier controllers used in an LLC-resonant half bridge converter

Figure 2. GREEN Rectifier controllers used in an LLC-resonant half bridge converter.

 I recently wrote and article on "Applying synchronous rectification in resonant half-bridge converters" for PowerElectronics.com, I invite you to check out for more details on how the GREEN rectifier controller operates in resonant half-bridge LLC converters.  

Also, here are a few reference designs from our PowerLab library…

  • PMP4332 - 5V80A Low Profile High Efficiency Power Supply
  • PMP5967 - 400VDCin, 12V@460W, LLC with Synch Rectifiers
  • PMP8920 - Universal Input TM PFC and LLC Converter
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