Enhance Synchronous-Rectification Control In Flyback Converters

Posted on Feb 5, 2014

The transition from diodes to synchronous-rectification (SR) MOSFETs in secondary circuits of flyback converters increases with each new generation of MOSFETs, improving performance at little or no cost penalty. SR MOSFETs can be more efficient than diodes, allowing lower operating temperatures and smaller heat sinks, or no heat sinks at all. Howe

Enhance Synchronous-Rectification Control In Flyback Converters
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ver, they require a control circuit to manage their switching behavior in order to emulate a diode. The usual synchronous rectifier control method in today`s commercial power supplies involves deriving the logic signal for the controller from the secondary of a current transformer. There is a better way, though. Traditionally, flyback converters were well suited for applications requiring power levels less than 150 W. Their major appeal was simplicity and low cost. Beyond 150 W, and certainly at power levels of 200 W and beyond, the half-bridge- and forward-converter were the standard topologies. The major problem with flybacks, whether they were implemented with diodes or SR MOSFETs, was semiconductor conduction losses. As with all isolating power-converter topologies, flybacks employ a transformer on the secondary, on which resides a rectifier. The simplest configuration uses a half-wave rectifier diode on either the high or low side ( Fig. 1a ). Synchronous rectification combines a MOSFET with some kind of control for turning the device on or off so that it emulates the diode commutation of the ac from the transformer. The synchronous approach provides greater efficiency, albeit with a corresponding tradeoff in complexity and cost ( Fig. 1b ). What kind of losses are we talking about For a diode, the forward-conduction power loss is simply the product of the forward voltage and current. For a MOSFET, it`s I2 RDS(ON)....

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