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A Novel Driving Scheme of Synchronous Rectifiers Suitable for LLC Resonant Converters (2006)

The higher efficiency improvements will be achieved by optimizing the rectifier stage. Thus, the way to keep efficiency is to substitute diodes by synchronous rectifiers, featuring lower voltage drop. Unlike PWM converter, the turn on time of SR and turn on time of primary side switches are not exactly in phase for resonant converters. As a result, generally, the SR and primary side switches may not share the same driving signal in resonant converters. A promising driving method is to utilize the drain to source voltage of the SR. The sensed Vds of SR is processed by control circuits to determine the level of the current. The SR can be switched in close proximity of the zero current transition. However, the accuracy of this driving scheme is highly affected by the package of the SR. Due to the inevitable package inductance, the sensed terminal drain to source voltage of SR is actually the sum of MOSFET resistive voltage drop and package inductive voltage drop. As a result, it leads to much less SR conduction time. CPES proposed a novel driving scheme to phase compensation driving scheme. An active phase compensation network is used to provide the true resistive voltage of Vds. Thus, the generated voltage reflects exactly the conduction period of SR. SR can be driven correctly. Both conduction loss and SR body diode reverse recovery loss can be greatly reduced. High efficiency can be achieved.

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