Active Output Stages Synchronous Rectifiers

For those applications where high efficiency is important, synchronous rectification may be used on the higher current (power) outputs. Synchronous rectifier circuits are much more complicated than the passive 2-leaded rectifier circuits. These are power MOSFE B, which are utilized in the reverse conduction direction where the anti-parallel intrinsic diode conducts. The MOSFET is turned on whenever the rectifier is required to conduct, thus reducing the forward voltage drop to less than O.f V. Synchronous rectifiers can be used only when the diode current flows in the forward direction, that is in continuousmode forward converters. 

A small Schottky rectifier with a current rating of about 20 to 30 percent of the MOSFET current rating (/d) is placed in parallel with the MOSFET s intrinsic P-N diode. The parallel schottky diode is used to prevent the MOSFET s intrinsic P-N diode from conducting. If it were allowed to conduct, it would exhibit both a higher forward voltage drop and its reverse recovery characteristic. Both can degrade its efficiency of the supply by one to two percent. 

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