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IR HEXFET™ Synchronous Rectification (KA-02061)

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IR HEXFET™ Synchronous Rectification (KA-02061)

It is replacing diodes with HEXFET™ MOSFETs. The forward voltage drop of a HEXFET™ MOSFET is current^2 X Rdson. In many cases this can be significantly less than the Vf of a diode having similar maximum current ratings. When rectifying AC voltages and currents, because the HEXFET™ MOSFET will only show a low forward voltage drop when it is fully enhanced, the gate to source control signal voltage will require synchronising with the AC input to be rectified. Also as HEXFET™ MOSFETs have intrinsic body diodes that conduct in the forward direction between source and drain with N types and between drain and source with P types, they must be placed in circuit with their body diodes in the same orientation as the diodes they are replacing, so as to prevent back feeds when the HEXFET™ MOSFETs are turned off. When fully enhanced (turned on) the conduction path through HEXFET™ MOSFETs are bidirectional with the body diodes bypassed by a very low resistance path (Rdson). Hence the need to synchronise the gate signals with the AC input applied to the rectification circuit. The added complication of synchronous rectification is only justified if the power dissipation and resultant temperature rise in an application is more important than cost.

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