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Synchronous rectifier FETs without turn-OFF diodes

cupoftea

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Hi,
Why do we see schems of eg synchronous SMPS with no diode turn-off for the synch fets?
Its well known that we often need to increase the gate series resistor so that turn on is slowed up.
However, we never want a synch rect fet to turn off slowly...because it lingering on may cause shoot-through currents.
So thus a synch rect fet needs a turn off diode, which bypasses any series high value gate turn on resistor.
But why do all the demo boards not feature this?
 

dick_freebird

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For high stepdown ratio and low frequency, perhaps the "off"
time is more than the sync FET needs to discharge through
the plain gate R. This is the opposite case from the high side
FET, where turnoff time should be snappy if you need to
keep duty / PW low.
 

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