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Reverse recovery in Phase shift full bridge SMPS?

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cupoftea

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Hi,
Page 12 of this, contains the following text, warning of the reverse recovery problem in PSFB, and why special FETs with low trr internal diode are needed....

Robust body diode with fast reverse recovery. Although that in normal operation the body diode current/charge are softly commuted, in some conditions such as startup, load transient, light load or low leakage inductance, body diode may have hard commutation, it may not have a channel conduction following its own conduction, or channel conduction might be too short and not enough to completely sweep out the reverse recovery charge, in such case, as the MOSFET turns off with a high dv/dt while there are still residual charge in the body diode region, the charge leaving the body diode P-region may bias the parasitic npn BJT, causing false turn on and destruction of the MOSFET.
..in the above, it doesnt make sense, because it speaks of a problem to the mosfet when the diode was conducting, and then the mosfet turned off......but that doesnt make sense, because if the mosfet was conducting, then the diode wouldnt have been conducting in the first place.

What are your thoughts?
 
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