The gates of the MOSFET must be precisely driven. If the gates turn off too
slowly, then a punch-through current can occur between power switch MOSFET
and the synchronous rectifier MOSFET. This is where both the MOSFETs are
momentarily conducting at the same time. This unrestricted current would add
to the losses and quickly lead to a failure. Figure 3–31 shows the critical periods
in the operation of the synchronous rectifier.
The hidden loss for synchronous rectifiers is the charge needed to drive the
gates of the MOSFETs. Refer to Section 3.7.2 for the information on the gate
charge calculations. Essentially, one must drive a gate whose capacitance ...
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