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PMP11107: Self-driven rectifier circuit in flyback designs (by D. Strasser)

Sven Pauli1
Prodigy 40 points
Part Number: PMP11107
Other Parts Discussed in Thread: PMP11065

Hi,

in the flyback converter reference designs PMP11107 and PMP11065 the author implemented a self-driven secondary rectifier:

It works reasonably well in situations where one does not want to employ dedicated ICs. But I am struggling a little to fully understand the circuit. My musings so far:

  • In principle I think Q1 is used to quickly rush out the gate charge from Q2 to turn it off.
  • R7 limits the static base current of Q1 to some quiescent level, and C10 provides an AC path for a much higher current peak to speed up the switching of Q1.
  • D6 might be there to protect the base-emitter junction of Q1 from being excessively reverse-biased (might otherwise die at some 6V or so).
  • R5 and D4 then seem to provide the turn-on path for Q2.

Does that remotely make sense?

Thanks a lot & kind regards,

Sven

  • +1
    TI__Intellectual 2420 points

    Sven,

    Yes, your analysis is correct.  This circuit was implemented to block the reverse transformer voltage from appearing on the gate pin.  D4 and D6 block the reverse voltage so the gate of Q2 is clamped to the source through Q1.  This is useful in wide input voltage converters where the reverse transformer voltage can exceed the FET gate voltage rating.  Even with standard input voltage ranges, the fast turn-off provided by Q1 helps to improve efficiency.

    Thanks,

    David

  • 0 in reply to David Strasser
    Prodigy 40 points

    Hi David,

    many thanks for the explanation.

    Also, I notice that I kind of neglected the reverse voltage, so again thanks for pointing this out.

    Kind regards,

    Sven