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PMP21783: Where is the resonant tank circuit?

Jamie Shaffer1
Prodigy 220 points
Part Number: PMP21783
Other Parts Discussed in Thread: UCC28742

This is described as a QR type flyback converter. However, looking at the schematics, I only see the FET and RCD clamp circuits connected to the primary, without any resonant capacitor or damping resistor. How is the resonance at demagnetization of the primary leakage inductance handled?

  • +1
    TI__Intellectual 1570 points

    Hi James,

    Sorry for the confusion, QR type is not an accurate way to describe this flyback. We will get this change made to that design's page.

    UCC28742 is a high-efficiency flyback controller that uses the resonant-ring to achieve valley-switching operation. The design will work like a typical DCM flyback, however the valley-switching will reduce the switching loss encountered.

    Thanks,

    John

  • 0 in reply to John Dorosa
    Prodigy 220 points

    Hi John,

    Thanks for clearing that up.

    Follow up question: Does the resonant frequency (for valley switching) thus depend on the transformer leakage resonating with the FET body capacitance? This seems to be the case. 

    Thanks,

    Jamie

  • 0 in reply to Jamie Shaffer1
    TI__Intellectual 1570 points

    Hi Jamie,

    Correct, the resonant capacitor will be the sum of the FET's output capacitance and the PCB's parasitic capacitance.

    Thanks,

    John