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UCC28056: UCC28056 question about FET Loss during reverse conduction?

Bob Lee62837
Mastermind 7230 points
Part Number: UCC28056

Dear support member,

My customer used UCC28056.

I have a question about page 32 of the data sheet.

(Data sheet 32page description)
『For best efficiency, use a MOSFET that incorporates a fast body diode.
Operation using discontinuous inductor current (DCM) from a low input voltage incurs additional switching power loss if a MOSFET with slow body diode is used. 』

(Question)
Since no voltage is applied during reverse conduction,
I think this FET does not cause recovery loss.

Is this notation incorrect?

Best regard.
Bob Lee.

  • 0
    TI__Mastermind 40430 points

    Hello Bob,

     

    Thank you for your interest in the UCC28056 TM PFC controller.

    The note on efficiency on page 32 is correct. Your conception of “no voltage applied during reverse conduction” is valid for transmission mode (TM) operation, where the MOSFET it turned on again while the body-diode is conducting the L-C resonant current.

    However, at lighter loads when operation shifts to DCM, the MOSFET turn-on is delayed to the next voltage valley as needed to maintain a better average input current wave shape.   During this interval, negative current in the body-diode rings back to positive current at which point we expect the body-diode to shut off and low amplitude ringing continues with parasitic capacitance.

    If the body-diode has slow recovery characteristics, a large positive current can build up in the inductor and increase the ringing current amplitude leading to higher losses when the diode finally does recover and turns off. Using a MOSFET with fast body-diode recovery will minimize this concern.

     

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Mastermind 7230 points

    Dear Ulrich Goerke,

    Thank you very much for your answer.
    I reply my customer your answer.

    I will feedback it.


    Best regard.
    Bob Lee.