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UCC24612: early turn-off

Chong Yu
Intellectual 1435 points
Part Number: UCC24612

Hi,

My customer need assistance.

When I used UCC24612 in an active clamp flyback converter, I found that premature turn-off occurred when the switching frequency was 500-600KHz (the power switch is BSZ018N04LS6 from infineon).

Please tell me whether you consider the problem of early turn-off when designing the chip, and how to solve this problem in system applications.

Please help. Thanks.

  • 0
    TI__Expert 7080 points

    Hi Chong,

    In ACF topology , SR FET not always conduct (turn on) during the whole high side FET turn on time , since the SR current equal to the resonant current minus demagnetizing current . The SR current will premature reach to zero special when the 1/2 resonant period(Lk&Cclamp) shorten than the PWMH on time .

    You have to probe the transformer primary winding current , low side FET Vds and SR gate driver to identify if it's a premature turn off or a normal behavior.

    Thanks.

  • 0 in reply to Jaden Ning
    Prodigy 20 points

    I am sure that it was abnormally turned off early. When I replaced the BSZ018N04LS6 with a directMOS with a smaller parasitic inductance, this phenomenon disappeared. This is an early turn-off caused by a larger di/dt. Replacing MOS is a solution, but the cost is higher. I would like to consult if there are other solutions.

    Thanks

  • 0 in reply to Minggang Chen
    TI__Expert 7080 points

    Hi Minggang,

    I have checked the datasheet of BSZ018N04LS6 , the Rdson is 1.8mohm, Basically , UCC24612 is commonly used on low output power application . it's typical turn off threshold is 9mV , that means SR turn off current is 5A and it will entry proportional gate driver when Isr= 27A.

    May I ask what is your output voltage and maximum load current , we recommend chose >4mohm Rdson SR FET .

    Thanks.

  • 0 in reply to Jaden Ning
    Prodigy 20 points

    OK, I see.Thank you very much!

    Vo=12V, Iomax=5A