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UCC24612: Malfunction with low on-resistance FET

TerryU
Prodigy 145 points
Part Number: UCC24612

Hi Experts,

We are designing a regulator board with UCC24612. Then we come across the turn-on issue.  It cannot work with TK30A06N1. Checking UCC24612-1 EVA, Evaluation board, it is used 19m ohm on-resistance FET, and our TK30A06N1 has 12m ohm on-resistance. In our trial, changing FET to higher on-resistance one, it works fine. Is there any limitation on the on-resistance? If so, is there any recommendation? And we need technical reason for that.

Regards,

  • 0
    TI__Mastermind 40070 points

    Hello User5151135,

    Thank you for your interest in the UCC24612 SR controller.

    There is some guidance on MOSFET selection given in the datasheet page 24, section 8.2.2.1.  The optimal Rds(on) value depends on the specific application, not simply what the EVM uses. 

    The reason for this is that this controller tries to regulate Vds to about -50mV and senses when Vds becomes less than -9mV as a condition to turn the SR Fet off.  If the Rds(on) is too low, this condition will be sensed too early, even at a high current and the SR will be turned-off too soon.  

    Regards,

    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 145 points

    Hi Ulrich,

    Thank you for your rapid response. I understand that.

    Thanks.

    TerryU