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TPS40041 - Should we use big Qg Bottom FET than Uppoer FET to prevent shoot through even if used controller of adaptive ant-cross function?

Tom Lee
Prodigy 80 points
Other Parts Discussed in Thread: TPS40041

I have some issue which no start up at full load(5Vin 3.3Vo 20A), the circuit has two same FET in upper and bottom side.

The start up is ok at 50% load condition, but fails at full load, so I am wondering it happens shoot through due to same Qg FETs(Qgd/Qgs >1).

The problem is gone if boot resistor increase, but efficiency hurt a little bit, so I am considering to change bottom FET with a FET of big Qg than upper FET and Qgd/Qgs<1.

The controller is TPS40041. I'd appreciate if somebody give some advice, Thanks.

  • 0
    TI__Genius 17240 points

    Hi Sookil,

    I am not convinced that the problem is caused by the shoot through. It is also possible that the noise on switching node causing short-circuit protection. It also can be solved by increasing boot resistor value. You can motinor the input current to confirm whether there is surge current due to shoot through. If that is the reason, you'd better change the bottom FET as you mentioned. Otherwise, if it is due to noise on switching node and you don't want to hurt the efficiency, you may be able to get rid of it by following layout improvements.

    (1) Increase the input capacitor and put high-frequency cap close to the drain of high-side FET.
    (2) Minimize the loop from the input cap to the drain of high-side FET, the source of high-side FET to switching node, switching node to the drain of low-side FET and the source of low-side FET to ground.
    (3) The connection between switching node to the SW pin of the IC should avoid parasitic inductance caused by traces and vias.

    Regards,

    Na