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FET with ultra low Qg in TPS40400 application

Tom Lee
Prodigy 80 points
Other Parts Discussed in Thread: CSD16323Q3

I am using CSD16323Q3 having ultra low Qg in POL, it shows higher efficiency due to fast gate drive than other competitors.

However it shows high FET spike so it needs FET snubber, it shows high output voltage noise so it needs more filter capacitances.

Currently the PWB trace of between input line GND and source pin of  bottom FET can not be shorten due to small PWB size and fixed pin out location.

I tried a slow FET having Qg=27nC, it shows less spike at most area but less efficiency as expected.

So is there any idea to get benifies of  both high efficiency and less noise? Thanks.

 

 

  • 0
    TI__Genius 17240 points

    Hi Tom,

    Thanks for using TI parts! Have you tried adding small boot resistor in series with the boot capacitor to slow down the turn-on speed of the high-side FET? Since it can only affects the turn-on edge but not turn-off edge, it may save some switching loss. But this is not as effective as snubber, you may still need snubber circuit with smaller snubber capacitor to meet the same damping on the switch node spike. All of this is a trade-off between efficiency and noise.

    Regards,

    Na

  • 0 in reply to Na Kong
    Prodigy 80 points

    Hi Na,

    Thanks for your response.

    Yes I tried some boot resistor and FET snubber too, but those are not enough to minimize output voltage spike at full bandwidth condition.

    In fact  output noise SPEC is below 30mVpp at full bandwidth.

    Only thing that shows the least spike is using slow( high Qg )FET, but the efficiency dropped much.

    Have TI ever consider  to design less noise concept of PWM IC? So we could use ultra low Qg FET without worring about high output noise.

    Thanks.

     

  • 0 in reply to Tom Lee
    TI__Genius 17240 points

    Hi Tom,

    You are right. Because of the noise introduced by the ultra low Qg of TI NexFETs, the PWM controller IC which was initially designed and verified with relative high Qg FETs now faces more challenges. For new controller or converter ICs, this issue will definitely be considered and evaluated thoroughly. Thanks again for your intests in our products and valuable suggestions!

    Regards,

    Na