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LM5170EVM-BIDIR: LM5170EVM-BIDIR

Steven ZF Liu
Prodigy 110 points
Part Number: LM5170EVM-BIDIR
Other Parts Discussed in Thread: LM5170

Hi TI engineer

I have a question about LM5170 evm board.

I replaced TVS diode SM5S36AHE3/2D to TPSMB20A(3 pcs) ,then I shutdown the circuit at output current 60A.

There is someing different on the waveform.We can see negative polarity voltage on the Vo1 waveform and positive polarity voltage on the Vo2 waveform.
I don't understand why the Vo1 waveform will drop to negative voltage.I think that someing wrong has be happened.

Please suggest how to solve or choose correct TVS diode.

Ch2:BRKG to BRKS

Ch3:Vo1

Ch4:Vo2

  • 0
    TI__Guru 68495 points
    Hi Steven,

    The negative voltage is because the parasitic capacitance resonates with the inductor, and it negative voltage is about the diode drop clamping by the TVS (it is a diode in reverse direction). The parasitic capacitance is the equivalent capacitor consisting of the TVS, and also the FETs. The resonance starts when Vo1 drops below the TVS's conduction voltage, leaving the the inductor and the parasitic capacitor resonates. Such negative voltage does not hurt because the CS pin rating can be -5V.

    Thanks,
    Youhao Xi, Applications Engineering
  • 0 in reply to Youhao Xi
    Prodigy 110 points

    OK.I understand that the CS pin rating is -5V and the parasitic capacitance resonates with the inductor.

    If Ihope to transform the waveform and let the Vo1 never drop to negative voltage, could you suggest solution such as increased TVS diode or substituted another spec TVS diode?

  • 0 in reply to Steven ZF Liu
    TI__Guru 68495 points
    I don't think you can easily clip the negative voltage to 0V with a passive device. The best Schottky diode has about 0.4V drop. Can you explain why this negative voltage is a problem?
  • 0 in reply to Youhao Xi
    TI__Guru 68495 points

    Hi Steven,

    May I assume the issue is resolved and we can close this thread here? You may reach TI FAEs in your region if you need onsite support.

    Thanks,

    Youhao