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TPS65983 FET Dead issue

Andrew Park1
Prodigy 460 points

Team,

Customer is using two FET on PPEXT line and one of FET on the VBUS side has been dead. we have suspected two cases which can cause FET damage and tested two items below.

- Inrush current check while driving PPEXT

- Drain current check while shortening the VBUS

I got two questions i don't understand regarding inrush current clamping and gate voltage.

please check the attachment and address this as soon as possible.

Thanks,

Andrew20161207_PD_FET fail issue test.pptx

  • 0
    TI__Expert 6430 points

    Hi Andrew,
    Your observation is true in the slides shared by you, that's the expected behaviour, because higher value of capacitor will need same current for longer duration of time to reach to the same output voltage. (Q = I * T)

    1. Why is the inrush current limited about 4.x A and how can TPS65983 do this?
    When VBUS is enabled, inrush current is limited to current limit value which is 4.x A in this case. Current in the FET is monitored using the sense resistor (5/10mOhm) and controling the gate drive of FETs to acheive current limit. It's an inbuilt analog control loop in our chip.

    2. Why is there 4V gate voltage after shortening the VBUS?
    TPS65983 tries to maitain the current by going into constant current mode when a short circuit is detected. In order to maitain this current you need to keep the FET turned on and for that you need some VGS, in this case 4V of VGS is required to maitain 4A of current from the FET and thats the reason you see 4V on the gate of FET.

    Let me know if you have any further queries.

    Thanks,

    Rahul

  • 0 in reply to Rahul Sharma16
    TI__Prodigy 460 points

    Hi Rahul,

    Thank you for your response.

    for the question 2, the VBUS goes low after shorting the VBUS.when does the PD controller stop driving gate voltage?

    I thnk PD controller should monitor this short on VBUS and stop to providing power to outside.but it seems there is a constant current.

    since there is voltage difference between drain and source while this constant current being flowed. and it will definitely damage to FET.

    do we have any way to protect FET?

    Thanks,

    Andrew