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UCC256403: Half-bridge LLC: Failed Low-Side FET and HO shorted to HB

Joshua Stump
Prodigy 140 points
Part Number: UCC256403

I have designed a half-bridge LLC using UCC256403. I recently had a unit that wasn't turning on. Tracked back to the LS FET was shorted G-S and D-S. As I was looking for other damage, also found that the high-side bootstrapped supply pin (HB - 14) measured 16Ω to the high-side gate waveform (HO - 15). The design requires external gate drivers, so really not even driving these pins hard.

My question is what failure mechanisms could cause HB to be shorted to HO?

Thanks for any insight!

  • 0
    TI__Guru* 86266 points

    Hello,

    Your inquiry has been received and is under review.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

    The most likely cause of the FET failure is over current, over voltage and/or over temperature.  More than likely your ISNS resistor was not setup correctly to protect the FETs from damage. The following link will bring you to an excel design tool that you can use to select the ISNS resistor based on your loading.

    https://www.ti.com/tool/download/SLUC675

    When it comes to the UCC256403 controller failing or being damaged this will occur if any of the absolute maximums or minimums on any pin is exceeded.   

    When it comes to using external gate drivers.  What did you do with the HB, HO and HS pins?  The HS will be on the switch node and if you did not use the HB and HO pins.   More than likely the high side driver was exposes to large negative voltage across it causing it to be damaged.

    Regards,