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BQ24735: Issue with input protection MOSFET failing and going short circuit

Paul Hackett
Prodigy 10 points
Part Number: BQ24735
Other Parts Discussed in Thread: CSD17307Q5A

We have implemented the design pretty much verbatim as per the BQ24735EVM-710 evaluation PCB schematic, including using the same peripheral components (see below schematic).

We have seen Q1 (CSD17307Q5A) fail on two prototype PCBs so far. Our supply input is 24VDC and we are using a 4S1P Li-Ion battery pack.

We are struggling to understand the cause of failure. If Vgs was being exceeded then we would expect Q2 to similarly fail given the drains and gates of Q1 and Q2 are connected together. The only other thought is the Vds voltage being exceeded. We are looking to change to a FET with higher Vgs and Vds, and perhaps place a transorb more locally close to the charger circuitry DC input.

Any other thoughts welcome.

  • 0
    TI__Genius 11385 points

    Hi Paul,

    There could be damage due to high power dissipation. Q1 is usually more susceptible to power damage than Q2.

    As VGS is getting higher and the MOSFETs are turning on, current start to go through both Q1 and Q2.

    For Q2, the body diode will start to conduct before the MOSFET fully turns on so the ID*VDS power will be smaller. For Q1, there is no body diode to conduct and the MOSFET has to go through a high VDS period (ID is the same) as VGS ramps up. There could lead to higher possibility of Q1 getting damaged compared to Q2.

    If that is the case, one option is to look at MOSFETs with better SOA curve.

    Thanks,

    Peng