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UCC256303: there has SR MOSFET damage issue, but change to UCC256403 can improve this problem.

Rock Shao
Expert 2995 points
Part Number: UCC256303
Other Parts Discussed in Thread: UCC256403

Hi,

My customer used UCC2564303 for 600W, 24V/15A design, there has SR MOSFET damage issue, but change to UCC256403 can improve this problem.

Abnormal conditions: 0A/0.5ms, 2A/0.5ms, SR MOSFET will break down after about 1~15 minutes
The LLC picture is normal when the duty is zoomed in to view the waveform, but the waveform is abnormal when the power shutdown.
When Chanel 2 LLC's low-side gate duty cycle waveform is abnormal, the gate corresponding to SR has turned on abnormally, causing SR MOSFET breakdown.
But why replacing UCC256403 can improve this problem, is the slew rate will be impacted?

LLC Gate.pdf

SR VGS.pdf

SR VDS.pdf

Regards and thanks

Rock

  • 0
    TI__Genius 9790 points

    Hi, Rock

    Your analysis is right, the slew rate detection is the major difference between UCC256303 and UCC256403.

    With UCC256303, the minimum detectable slew rate is 1V/ns, if the slew rate is below 1V/ns, the MOSFET cannot be turned on at right moment, this lead to ZCS switching of LLC MOSFET which lead to very high current spike of resonant current. and there will have very high current on SR current.

    With UCC256403, the minimum detectable slew rate is 1V/ns, if the slew rate is below 0.1V/ns which can improve the SR failure.

    Yunsheng

  • 0 in reply to Yunsheng Qu
    Expert 2995 points

    Hi Yunsheng,

    How to solve this problem without replacing the IC? Did you have any ideas? Is the need to choose LLC or SR MOSFET Co or Rds(on).Thanks.

  • 0 in reply to Rock Shao
    TI__Genius 9790 points

    Rock,

    It is not easy to avoid missing slew rate detection with UCC256303. Here is what I suggested:

    1. Decrease Lm

    2. increase Burst mode threshold

    3. use MOSFET with smaller Coss.

    Yunsheng