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UCC24624: MOS tube burned

Timsen
Expert 1660 points
Part Number: UCC24624

Dear all：

          I used UCC24624 to design a 500W power supply and it works normally.However, there is a situation now. After I removed UCC24624, no load was added. At this time, the power was switched on, and the MOS tube attached to UCC24624 burned out.Is my circuit design defective?Please help to confirm. Attached is my schematic diagram

Thank you！

500W.pdf

  • 0
    TI__Mastermind 40580 points

    Hello Timsen,

    Thank you for your consideration of the UCC24624 dual SR controller.

    I do not believe that your circuit design is defective. Certainly, it works well with the UCC24624 in place.

    I can think of 2 possible causes of the MOSFET failure when the UCC24624 is removed:

    a) At no-load condition, an LLC converter typically runs at a very high frequency.  It is possible that the reverse-recovery time of the MOSFET "body-diode" is too slow for this frequency and the two legs of the output winding start to cross-conduct and sustain high peak cross-conduction currents which results in failure by overheating the junction(s).  If this is the case, you may have to find faster recovery MOSFETs.

    b) When the UCC24624 is removed, there is only a 10K pull-down to GND on each pair of MOSFETs.  This may not be low-enough impedance to prevent charge-up of the gates during high-frequency no-load switching.  10K is a reasonable pull-down in static conditions, but the switching voltage may induce sufficient Vgs to partially turn on these MOSFETs and lead to overheating and failure.  When the UCC24624 is in place, the gates are held low by the SR driver, so no problem occurs. 

    This can be tested by shorting out each of the 10K pull-downs (R405 and R406) and running without UCC24624 at no-load.
    If the MOSFETs do not fail, then (b) is probably the reason for failure.  If they still fail, then (a) is probably the reason. 
    (At this time, I can't think of a third possibility.)

    Regards,
    Ulrich