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UCC28950: When going from discontinuous mode to burst mode, the MOSFET is destroyed

Kaji@PAN
Mastermind 7775 points
Part Number: UCC28950

Hi,

I am currently using the UCC28950 device to create a circuit and verify its operation.
During operation, the MOSFET will be destroyed if it goes from discontinuous mode to burst mode.
By the way, the circuit I made is not synchronously rectified.

So, after making some constant changes, the resistance connected to the TMIN pin was changed from 27kΩ (minimum ON width 160ns) to 110kΩ (750ns), and the damage disappeared in burst mode.

However, the reason why changing Rtmin does not destroy it is unknown.
Could you tell me the possible factors for this reason?
Also, please let me know if there are any points I should check.

(Factors I am thinking about now)
The circuit this time is switching at 100kHz, and Ciss uses a FET of about 2000pF, but I think that if the minimum ON width is too narrow, switching will not catch up and it will not operate normally. For example, OUTD turns off while OUTA is on.

Best regards,

  • 0
    TI__Mastermind 40120 points

    Hello Kaji-san,

    Thank you for your interest in the UCC28950 Phase-shifted Full-Bridge controller.

    I will refer your question to an expert in this part. Please allow a day or two for a response.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    TI__Mastermind 25570 points

    Hello Kaji-san

    You may well be correct about the timing -

    If the Tmin is too small the system may be spending too large a proportion of its time changing state (switching transitions) and proportionally too little time actually transferring energy from primary to secondary. If this is the case then the switching losses will increase so it may be that the MOSFETs are getting too hot. Increasing Tmin will change the proportion of time spent changing state to that spent transferring energy so the MOSFETs should run cooler.

    If you have a current probe you should look at the transformer primary current - see if the primary current can change direction fully during the on time (either positive or negative voltage across the transformer) the reason for this to be of interest is that the time needed for the transformer primary current to reverse is set by the input voltage and the leakage inductance (V = L*di/dt) and no energy is transferred from primary to secondary until this change has completed.

    another possibility to look at is if there are any large voltage spikes on the Vds waveform during operation.

    Please feel free to post some waveforms - Vds of the MOSFETs, I_pri, Vgs would all be of interest.

    Regards

    Colin