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UCC28950: Worst Case Power Dissipation for clamp diodes Db and Dc

Frank Marrone
Prodigy 10 points
Part Number: UCC28950

In the UCC28950 datasheet section 8.2.2.5 says the maximum power dissipation for clamping diodes Db and Dc is given as  = .5*Ls*I^2prms*Fsw   

What are the conditions where this worst case power dissipation can occur? 

The section in the datasheet says to use MURS360 SMC rectifiers.  However, based on the calculation for maximum power dissipation the power can be well over 10 watts which is not going to work with the diodes recommended.  Please advise!!

We have observed that certain conditions (abnormally low low output voltage operation) these rectifiers can overheat and fail.  Is this expected?  How can this be mitigated?

  

  • 0
    TI__Expert 6080 points

    Hello Frank,

    The purpose of the clamp diodes (DB and DC) is to prevent parasitic ringing on the transformer primary which would be reflected into the transformer secondary and possibly overstress the rectifiers. The ringing is due to the resonances between the shim inductor Ls and the stray circuit capacitances.

    This phenomenon is described by Wu in IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 5, MAY 2010

    May I ask you to share the design spec like output power/voltage, shim inductance, Lm and leakage inductance of the transformer. It would be useful to look at the waveforms across the diode and the FETs at the time of breakdown.

    Regards,

    Sonal

  • 0 in reply to Sonal Singh
    TI__Mastermind 25570 points

    Hello Frank

    Unfortunately I don’t have a quantitative treatment of the operation of these diodes – you will find some information in a paper by Redl et.al. at https://ieeexplore.ieee.org/document/388499 but I don’t think it fully answers the question.

     

    One thing is, At low output voltages the power stage is operating at narrow duty cycles so the clamp diode may still be conducting at the time when the bridge switches from the passive state where the current is circulating around the primary QA/QC (or QB/QD) pair to the active state where the QA/QD (or QB/QC) pair is on. This would cause significant reverse recovery losses in the clamp diode and this might be the reason for the increased losses. The authors of the paper suggest adding a resistor to shorten the time needed for the diode current to decay so this might be something to consider. If reverse recovery losses are indeed the cause of the diode failure then there would also be the possibility of using SiC diodes.

     

    I’d suggest that the customer probes the currents in the diode.

     

    If the diodes are ok at full load then they should be ok at light load and I wouldn’t suggest changing them without knowing why they are failing.

     

    I haven’t encountered this issue previously because most of the PSFB applications I have encountered have been for fixed output voltages – or battery chargers where the output voltage does not vary greatly.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Mastermind 25570 points

    Hello Frank

    We haven't heard back from you for a while so I assume the question has been resolved so I am closing the thread. If I am mistaken you can always open a new, linked thread.

    I've added a task to put together some quantitative treatment of this question but it may be some time before I get to it.

    Regards

    Colin