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UCC28951: Trouble achieving ZVS past light load

Brian Collins
Prodigy 20 points
Part Number: UCC28951

Tool/software:

Hi,

 

I am designing a 5kW phase-shifted full bridge regulator using the UCC28951. I am having trouble maintaining soft switching on my A/B transistors, which is preventing me from reaching high loads. I can achieve ZVS at light loads, but lose it as my currents increase which seems backwards to me.

 

On my primary side, I am using silicon carbide FETs (~20pF Coss) with parallel silicon carbide diodes (~20pF junction) to mitigate reverse recovery. Clamp diodes are also silicon carbide.

 

I've noticed significant asymmetry in my AB and CD legs on transitions - more significant than in any reference material I can find. This is coupled with shim inductor current that also looks inconsistent with reference material. AB transitions cause a complete depletion of shim inductor energy that does not seem to line up with the capacitances I have.

 

I attached a simplified schematic and a screenshot of the behavior at 20% load showing my AB and CD transitions as well as shim inductor current.

 

My question is: does my AB waveform look normal and I just need to do some tuning, or is there likely something else wrong in my system?

 

 

 

 

Thanks!

Brian

  • 0
    TI__Guru* 86266 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

    When I look at your switch node it does not look like that your turn on delay for A, B, C and D are setup correctly.  It needs to be set for 1/4 the resonant ring period.

    Because your delay is to long the amplitude of the resonant ring will increase with load.  This is why switching losses look like they are getting worse with load.  If you reset the turn-on delay to 1/4 the tank frequency the design will valley switch at light load and ZVS as the load increases. 

    The following will bring you to an application note that goes through the step by step design process using a fixed delay approach using the UCC28951.  It is setup to achieve ZVS down to 50% load.  If you want to achieve ZVS at lighter loads you will need to adjust Ls.  I think you will find this application note useful in your design and debug process.

    https://www.ti.com/lit/pdf/slua560

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

    Regards,