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Original question:

UCC28951: UCC28951 Phase C, D duty cycle is not ~50%

UCC28951: UCC28951 Phas A B ZVS issue

Bill Xu
Intellectual 1520 points

Part Number: UCC28951

Dear PL AE

I design in UCC28951 to implement output adjustable power supply in range 0~500V. during debug,  I found that MOSFET temperature rise of phase C & D is less than phase A & B. then I check phase B MOSFET's gate and drain waveform. it seems that phase B MOSFET sometimes can implement ZVS, sometimes could not implement ZVS. see below picture.

Phase ZVS

  Green-PhaseB Gate--Pink--PhaseB Drain--70W87V.jpg

Phase B non-ZVS

Electrosurgical Unit Energy Platform Rev0.1.pdf Green-PhaseB Gate--Pink--PhaseB Drain--70W87V--nonZVS.jpg

my question is: how could I keep Phase B always to implement ZVS? attach my schematic for reference. note: I use MOSFET replaced GaN in my debug. thanks.

 

regards,

bill

 

  • 0
    TI__Intellectual 1170 points

    Hi Bill,

    Thanks for the waveforms.

    From the waveforms you have shared, I can see the switching is happening at optimal point even for non ZVS case, that is valley or peak of the Vds. Please confirm the following.

    1. Please confirm, is it the trailing leg you meant by phase B?

    2. Please confirm whether you are losing ZVS capability as the load reduces

    If the answer to above questions is yes, then it is expected to have this behavior due to shim inductor not having enough energy to charge/discharge the Coss of the trailing leg MOSFETs under light or no load. The ZVS happens first for leading leg and under light load, shim inductor may not have enough energy to get ZVS for trailing leg. One way to solve this is by increasing the shim inductance value and thereby making sure it stores enough energy to get ZVS for both legs even under light load but please note that this will increase the conduction losses since increasing shim inductance will lead to higher primary side circulating current.

    Regards,
    Deepak K 

  • 0 in reply to Deepak K
    TI__Intellectual 1520 points

    Hi, Deepak

    got it and thank you so much for supporting! 

    but why temperature rise of MOSFET of Phase A, B is higher than phase C & D? how could I keep MOSFET's temperature of phase A & B is low as phase C & D? big thanks.

    regards,

    Bill 

  • 0 in reply to Bill Xu
    TI__Intellectual 2470 points

    Hi Bill,

    I’m assuming that “Phase A” and “Phase B” correspond to switches U8 and U10 in your schematic. Since these switches are driven by OUTA and OUTB, they form the trailing leg. Because the shim inductance is low, the trailing leg isn’t achieving proper ZVS, which is why it’s heating up more than the leading leg (switches C and D). To address this, you can either increase the shim inductance or improve the cooling for those switches to keep their temperature down.

    Regards,

    Anil