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UCC24612: UCC24612-1 vs. UCC24612-2 : Which should be used with GaN switching devices (primary and secondary) in an LLC switching 250kHz? Why?

UA&A_CodyDowling
Expert 1006 points
Part Number: UCC24612
Hello,

We have the first protos of a power supply back and making power in the lab. It uses GaN devices in the half-bridge LLC as well as the synchronous rectifier secondary. The LLC fsw is 250kHz.

We are going to spin the board and make some thermal improvements, and while we have a chance to update anything else in the design that needs attention, I was reviewing the SR section and I ran across Table 1 in the data sheet:

Since we are using GaN devices in the SR stage, should we be using the -2 variant of the device instead of the -1 that we're using currently?

What are the trade-offs of one vs. the other? Where will the shortcomings of the -1 variant surface in the current design?

I'm hesitant to change something that appears to be working, but now is the time if it's a necessary change.

Thanks for your help!
  • 0
    TI__Mastermind 27145 points

    Hi Cody,

    The larger minimum on time of the UCC24612-2 can be useful for making sure the SR doesn’t turn off too early and cause the secondary current to ride the diode you have in parallel to the SR GaN FET longer that it needs to. Where this can be a problem is if the minimum on time is too long and the SR doesn’t turn off when it is supposed to. The smaller minimum on time of UCC24612-1 can make it more susceptible to any ringing on the VD pin and result in the SR turning off early.  

    For low frequency LLC, the UCC24612-2 is favorable but for high frequency LLC designs where the initial switching frequency can be quite high, the UCC24612-1 is more preferable for the smaller propagation delay and smaller minimum on time. For this case where the LLC resonant frequency is 250kHz, I would say stick with the UCC24612-1 if you are seeing good performance. 

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Expert 1006 points

    Hi Ben, 

    With apologies, I have a typo in my original message. I meant to say that we are currently using the "-2" variant in our design and my question is whether we should consider swapping it over to a "-1" with this spin? Its seems maybe the answer is yes, we should- right?


    Should I take a look at the VD net to see what the ringing looked like before we do? Any other tests that might give us confidence in the choice?


    Thanks again!

  • 0 in reply to UA&A_CodyDowling
    TI__Mastermind 27145 points

    Hi Cody,

    In that case, I would say take a look at the SR gates for both controllers. Look for cases where both SR gates are turning on at the same time (this can happen if the minimum on time is too long and the other primary side FET has already turned on). I would expect startup to be the worst case due to the high switching frequency but also check at light load. The UVLO would prevent the SR controllers from operating at the initial switching pulses where the frequency is highest but once the REG pins are able to get above 4.5V, then you should start seeing some switching action from the SR.

     If you aren’t seeing cases where both SR gates are turning on at the same time then I would say stick with the UCC24612-2. My concern with the UCC24612-2 is just making sure the minimum on time isn’t too long to cause issues at this switching frequency.

     Swapping to the UCC24612-1 would make the SR be better equipped for the higher frequency but it’s also going to make the SR controller more susceptible to turning off too early as it has a smaller turn-on blanking time. Turning off too early is going to be more preferable than turning off too late but if you aren’t seeing any issues with the UCC24612-2 during your testing, then I don’t see a reason to make the swap.

     Best Regards,

    Ben Lough