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UCC28951: No Synchronous Rectifier Drive

Ken Hayden
Prodigy 30 points
Part Number: UCC28951
Other Parts Discussed in Thread: UCC28950

I'm attempting to configure a 300W 300V to 24V DC-DC converter using UCC28951 to run as a slave at 937.5kHz sync frequency.  I've patterned the design after Figure 56 of  data sheet "SLUSCK4A –OCTOBER 2016–REVISED FEBRUARY 2018", with FETs, gate drivers, and magnetics selected for the higher speed.  I have the circuit basically running, but with extremely low efficiency (50%) and no drive to the synchronous rectifier FETs. (That is, OUTE, OUTF never activate.) My understanding is that this could indicate inadequate current sense signal at CS input.  But I have a reasonably decent ramp waveform that peaks in the 1 to 2 volt range.  What am I missing?

  • 0
    TI__Guru* 86266 points

    Hello,

    The LSUSCK4A does not give a complete schematic.  The following links will bring you to an application note and 600 W evaluation module (EVM) for the UCC28950.  They UCC28951 is a pin for replacement for the UCC28950.  The only difference is the UCC28951 can be designed for slightly more duty cycle.   This information can be used to help you with your design.

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

    https://www.ti.com/lit/pdf/sluub02

    There is also an Excel design tool that you can find at the following link that will help your with design as well.

    https://www.ti.com/lit/zip/sluc222

    When it comes to the SR drivers not turning.  This could because you don't have your DCM comparator setup correctly.  This comparator is used to turn off the SRs before the inductor because negative.  If this is not setup correctly the FETs will never turn-on.

    It also maybe worthwhile for you to order one of the UCC28950 EVMs to evaluate the design.  You can replace the UCC28950 with the UCC28951 and the design will still function the same:)

    Regards,

  • 0 in reply to Mike O'
    Prodigy 30 points

    Thank you for bringing these three documents to my attention; I had not encountered any of them yet.  I immediately began entering parameters I know offhand into the spreadsheet, and it accurately predicted the large losses I'm witnessing.  That's encouraging and helpful.  I will continue studying all of these documents.

    Regarding the incomplete schematic in LSUSCK4A,  I should have mentioned that I based my design not just on figure 56, but on both figures 55 and 56, plus additional circuitry for boot-up  - so I do think I have a complete circuit to work with.

    Regarding the DCM comparator, I have the DCM pin biased at 0.25V and the CS input is driven beyond 1V on every cycle.  Shouldn't that be sufficient to engage the synchronous rectifier drives, or do I misunderstand that function?

  • 0 in reply to Ken Hayden
    TI__Guru* 86266 points

    Hello,

    Double check the CS signal vs the DCM threshold.  If CS is exceeding DCM you should have Out E and Out F outputs?  Start at the UCC28951 and work your way to the gate driver outputs.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 30 points

    I do have Out E&F now, and the power supply seems to be up and running well enough to finish tuning it with the help of SLUC222D. The loop control is slightly unsteady and the efficiency is closer to 83% then the 93% I'd like, but with a little work I think it will come around.  Thanks, Mike!