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Original question:

PMP6712: pmp6712 active buck snubber operation

PMP6712: What is the function of the C80?

Ching He You
Prodigy 30 points
Part Number: PMP6712
Other Parts Discussed in Thread: UCC28950

Hi all,

I had made a simulation with PSIM of the active buck snubber, but I can't understand the function of C80.

Is it just like C36, C37, C72, C73 to store the energy which comes form spike? 

So why should we put a capacitor across the SNUB and Vo?

On the other hand, I think maybe C80 can help Q27 to achieve ZVS, but I can't see the ZVS in the simulation.

Thanks for reading question.

Best regards,

Mike Yu

  • 0
    TI__Genius 16725 points
    Hello Mike,

    I am not sure of the purpose of C80, it seems to be in parallel with C81 and from Vcc to GND of the CD4050. It is a good guess that it is used to smooth out the signal between those nodes.
    Where did you grab this design from? Maybe we can find the team who designed this and ask them.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Prodigy 30 points

    Hi Karan,

    I am designing the FBPS with UCC28950, and I found out that the buck snubber circuit of Ti design is unique.

    I had found an paper on IEEE which designed a active buck snubber for FBPS, but it clamping capacitor is putting between SNUB and Gnd. (fig1) (The switch of that active buck snubber is control by S1~S4.)

    Then I had two questions, we already had C36, C37, C72, C73, so why should we place C80?

    Why C80 placed between SNUB and Vout, but not placed between SNUB and Gnd?

    Thanks for reply the question :-)

    Mike 

  • 0
    TI__Intellectual 1235 points
    Hello Mike,

    C80 is a local bypass for the Zener reference, comprised of R99 and D22. For proper switching, the inverter input D must respond the SNUB voltage ripple and reject any switching noise spikes. It uses the hysteresis of the Schmitt input for switching thresholds. Component placement and layout may affect the operation, since the buck snubber control is referenced to VOUT, which is some distance from the ground referenced SNUB caps C36, C37, C72 an C73.

    Regards,
    Bob Sheehan
    Power Design Services
    Texas Instruments
  • 0 in reply to Bob Sheehan
    Prodigy 30 points

    Hi Bob,

    Thanks for your reply, it helps a lot.

    Best regards,

    Mike