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UCC28780: UCC28780 - passive clamp flyback

dariusz jewula
Prodigy 110 points
Part Number: UCC28780
Other Parts Discussed in Thread: LM5023,

Hi,

Is it possible to use UCC28780 in classic passive clamp flyback topology? I know it doesn't make sense to use it with UCC28780 due to lower efficiency etc. however it is mostly for evaluation purpose and easy check some parameters between PCF and ACF using the same IC.

Best Regards

Darek

  • 0
    TI__Mastermind 26750 points
    Hello Darious,

    Yes, it IS possible to use the UCC28780 in the passive-clamp topology. Instead of driving the ACF high-side clamp-FET, the PWMH output is used to drive a synchronous rectifier MOSFET on the secondary side through a primary-to-secondary isolator and gate-driver.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 110 points

    Hello,

    Thank you for clarification. I understand that leaving PWMH not connected and using second rectifier controller in PCF topology will be also possible? (the same scheme like for example UCC28780EVM-021, the only difference PWMH connection).

    Best Regards

    Darek

  • 0 in reply to dariusz jewula
    TI__Mastermind 26750 points
    Hello Darek,

    Technically it is possible to leave PWMH unconnected, but then you lose the "negative current" function that produces ZVS. Without this you are basically turning the UCC28780 into a Transition-Mode QR valley switching controller like the LM5023. That's okay if that is what you want to do for comparison purposes.

    By the way, with a passive clamp you need to have a means of dissipating the clamp energy, either with a TVS or with a resistor in parallel with the clamp cap. Simply disconnecting PWMH from the high-side MOSFET drive of the UCC28780EVM-021 will not limit the voltage build up on the clamp.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 110 points

    Hi,

    Thank you for helpfull answer.

    Best Regards

    Darek