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UCC28780: Simulation puzzle for ACF(simplise)

fleming fu
Prodigy 130 points
Part Number: UCC28780

I get a version of ACF with GAN used by UCC28780(simplise).

The related result as below:

I wander the below questions:

1) what's the QH close conditon? when sencodary current to zero? Or only Im reached Im(-)?

  • 0
    Prodigy 130 points

    1)  what's the QH close conditon? when sencodary current to zero? Or only Im reached Im(-)?

    2)  From the simulation result, the QH close current is not zero. Then the QH is not ZCS? it's different as our design.

    3)  Also the secondary diode conduct current is not n*Ipri? Much current flow to the clamp current?

    4) When QH close, the secondary current is not zero. Then secondary diode is  not ZCS? The current decline slope is also strange.

    I get the result from the simulation result. It's the real wave or much different as the reality?

    Would you please help me about the puzzle?

  • 0 in reply to fleming fu
    Prodigy 130 points

    hi TI team,

    Would you please help me about the puzzles?

  • 0 in reply to fleming fu
    TI__Mastermind 40120 points

    Hello Fleming,

     

    Thank you for your interest in the UCC28780 ACF Controller.

     

    The answers to your questions are:

    1. QH closes (turns off) when the proper Im(-) level is reached for the specific bulk voltage condition. An internal tuner circuit regulates this level and extends or contracts the PWMH on-time to match the conditions for ZVS.

    2. No, QH does not operate in ZCS.

    3. Yes, some of the Ipri current flows to the clamp capacitor, and some is diverted to the parasitic capacitance of the output rectifier (Coss in a MOSFET, Cj in a diode). Therefore the measured peak of Isec may not = n*Ipri.

    4. In that simulation, the secondary current is not zero. This leads to non-ZCS in the output rectifier. For a diode rectifier, this is not a big problem, except for reverse recovery (if any).
      For an SR-MOSFET, this situation can have high losses and possibly shoot-through if the QL turns on right away. Non-ZCS in the output rectifier normally should be avoided.
      The current decline is not a traditional flyback slope of straight line.
      In ACF this current is a portion of a sinusoidal waveform. In this simulation the resonant capacitor is too big, so the resonant waveform has a low frequency making it look like a strangely curved slope.

    5. The simulation waveforms you have are basically very close to reality, although real world waveforms normally have more noise and ringing on them. Since the ACF topology uses parasitic inductance and capacitance to recycle energy, some deviations from reality can occur when the MOSFET model does not accurately reflect the highly-non-linear Coss curve vs. Vds.

       

      Regards,
      Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 130 points

    Dear Ulrich,

    I'm very appreciated for your answer.

    Still I wander that the secondary diode is ZCS or not in real design. For in simulation, it's not ZCS. Then in real product, is it ZCS? How can we avoid the non-ZCS in the secondary diode?

    Best Regards

    Fleming

  • 0 in reply to fleming fu
    TI__Mastermind 40120 points

    Hello Fleming,

    The secondary diode current can be shaped to be ZCS or non-ZCS by choice of certain components, in both real design and in simulation.

    In your simulation, I mentioned that the clamp capacitor value was apparently too high. If you reduce the value, the secondary current will peak higher and resonate faster and look more sinusoidal.  This cap value can be chosen to achieve ZCS.  Real world will follow the simulation.

    Note:  sometimes it may be a disadvantage to having ZCS all the time. There are cases where it is advantageous to have some non-ZCS of the secondary current.  Since QH is on longer at high line (to increase Im(-)), tuning C_clamp at high line may still have non-ZCS at low line, when QH is on for shorter time.  To achieve secondary ZCS under all conditions, C_clamp must be chosen at the lowest line input condition.  It will always have ZCS across the whole line range.  BUT... the peak and rms is higher so resistive losses are higher. 

    At times, one may achieve a better balance of losses with lower rms (higher efficiency) if ZCS is set to occur just as QH turns off at 230Vac input.
    There will be non-ZCS at low line, but reduced core loss and rms loss can more than offset the non-ZCS switching loss of the rectifier.

    This is not easily quantified "on paper", and should be optimized during prototype evaluation. But the simulation can give you a good idea of what waveshapes to expect in the real world circuit for the given set of component values.

    Regards,

    Ulrich

    . 

  • 0 in reply to Ulrich Goerke
    Prodigy 130 points

    Dear Ulrich,

    Appreciated for your detail explaination.

    I got the concept now.

    Thanks again.

    Best Regards

    Fleming