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UCC28782: Unstable operation

Kaji@PAN
Mastermind 7795 points
Part Number: UCC28782
Other Parts Discussed in Thread: LMG3616, LMG3622

Tool/software:

Hello,

I am an FAE at a distributor that handles TI products.
I previously asked a question about an unstable operation in the following thread.
I have obtained the results of my customer's verification of this instability.
My customer changed the secondary resonant circuit.
Co1 was 2000uF, but based on a calculation tool, this value was changed to 120uF and 10uF.
The result was that it did not work properly and would latch-stop even with a small load.
We are not sure what the cause was, but we are currently considering using the original 2000uF primary resonance for the time being.
My customer also tested other solutions and found that they were effective in resolving the unstable behavior.
They are using LMG3616 as the switching element for the active clamp, and added 470pF between the drain and source.
This resulted in stable operation, as shown in the waveform below.
It was confirmed that the unstable operation was caused by voltage changes between the drain and source of the high-side switch.
What do you think could have been the factor that improved operation in this case?

Best regards,

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1494717/ucc28782-about-ucc28782bdlrtwr/5792059?tisearch=e2e-sitesearch&keymatch=ucc28782#5792059

  • 0
    TI__Mastermind 40550 points

    Hello Kaji-san, 

    I'm sorry, but I don't understand this statement: 

    Kaji@PAN said:
    It was confirmed that the unstable operation was caused by voltage changes between the drain and source of the high-side switch.

    The voltage between drain and source of the high-side switch is normally always changing drastically every switching cycle. 
    Every time the switch turns on, the Vds drops close to 0V, for example. 

    I think that maybe too much voltage change across the clamp capacitor from cycle to cycle may have caused the instability, but that is just my guess. 
    I am not sure why adding 470pF across the high-side switch would solve this problem.  Maybe my guess is wrong. 

    Is the low-side switch still the LMG3622, as indicated our the previous thread? 
    Was the change of Co1 from 2000uF to 120+10uf the only change made to the ACF design, or were all (or most) of the component values implemented from the Excel tool that I had provided?

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Mastermind 7795 points

    Thank you for your reply.
    Regarding the adjustment of Co1, the customer only changed Co1 from 2000uF to 120uF (or 10uF).
    Is just changing Co1 not enough?
    Best regards,

  • +1 in reply to Kaji@PAN
    TI__Mastermind 40550 points

    Hello Kaji-san, 

    Because the ACF relies on resonance between the transformer leakage inductance and the resonant capacitor (Co1 in Secondary-side Resonance, or Cclamp in Primary-side Resonance), changing one thing usually affects several other things.  The RDM resistor must also be programmed to the correct value for the new resonance.  RTZ value is affected when the switched node capacitance changes.  The output inductor needs adjustment. 

    That is why I provided the revised Excel Calculator tool in the previous E2E post. 
    With the change of high-side switch, it should be revised again, but it probably wouldn't change much. 

    Regards,
    Ulrich