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UCC28782: Question about the operation of active clamp flyback

Kaji@PAN
Mastermind 7775 points
Part Number: UCC28782
Other Parts Discussed in Thread: ISO7710

Tool/software:

Hello,

I am an FAE at a distributor that handles TI products.
I have received three questions from my customer about the operation of an active clamp flyback.
This question is the same as the following thread.
(Question 1)
I am experiencing a phenomenon where the mode switches back and forth between LPM and ABM, and between ABM and AAM.
I have attached the waveforms.
Even if the load is constant, the mode switches back and forth between LPM⇔ABM and ABM⇔AAM at the time of mode transition.
Are there any reasons why this might be happening?
Could you tell me if there is any way to improve it?
(Question 2)
A ringing current flows between low side turn-off and high side turn-on.
I have attached the waveforms.
The area indicated by the red frame.
It is unclear whether this is due to superimposed noise or the effect of the resonant current in the high-side loop.
(Question 3)
When ABM is on and Nsw is 3, the second Hi side gate signal is not output.
Waveforms are attached.
Since it has been confirmed that a signal is being output from the UCC28782 PWMH, it is assumed that this is due to the fact that no signal is being output from the ISO7710 out.
Changing the capacitor between VCC2 and GND2 of the ISO7710 has no effect.
Please let me know if there is any way to improve this.

Best regards,

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1521840/ucc28782-unstable-operation

Questions about Active Clamp Flyback.pdf

  • +1
    TI__Mastermind 40100 points

    Hello Kaji-san, 

    Thank you for providing waveforms showing the issues that your customer's questions are covering.

    For Question 1:  This mode-switching may be caused by noise currents being coupled into the FB input.  If the load and line are constant, the FB current should also be constant (except for some small ripple).  If noise is altering the FB current, it will cause the control loop to shift around through the ABM and LPM modes in a manner that is as random as the noise.  Please check to see if some variable signal may be coupling to the opto-coupler current either at its output or at its input.

    For Question 2:  I believe there is a high-frequency resonance (~29MHz) on the primary side that is causing the ringing seen on the T101 primary current (slide 4 and slide 6).  I don't see a significant amount of ringing on the secondary current, so it appears to be confined to the primary side.  It also does not appear on the Vsw voltage.
    Therefore it may be self-resonance between the primary leakage inductance and the primary winding capacitance.  At 29MHz, the other circuit capacitances seem to be too high to account for that high of a frequency.    

    For Question 3: In slide 6, it can be seen that there is no secondary current (red trace) during the 2nd pulse, but there is during the third pulse.  That tells me that the primary current of the 2nd pulse is not going to the output, but all of it is going into the clamp capacitor.  It would do this if the clamp cap voltage is lower than the reflected output voltage.
    The primary energy goes to raising the clamp cap voltage high enough to be able forward bias the secondary winding and move energy out to the secondary output caps. 

    So it is not clear that the ISO 7710F output is or is not providing a gate-drive to the high-side switch.  I think that it is.  I think that the clamp cap voltage is too low to transfer energy/current from the primary to the secondary during the 2nd pulse.  It may be that the clamp cap value is too low to retain sufficient voltage after the 1st pulse energy rings to the secondary.  This cap value may need to increase. 

    Regards,
    Ulrich