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UCC256303: Output voltage not regulating

Francesco Giuliani
Prodigy 70 points
Part Number: UCC256303
Other Parts Discussed in Thread: TLV431, UCC25630-1EVM-291, TL431

Hi,

we are building an LLC power stage with the following specs

VIN,NOM = 310VDC, VOUT = 85VDC, POUT = 600W

Simplified schematic attached for more details

After some initial fixes, we are now experiencing the following issues:

  • Controller is switching but output voltage is not regulated.
    Switching frequency keeps reducing (and output voltage increasing, as expected) even well after target voltage is reached
  • Without the Zener circuit to prevent optocoupler deep saturation, the controller does not start at all.
  • Switching frequency changes with controller temperature (higher temp -> lower switching freq, and vice versa)

Here some scope acquisitions (@VIN = 280V POUT = 110W). 

We thought of a problem on FB pin or mistakes on our voltage compensation loop.

Any suggestion or recommendation is welcome.

Many thanks.

  • 0
    TI__Genius 12865 points

    Hi Francesco,

    Thanks for reaching out to us. After Simulating the feedback circuit with your parameters, I observed that Voltage between cathode and anode of the TLV431 is exceeding recommended voltage range (Vref-6V). 

    So, That's the reason the feedback is not able to regulate the output voltage.

    For designing feedback components around TLV431 and optocoupler, you can refer the following link. 

    Shunt Regulator Design Procedures for Secondary Feedback Loop (ti.com)

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    TI__Guru**** 189399 points

    Hi, Francesco,

    Let us know if you have further questions. It looks like this thread is resolved now.

    Best regards,

    Don

  • 0 in reply to Manikanta Pallantla
    Prodigy 70 points

    Thanks for your recommendation.

    We added a Zener diode to get a fixed bias and avoid overvoltage on TLV431 cathode.
    Voltage regulation works better as controller is able to reach the 85V set-point.
    Not sure that every issue is fixed so we might have further questions in future.
    But at least it seems we have one less thing to worry about.

    Given the component values used in the feedback loop of the UCC25630-1EVM-291, if you run the same simulation I would expect the voltage between cathode and anode of the TLV431 to exceeds recommended voltage range (Vref-6V) also in the evaluation module. Am I right to suppose so? In that case, why is not creating any issue there?

    Best regards

  • 0 in reply to Francesco Giuliani
    TI__Genius 12865 points

    Hi Francesco,

    I am glad that connecting zener resolved the issue which should limit the cathode voltage.

    I have rerun the same simulation with the evm values. I found out that voltage rating is exceeding TLV431's cathode recommended voltage.

    Then I realized that I was running the simulation in open loop mode (no connection between ref and cathode). I am sorry about my mistake.

    For open loop characteristics following reference was useful. 

    Using the TL431 for Undervoltage and Overvoltage Detection (Rev. A)

    So to verify the TL431 feedback design and component voltage limits, closed loop simulation is recommended. 

    Either by inserting power stage or by giving a reference voltage like below.

    Here I am using one of the Bosso's models where output voltage needs to be regulated at 12V.

    Here for output voltage of 12V, we can see that cathode voltage across TL431 is within the limits while operating in the closed loop. Hope this helps you.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 70 points

    Hi Manikanta

    thanks for your reply.

    Your explanation make sense. However, in the results you posted the cathode voltage across TL431 is about 10.4V (that is above maximum recommended voltage). Did you get them from closed loop simulation? Or is that the open loop case?

    Regards

  • +1 in reply to Francesco Giuliani
    TI__Genius 12865 points

    Hi Francesco,

    The device that I used was TL431 whose recommended voltage is Vref to 37V. 

    The simulation results that I have presented in my last post are closed loop. Thank you.

    Regards

    Manikanta P