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UCC28950 PWM Turn off

SHIANG KAI HUANG
Prodigy 80 points
Other Parts Discussed in Thread: UCC28950

Hi Everyone,

Here is my SPEC

Vin 135V , Vo 12V 45A  Phase Shift full bridge
 

For test Over Voltage Protection, we found a problem.

When it work on no load, by adjust variable resistor output voltage will rise until trigger the external OVP circuit than turn off UCC28950.

But within load lower than 10%, after we adjust variable resistor, the PWM signal  will shutdown automatically even the output voltage not rise to our target (14V), it shutdown at 12.6V

We went to know how it will turn off  PWM automatically?

Thanks

UCC28950 FAILE.pptx

 

  • 0
    Prodigy 80 points
    We found SS pin voltage will go down when PWM turnoff, but don't know why SS pin go down?
  • 0 in reply to SHIANG KAI HUANG
    TI__Mastermind 31580 points
    Hi Shiang,

    I have asked one of application team to respond to this post, you should see a response later today.

    Regards

    Peter
  • 0 in reply to SHIANG KAI HUANG
    TI__Intellectual 1550 points

    Hi Shiang,

    Based on the slides you attached, my understanding is that the OVP circuit is implemented with the ON_OFF signal all the way to Q27. When Vo > 14V, ON_OFF signal goes high and eventually turns on Q27 and then pull SS pin low which shuts off the PWMs. The problem you are having is that at high load, ON_OFF is low, and Vbe of Q27 is low (i.e., Q27 is off) according to the waveform on page 1 of your slides. But the SS pin is pulled low somehow. Correct me if my interpretation is wrong.

    With that, I suspect that some other faults, such as OCP, may be falsely triggered at high load current which causes the SS pin to be pulled down and then shuts off the PWMs. Please check on this and keep me updated.

    Regards,

    Wangxin

  • 0 in reply to Wangxin Huang
    Prodigy 80 points

    Hello Wangxin,


    Thanks your reply.

    We found the cause,  Vcc of IC UCC28950 was affected by other circuits when power supply came to OV.

    Now it can work after we solve Vcc problem. 

    Thanks your reply again!

    Shiang Kai