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UC3842: Feedback circuit is not working properly with shunt regulator

Manjunath MN
Prodigy 30 points
Part Number: UC3842
Other Parts Discussed in Thread: TL431, UC3844

When I use POT and give feedback voltage the design works fine as expected but when I try to connect the same to feedback with shunt regulator it does not work as expected .

Please have a look at my design and check if I have missed anything or values.I have attached schematic for reference.

In detail When connect using potentiometer I will get 12V output as expected but using shunt regulator I am only getting around 1V to 3V.

Feedback circuit is from TI application note 

Input : 200 - 300 Vdc

Output : 12V

6428.2.pdf 

  • 0
    TI__Genius 13660 points
    Hi Manjunath,

    I am sorry that I am very unclear to the problem you stated.
    The feedback circuit seems right. Could you explain more about your issue that how POT and shunt regulator connect in the circuit? Did shunt regulator mean D17 TL431? If you can post these two connection circuit SCH below respectively, it will be greatly appreciated.

    Regards,
    Teng
  • 0 in reply to Teng Feng
    Prodigy 30 points

    Thank you for the reply,

    Yes shunt regulator means TL431.

    Both POT and TL431 are not connected same time.

    When I remove feedback circuit and change voltage across Vfb pin using POT I can see change in output voltage.

    But when I connect feedback voltage and check , I am not able to get output.

  • 0
    Prodigy 30 points
    Fond one problem D15 is connected to Analog ground instead of digital ground.
    But still this does not solve my problem as uc3844 restarts again and again
  • 0 in reply to Manjunath MN
    TI__Genius 13660 points
    Hi Manjunath,

    I think that maybe during the system start-up, the bias voltage to VCC doesn't reach its corresponding turn off threshold (10V), or VCC donsn't provide enough energy then drops below the UVLO-off threshold. Then the device restarts by high-voltage start-up.
    Besides, I aslo wanna insure that if the over-current protection is activated.
    Could you please capture the waveforms of VCC voltage, Vfb and Isense pin voltage?
    In your circuit I found there is no the bleeder resistor, placed between the gate and the source of the MOSFET should be used to prevent activating the power switch with extraneous leakage currents during undervoltage lockout.

    Regards,
    Teng
  • 0 in reply to Teng Feng
    TI__Genius 13660 points
    Hi Manjunath,

    I haven’t heard back from you, I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if the thread has locked due to time-out). Thanks.

    Regards,
    Teng