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UC3843A: UC3843A

ahmed hamdy
Prodigy 20 points
Part Number: UC3843A

Greetings!

I have a PLC Power supply with multi output values (6v, 16v, -16v). The problem is: the UC3843A controller doesn't start and voltage at VCC pin always 7.4V so it is always in UVLO status.

Power supply specifications:

1- it is non isolated power supply. 

2- VCM (Controlling MOSFET).

3- Feedback circuit using Optocoupler.

Is the UC3843A defected? 

  • 0
    TI__Mastermind 34465 points

    Hello Ahmed,

    There is probably an issue in your circuit that is pulling VCC below the UVLO threshold.

    Without being able to review the schematic I can only guess there is some issue at startup.
    If the capacitor on VCC is set too low and the startup resistor from VIN to VCC set too high then VCC will discharge before the system has full started because the auxiliary winding on the transformer is not yetable to supply the bias voltage.

    You can post the schematic or check yourself if it is possible to maintain VCC greater than UVLO with an external bias supply.

    It is very unlikely that the UC3843A is defective. unless perhaps it was damaged during your tests.

    Regards

    John

  • 0 in reply to John Griffin1
    Prodigy 20 points

    Hello John,

    Best regards for you respond. But sadly it is PLC power supply come with a customer's production line. So i don't have a schematic for it.

    But i will check what you suggested and tell you the results. And one more thing, the transformer is buzzing. I changed the transformer with a working one and still buzzing. The weird thing that i know when the transformer is buzzing then the IC controller is running with a very low frequency!!. which is weird because my IC is in UVLO status. And i changed the MOSFET so that i kill any doubt and it is still buzzing. :D

    Note: During the current status of the power supply it produce voltage but not the rated voltage. Rated voltages (+6v, +16v, -16v) and produced voltage (+3.32v, +12.2v, -12.2v). Wired thing huh? and remember my VCC is 7.42v.

    Regards

    Ahmed

  • 0 in reply to ahmed hamdy
    TI__Mastermind 34465 points

    Hello Ahmed,

    Glad I was able to give you some help.

    I will close this post .

    Please open a new post if you need further help.

    Regards

    John