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OCP not triger as

ongcharoenret
Intellectual 1960 points
Other Parts Discussed in Thread: UCC3895

 

Dear Comunity,

my customer have OCP(over current protection)problem in their power supply(AC-DC) which using UCC3895 as PWM  controller(-54V output). the issue is OCP change out of spec in high line input voltage. initially guessed OPP of the unit but after measurement of the CS signal (Pin12),the CS peak voltage is just only 1.4V(over current threshold (2.5V) and peak current threshold (2.0V)) before -54V output shutdown. so anyone give an advise what case OCP trig point change(out of spec) in high line input. I attached schematic for reference.

Thanks.7888.SC UCC3895.xls

  • 0
    TI__Mastermind 25130 points

    Dear Sir,

    I expect that you are getting noise spikes on the CS pin. Using a high speed scope and no band width limiting connect the scope ground to pin 5 and measure the voltage on pin 12 using tip and barrel techniques. See how much noise is present.

    In your design you have pin 20 connected through R603 to Vref. At start up and any time the optocoupler is not conducting enough current you will have teh EAP pin go out of the common mode range. This can result in erratic operationof the outputs. I would suggest that there be an additional resistor in parallel with the optocoupler transistor to prevent this from happening.

    Ensure that the quiet ground (pin 5) and the power ground (pin 16) are connected at only one point and that is directly under the UCC3895. You should review the layout recommendationof SLUA501
    http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slua501&fileType=pdf 

    Regards,