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UCC256403: Regarding The optocoupler in Feed back circuit

hema danda
Prodigy 160 points
Part Number: UCC256403

Tool/software:

Hi 

I am Using PC817 as my feed back optocoupler whose input connected to CC CV loop outputs with OR Gates

By seeing the Below image As one of the Diode conducts the Base current would be approximately ( 0.94mA)

From the Data sheet of UCC256403A the IFB current maximum the IC can provide is 164uA for UCC256403

So CTR of Optocoupler should be less than 20 percent ( IC/IF) ..Is my understanding correct??

I found from the Data that CTR varies with respect to Temeprature 

one more question is Should my optocoupler operate in saturation mode or linear mode?? 

  • 0
    TI__Guru* 77475 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 77475 points

    Hello,

    Please see my comments below.

    Regards,

    -------------------------------------------------

    Hi 

    I am Using PC817 as my feed back optocoupler whose input connected to CC CV loop outputs with OR Gates

    By seeing the Below image As one of the Diode conducts the Base current would be approximately ( 0.94mA)

    > The maximum current that can go through your opto diode is (12V - 0.6V -0.3V)/8.25 k ohm = 1.35 mA

    From the Data sheet of UCC256403A the IFB current maximum the IC can provide is 164uA for UCC256403

    > To control the switching frequency the LLC will vary the current coming out the FB pin from 0 to - 164 uA

    >IFB = - 164 uA the LLC will be operating at max frequency

    >IFB = 0 uA the LLC will be operating at min frequency.

    So CTR of Optocoupler should be less than 20 percent ( IC/IF) ..Is my understanding correct??

    > If your CTR is 20% you will need 820 uA of opto diode current to pull 164 uA of FB current out the FB pin.

    I found from the Data that CTR varies with respect to Temperature 

    one more question is Should my optocoupler operate in saturation mode or linear mode?? 

    > This will depend of the feedback loop.

    > If your opto is demanding more current than 164 uA coming out of the FEED back pin the opto will be saturated.  That will only happen if your output voltage is high trying to correct the voltage loop.