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TL431: TL431 Voltage?

Leonardo dicaprio
Prodigy 70 points
Part Number: TL431

Hello.

I used TL431.

This Circuit.

What is the voltage at this point?

I don't Konw....

Thanks.

  • 0
    TI__Expert 3835 points
    Hi Leonardo,

    This depends on the bias current that is going through the TL431 for regulation.

    To determine that you would need to know what the optocoupler is connected to on the primary side. If it is connected to a controller, the pull-up resistor and voltage on the COMP pin will determine the range of collector current will be on the optocoupler. You would also need to look at the optocoupler datasheet to determine the current transfer ratio and the forward voltage of the diode.

    In the end, the cathode voltage varies and will change, but you can get a estimated range.

    The 300 ohm in parallel with the diode is kind of odd, what is it used for? Is it for TL431 regulation during transients or off states? What is the application of this power design?

    Regards,
    Ethan
  • 0 in reply to Ethan T
    Prodigy 70 points

    Thank you for your reply.

    The primary side circuit is as follows.

    PC817 CTR = Min50%, at If = 5mA

    Optocoupler Opaerating State   

    PC817  Output side Ice = 0.22mA, 

    Input side = 0.22mA / 0.5(CTR 50%) = 0.44mA

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

    I want cathode voltage.... get a estimated range..

    R2.. remove...

    Vr1 = 600*0.44mA = 0.264V

    optocoupler VF = about 1V

    12-1V(Vf) - 0.264V = 10.736V  right?

    Thanks.

  • 0 in reply to Leonardo dicaprio
    TI__Expert 3835 points

    That is correct. I would not remove R2 in your final design, as 440 uA is not enough to properly bias the TL431 in worst case conditions. 

    Hope this helps,

    Ethan

  • 0 in reply to Ethan T
    Prodigy 70 points
    Hi.
    In actual operation, the TL431 continuously feeds back the voltage. ( TL431 Pin number 1.)
    Since the current that flows depends on the feedback voltage The measured voltage value differs from the theoretical value.

    But I understood the theoretical working principle.

    It really helped me to understand the principles of operation.

    Thanks.