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I am learning how to design a flyback converter power supply. I am using a reference design from TI PMP11047.

Detail schematic can be found here: https://www.ti.com/tool/PMP11047#description

I have a question on the feedback circuit. I understand that when the voltage increases above 24V, the reference voltage at TL431 is 2.5V, making it conduct and turn on the LED inside the opto-isolator. What is the purpose of D10, R28, D11, and C15? enter image description here

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  • \$\begingroup\$ It is a cheap soft-start circuitry, closing the loop from the secondary side upon start-up. You can find design guidelines regarding the TL431 used in power supplies in a seminar I built some years ago. \$\endgroup\$ May 4, 2023 at 20:14
  • \$\begingroup\$ @VerbalKint Thanks! Do you also have a seminar on how to design a flyback converter? I am looking for methods to calculate peak current, switching frequency, how to decide turn ratio.. these kinds of things \$\endgroup\$
    – HV16
    May 4, 2023 at 20:23
  • \$\begingroup\$ You can check my webpage and download The Dark Side of Flyback Converter seminar. \$\endgroup\$ May 4, 2023 at 20:27
  • \$\begingroup\$ @VerbalKint: that's exactly what I need! Appreciate it. \$\endgroup\$
    – HV16
    May 4, 2023 at 20:45

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Looks like during startup D10 and C15 provide an initial path for LED current during the time when there's not yet enough voltage to activate the TL431. That probably prevents an odd spike in the output voltage, providing a smooth linear startup waveform.

C15 eventually charges up above the TL431 cathode, reverse biasing D10 so it no longer affects the circuit.

D11 and R28 discharge C15 on shutdown so that on the next startup the circuit operates as intended.

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