I used this schematic to build a half bridge power supply, but I have some problems and need some help.

The first problem is feedback.

I think this feedback is not good because with this feedback the transformer produces a noisy sound like "tik-tik-tik"about twice every second and the voltage changes beetwen 40 to 45 in each rail.

I read some where that this feedback doesn't control the duty cycle and it just turns the PWM on or off but I need a feedback that controlx the output voltage by decreasing or increasing the duty cycle.

Without feedback there is no noisy sound and the SMPS works fine. When I connected a 300 watt load the voltage dropped about 20 volts.

Pleasehelp me to change the feedback.

What should I do?

I changed this schematic a little bit.

  • My primary winding has 19 turns. The secondary is 8*2.
  • I used an ETD 39 core.
  • The target output voltage is 45*2.
  • I changed the frequncy to 80 kHz.

enter image description here

  • \$\begingroup\$ You have secondary side optically coupled feedback, though it's kind of crude and only active on the positive output. Temperature stability will not be great due to the zener references. You might want to look at optical feedback coupled with a TL431 type shunt-regulator. \$\endgroup\$
    – John D
    Feb 8, 2019 at 21:39
  • \$\begingroup\$ can you help me about tl431 feedback?can you explain by a schematic? \$\endgroup\$ Feb 8, 2019 at 22:15
  • 2
    \$\begingroup\$ You can find lots of examples by Googling "Optocoupler TL431". Here's a presentation on how it works from ON Semi: onsemi.com/pub/Collateral/TND381-D.PDF \$\endgroup\$
    – John D
    Feb 9, 2019 at 20:20

1 Answer 1


this circuit is more promising . both of the resistors are 470K . and after the pot output dont forget to use a zener diode 12-24v.

enter image description here


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