I am using PI Expert Online (this) to design a smps circuit for 5V, 2.5A output. I already have TOP258PN IC with me so built circuit for that.


Change from schematics: I am not using choke and using SR360 as output diode.

I am able to get 5V output when no load is connected. But as soon as I add load (5 ohm test load for 1 A current), I get current at right value but within few second voltage drops to zero and transformer makes noise. And then it again goes up and then down and it continues. Just before transformer goes to zero, it makes sound. I made transformer design as given by the PI Expert software (which says Design Passed).

Is it problem in transformer or some other thing like switching?

Leakage inductance is estimated to be 93 uH but is 124 uH in real. Will that be a problem?

  • 3
    \$\begingroup\$ Impossible to deduce unless you provide some waveforms. Flyback transformers (coupled inductors) are notoriously finnicky (very sensitive to construction methodology). If you can't show us some waveforms the answers will be speculative only. I suspect your transformer isn't properly designed/built and the TopSwitch is hitting peak current limit, but it's impossible to prove without more data. \$\endgroup\$ Commented May 16, 2019 at 13:47
  • \$\begingroup\$ What are the power ratings on your transformers? \$\endgroup\$
    – Voltage Spike
    Commented May 16, 2019 at 14:46
  • \$\begingroup\$ I gave transformer for development with EE28, 3F3 core. It was rated at around 39.5 W. How to check after receiving transformer? \$\endgroup\$
    – Akshit
    Commented May 16, 2019 at 15:02
  • \$\begingroup\$ Leakage inductance is estimated to be 93 uH but is 124 uH in real. Will that be a problem? \$\endgroup\$
    – Akshit
    Commented May 17, 2019 at 6:10
  • \$\begingroup\$ I'm voting to close this question as off-topic because it was self-answered and nobody else understood what the problem was to begin with. \$\endgroup\$
    – Dave Tweed
    Commented Jun 9, 2019 at 21:31

1 Answer 1


Strange but using RCD clamp instead of Zener Clamp solved the issue for me.


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