enter image description here

I am doing the hardware implementation of the flyback transformer as per the attached figure, but I am not getting the proper voltage waveform at Vs and Vd.

The frequency is 20 kHz, the turns ratio is 8.34, and the MOSFET used is IRF840. I have connected a 100 Ω (2W) resistor in series with the primary winding.

enter image description here

The oscilloscope figure shows the output waveform I’m getting, which is not coming as per the simulation as for Vs I should get -24 V to 24 V but I’m getting 7 V to -24 V. The left side of the CRO shows the reading:

enter image description here

  • 1
    \$\begingroup\$ What makes you think you are getting the wrong voltage waveforms? What voltage waveforms do you expect? \$\endgroup\$ Feb 26 at 3:38
  • \$\begingroup\$ sorry , I missed adding CRO plot. \$\endgroup\$
    – silvi09
    Feb 26 at 4:31
  • \$\begingroup\$ Just a hunch, check your switching frequency and look at your rectifying diode. \$\endgroup\$
    – The-Duck
    Feb 26 at 8:10
  • \$\begingroup\$ You haven’t entered any inductance of your transformer in LTspice so your turns ratio is 1:1. \$\endgroup\$
    – winny
    Feb 26 at 9:01
  • \$\begingroup\$ You don't show values and the duty cycle is different. But you were already told what the problem is with your LTspice setup. It loks like you even have another question which had various suggestions in the comments. And still you're asking yet another one? Do you realize that people could interpret this as mocking their time and efforts into helping you? \$\endgroup\$ Feb 26 at 9:53

1 Answer 1


I have connected a 100 Ω (2W) resistor in series with the primary winding.

That's your main problem in not getting the required positive output voltage. It's forming a potential divider with the load resistor of 1000 Ω but, because there is a turns ratio involved, the 100 Ω is seen as 8.34 x 8.34 x 100 Ω at the secondary. This is 6956 Ω hence, it forms a potential divider with the 1000 Ω load.

So you expect to see about 24 volts but, with the 100 Ω series resistor it's going to be much lower.

Of course, I'm not ruling out other problems; the one above is glaring.


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