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When loading flyback converter with heavy load, waveforms are as expected. But with light load, waveforms are not as expected. What can be the reason ? Duty cycle is less than 50% so no slope compensation issue.

(Attached one is waveform of drain to source of MOSFET)

  • 3
    \$\begingroup\$ Looks perfectly normal for valley switching. Is your shift key broken? \$\endgroup\$
    – winny
    Commented Apr 28, 2018 at 12:40

1 Answer 1


On light loads the energy required to be transferred is low hence, the duty is low. This of course means that the MOSFET is on for a shorter time period and the energy stored by that inductor during charging is released into the secondary with plenty of time to spare before the cycle repeats. The picture below shows a switching cycle with varying loads. For clarity only the primary and secondary currents are shown and it is assumed to be a 1:1 transformer: -

enter image description here

Picture source - hit the "flyback DCM" button.

As you might imagine on very light loads the charge time is very short and the flyback time is equally shortended. This leaves plenty of time left (hold time) doing nothing however, there is always a little bit of residual energy left in the inductor and now that the MOSFET is off and the secondary diode is not conducting, the only path for this residual energy is to form a tuned circuit with the MOSFET drain-source leakage capacitance (several hundred pF usually).

So the waveform you see is a decaying sinewave formed from the residual current in the transformer (aka residual energy), the magnetization inductance of the primary and the DS capacitance.

It's very normal to see and only happens in discontinuous mode (DCM) because neither the MOSFET nor the diode are conducting in the hold period.

I assumed this type of circuit when explaining the above: -

enter image description here

EDIT section

This is to explain the double charge cycle followed by the extended hold period (pulse skipping): -

enter image description here

  • H means "hold"
  • C means "charge"
  • F means "flyback"
  • \$\begingroup\$ actually what you are saying is absolutely right. but my problem is when I am connecting a light constant load I am observing two different duty cycles in waveform \$\endgroup\$
    – vck
    Commented Apr 28, 2018 at 13:05
  • 1
    \$\begingroup\$ What is happening is that your converter is entering pulse skipping mode. You get two consecutive charge cycles followed by a sustained off (or hold) period. This can happen. \$\endgroup\$
    – Andy aka
    Commented Apr 28, 2018 at 13:16
  • \$\begingroup\$ Andy, is there any way to avoid pulse skipping problem? \$\endgroup\$
    – vck
    Commented May 2, 2018 at 3:14
  • \$\begingroup\$ @vck The easiest way is to not think of it as a problem. If it is an actual problem then please explain what it does to the circuits you attach. \$\endgroup\$
    – Andy aka
    Commented May 2, 2018 at 9:47
  • \$\begingroup\$ My flyback converter has three output. 45v,2A,100v,1A,25v,1A. Circuit was ok but suddenly once output diode of 45v and 100v output got damaged and mosfet also failed. So I changed that components and now it is working properly but I am searching for root cause of failure \$\endgroup\$
    – vck
    Commented May 2, 2018 at 10:07

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