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I am working on buck converter but stuck on at the basic results.

I simulated my circuit in OrCad.

Attaching my circuit diagram and waveforms.

The problem is with the voltage at source of mosfet. Initially, the m source voltage goes down as gate goes down but just after a little time it is keep turning high(with ringing) while gate is still low.

Can some one please explain why this is happning.

I have fabricated this circuit also, same thing happaning . I am attaching those DSO waveforms too.

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OrCad circuit diagram

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DSO waveform of implemented circuit

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OrCad circuit Simulated WaveForm

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First things first - you are not driving the MOSFET correctly to achieve reasonable performance. The gate voltage you apply is 5 volts relative to ground and your drain is connected to 12 volts. This means that you can never fully turn on the MOSFET and you'll get really poor efficiency. Try activating the gate with a drive voltage that reaches 15 volts OR better still apply a floating gate-source drive voltage by connecting the negative end of V1 directly to the source.

I understand that the above recommendations make it difficult to achieve in your physical set-up but nobody who knows about these things has said that making an efficient buck converter (or any switching converter) is easy.

As for the ringing, that is due to the parasitic capacitance between drain and source causing a 2nd order damped oscillation (in conjunction with L1) when the MOSFET switches off. Pretty normal for most switching converters.

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  • \$\begingroup\$ So much thanks to answer. I got you. Now i have a doubt 1st- why this ringing is not at initial waveforms(as seen it is coming after some switching periods )? 2nd- Another thing i observe that when i increse switching frequency the ringing not happanes. 3rd - Ringing while on time is different then it is at off time? Are there diffrent reason for both situation?Like in my case turn on Ringing is not happning. it is after turn off. 4th- Why ringing is not happaning just after the turn off (It is taking some time right?) . IS there any documents you can link to read.again Many Many Thanks. \$\endgroup\$ – Ashish Jha Jan 20 at 13:30
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    \$\begingroup\$ Ringing occurs when the mosfet is off and after a small period of time when there isn’t enough energy left to forward bias D1. You should also use a faster diode rather than the really slow 1N400n series. \$\endgroup\$ – Andy aka Jan 20 at 14:29
  • \$\begingroup\$ Try this picture - top is CCM and bottom is like yours DCM \$\endgroup\$ – Andy aka Jan 20 at 14:52

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