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I'm studying AC adapters, and to check about waveforms while changing parameters I did a simulation of this circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

Then I wanted to plot also the rectified waveform, so I added a second diode and a big resistance (sorry if this circuit doesn't use the internal CAD, but I was unable to find how to insert two schematics: if I tried to insert a second one, the first was deleted):

enter image description here

so on the 1G resistance there should be the rectified voltage, and the resistance should be big enough to not influence the circuit.

It seems to work, but what I don't understand is that there seem to be a disturbance on the secondary of the transformer, and on the rectified waveform (green on the left graph is the primary of the transformer):

enter image description here

So the secondary voltage has the positive part of the wave distorted, and the negative which is thicker, like there was some very rapid oscillation; and the rectified wave has the same distortion.

I tried experimenting with the simulation, and it seems to be due to the capacitor, since if I delete it the waveforms are a non distorted sines and the rectified waveform with no distortion; I'm not sure if this is something real, or an error of the simulator.

As for the diode, there is a parameter I don't know ("Current At Above Voltage"), but their model as given in the simulator is:

  • saturation current: 171 nA
  • forward voltage: 806 mV (but the distortion are the same with 0)
  • current at above voltage: 1 A
  • emission coefficient: 2

Are those distortions something to be expected? If so, why?

As simulator I used Circuit JS.

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    \$\begingroup\$ This is LC distortion from the transformer and the cap. It's common. \$\endgroup\$
    – Aaron
    Apr 27 at 13:58

1 Answer 1

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Are those distortions something to be expected? If so, why?

The disturbance at the top of the positive peak is fundamental to any practical rectifier and smoothing capacitor. Once the capacitor has charged to a significant voltage, the diode won't conduct to inject more charge into the capacitor until the diode's anode voltage has reached slightly above the charged capacitor voltage. At that point, a sudden injection of current occurs and this can be quite high in peak value.

The net result is that the impulse of current causes significant volt-drops in the transformer and diode resulting in the waveform distortion you see. If you also plotted diode current you would see what I mean.

I expect that the high frequency artefacts you get are as a result of the simulator although, there will be some ringing due to parasitic inductance in a real circuit - maybe you have a bit of both - it all depends entirely on your models.

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  • \$\begingroup\$ I plotted the current, thanks: i.imgur.com/CXdpoat.png. \$\endgroup\$
    – Mauro
    Apr 27 at 20:54
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    \$\begingroup\$ @Mauro so, has this answered your concerns? \$\endgroup\$
    – Andy aka
    Apr 27 at 22:24
  • \$\begingroup\$ It did, thanks. \$\endgroup\$
    – Mauro
    Apr 28 at 8:09

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