Electronic – SMPS, output ringing

dc/dc converterringingswitch-mode-power-supply

I built an SMPS (see here for details), and so far it's been working good. But I noticed a strange oscillation (or ringing?) at the output.

Here's what it looks like, with a 4 amps load:

enter image description here

Now, you'll see a HF oscillation when the switch turns on (right at the center of the image, for example). I measured this at 15MHz and figured the MOSFET needed a snubber. I added one (10nF + 10R in series, across D and S), and the 15MHz ringing went away completely.

But I still have that "larger" ringing that can be seen in the picture, around 3.5-4MHz.

What is this ringing?

Is it harmful?

If it is: How can I fix it?

EDIT:
Here are some measurements (transient response, voltage across MOSFET and diode, etc). http://imgur.com/a/Qs351

Best Answer

The inductor has got parasitic capacitance that forms a resonant circuit at a high frequency. The action of switching current into it and turning it off ALWAYS causes this to happen on pretty much ALL power supplies of this type. The leakage capacitance of the series MOSFET also interacts with the inductance to modify this too.

The inductor is 27uH and maybe the MOSFET has 200pF D-S capacitance - this would cause a resonant frequency of: -

f = \$\dfrac{1}{2\pi\sqrt{LC}}\$ = 2.16 MHz

OK I'm just guessing at what \$C_{DS}\$ is but it's definitely in the same ballpark as you figures. You don't need to add a snubber - doing so will reduce the efficiency of your circuit and what are you trying to "protect" against?

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