Ringing on inductor current waveform (buck converter)

Thread Starter

HRN88

Joined Jul 21, 2020
24
Hi guys!

I'm working with the power stage for a buck converter, but when I did some measures with the scope I saw thet my inductor current waveform has a thing that I think are ringing spikes. Of course this is not a perfect triangular wave like in simulations hahaha.

Background:
The green waveform is the current on the buck inductor through a current probe I measure both at the start node and output node of the inductor, the result is the same.

The yellow waveform is the gate of the switching MOSFET referenced to GND.

Below image is for a full load in CCM. Vin = 70V, Vout= 30V and Iout = 2A, the output power is about 60W
50load30v1a3.png

Now if I change to work at less load
Vin = 70V, Vout= 30V, Iout = 200mA (10% of max Current that is 2A) Pout = 6W

As you can see, the spikes of this waveform has more ringing even distorts the shape of the triangular wave :(

50load30v1a4.png

My concern is that the GATE Mosfet waveform has a very low ringing but the inductor ringing is bigger, does this ringing due to the physical construction of my inductor?
I made this inductor with multi strand wire, 22 strands of 34AWG (22/34) and the bobbin former has two chambers. To reduce the parasitic capacitance of the inductor.

I think that this ringing is due to the parasitic capacitance it isn't?

Ty guys.
 

crutschow

Joined Mar 14, 2008
30,103
I think that this ringing is due to the parasitic capacitance it isn't?
Yes, likely due to stray capacitance, which can generate a tank resonant circuit with the inductance.
A small resistor (e.g. 100Ω) in series with a small capacitor (e.g. 1-10nF) connected from the inductor input (MOSFET output) to common can usually damp that.
You may have to experiment with the resistor and capacitor values to get the best results.
 

Irving

Joined Jan 30, 2016
2,771
Can you show your schematic.

Ringing is generally as a result of parasitics and it can be hard to pinpoint exactly the cause. Often they are harmless, though they do represent inefficiencies and unwanted losses.

Your buck converter is variable frequency rather than variable duty cycle?
 

Thread Starter

HRN88

Joined Jul 21, 2020
24
Can you show your schematic.

Ringing is generally as a result of parasitics and it can be hard to pinpoint exactly the cause. Often they are harmless, though they do represent inefficiencies and unwanted losses.

Your buck converter is variable frequency rather than variable duty cycle?
Hi, I think the buck is both variable and duty cycle frequency, you can se that in the above image, when changes the output load there are a change on the control frequency from 117kHz to 193kHz approx. this to control the output current, but the duty cycle remains constant to the output voltage.

For the moment I don't have the schematic but maybe I'll do it one.
 

Thread Starter

HRN88

Joined Jul 21, 2020
24
I adjust the scale to see more clearly the inductor current waveform, and for a bit moment I was thinking that maybe the inductor is saturated?

50load30v1a.png
I don't know I compared with this image of a real inductor saturated.
1655402665888.png

Another question.
This is the way in that I put my current probe. Do you know if there are a chance that it is catching noise from another part of the circuit.
Are there another way/technique to put that probe?
1655402891621.png

Ty guys!!
 

Thread Starter

HRN88

Joined Jul 21, 2020
24
Those off-board wires won't be helping... :)

Have you got a snubber network?
AHHH you're right I used them to connect my oscilloscope probes. And the green wires are the JTAG of the MCU

Look this is the schematic (very very simplified), I focused to show the snubbers..

1655410536259.png
 
Last edited:

Thread Starter

HRN88

Joined Jul 21, 2020
24
How did you come to the R & C values for the snubbers?
Ok. actually is my first attempt to design a snubber. I've been investigating about.
This first was a try and error approach using fcut = 1/(2*Pi*R*C), but I know that this is an incorrect approach.

Yesterday I found this application note, so I will try to do it.
https://assets.nexperia.com/documents/application-note/AN11160.pdf

May I question, do you know some approach that you share with me?

Ty Mr. Irving
 

Marley

Joined Apr 4, 2016
476
A little bit of ringing like this is expected and not necessarily a problem.
It can be minimised with a good PCB layout. Using a ground plane, short connections and good quality low ESR capacitors (electrolytic capacitors should have SMD ceramic capacitors in parallel).
Your oscilloscope probe setup can create ringing on the 'scope display that is not actually there. Solution: short probe ground lead and no "loops" in the probe wires.
 

Thread Starter

HRN88

Joined Jul 21, 2020
24
I see an inductor with two windings. Not a repurposed common-mode choke, I hope?
Hi dude, not is a repurposed magnetic. Its a coil former with two chambers where I divided by two of my calculated 170uH with 24 turns, so each chamber has 12 turns of 22/34 muti strand wire. I did that to try to reduce the parasitic capacitance of the Litz wire hoping to reduce the range where the self resonant of the inductor.
 

Ian0

Joined Aug 7, 2020
5,510
Hi dude, not is a repurposed magnetic. Its a coil former with two chambers where I divided by two of my calculated 170uH with 24 turns, so each chamber has 12 turns of 22/34 muti strand wire. I did that to try to reduce the parasitic capacitance of the Litz wire hoping to reduce the range where the self resonant of the inductor.
Good batting, Thinkman.
 
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