Transistorgrab

Some time ago I built a bench power supply utilizing power supply modules DPS3003 which I bought on Aliexpress.

Some weeks ago I got a new oscilloscope Rigol DS1104Z and one of my first measurements was the output voltage of this power supply:

The voltage ripple is fine with about 25 mV. But what was really outstanding was the switching noise with about 900 mV peak to peak. This is with a 30 Ω load and 12 V output voltage.

Searching for a solution I found several forum posts that proposed just filtering the output with a LC filter and that’s it.

However, I wanted to eliminate or at least reduce the noise right at the source.

This is quite easy to achieve (⚠ Attention: void warranty ).

To reduce the ringing noise in a buck converter you need a snubber network from the switching node to the board ground.

I could not see such a thing on the module. The switching node is at a corner of the power PCB and easy to access:

Its on the upper right corner of the device when you look at it with the heatsink on the upper edge. The switching node is at the upper lead of the big inductor, the copper plane at the edge is ground.

By removing a small area of the solder resist on both copper areas we provide the place for the placement of the snubber:

With some testing I decided the values for the snubber for best performance were: 2.2 Ω for the resistor and 620 pF for the capacitor. The 620 pF were achieved by connecting a 470 pF and a 150 pF in parallel.

Adding this resulted in a much lower output noise of about 300 mV peak to peak:

I could not get a better result, but I also was not willing to spend too much time on this part.

I had two ferrite rings lying around that seemed perfect for the next step: designing the output filter.

After some testing 2 windings with each output around the core provided the best result. About 200 mV peak to peak:

I then added a capacitor of 100 nF between the both outputs and had a really nice result. About 100 mV peak to peak noise:

After that I did it for a second module with similar success.

This module showed lower ringing noise from the start of about 550 mV peak to peak.

After applying the filters on the PCB and on the output this was reduced to about 90 mV peak to peak.

However, this output showed some minor oscillations that were not visible before applying the filters.

And finally here are both power supply outputs both with identical load side by side:

For completeness here the schematic for the power supply:

And some impressions:

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