Computing stuff tied to the physical world

Switching glitches

In Hardware on Jun 11, 2012 at 00:01

The dual power supply described yesterday had a nasty spike every 5 µs. I tried damping them with one of these:

Varkensneus

(they are called “varkensneus” – pigs nose – in Dutch, ’cause that’s what they look like, seen from the end)

But the results were not very substantial when adding one to the supply output. When I added one on both the input and the output of the 7812 regulator, things did improve a bit further:

SCR15

The yellow trace is the output with ferrite core between the DC-DC converter output and the 7812 linear regulator input, and the blue trace is from a second ferrite core added at the end, i.e. the linear regulator’s output pin.

Note the scale of this oscilloscope capture: 10 nsec/div, so this is a 100 MHz high frequency signal of about ± 200 mV. The second ferrite core almost halves these spike’s amplitudes.

In conclusion: these are very brief ± 100 mV glitches, super-imposed on the +12V supply output voltage – i.e. about ±1% of the regulated supply output voltage. The glitches don’t change much with a 1 kΩ load, i.e. 12 mA.

It’s an artifact of the switching inside the DC-DC converter – looks like there’s not much more I can do about it!

  1. 100 MHz, wow. I wonder if the right bypass caps might help. Power supplies are not my specialty, but I seem to recall that some caps handle such high frequencies better than others.

  2. for my education : why would a simple rc filter not be a good solution ? since this delivers DC, the suppression of high freq signals could/should be enormous, even with -3dB per decade.

    • That was my thought too. A simple low pass filter would squash those nasties.

  3. As I’ve read up on these ferrite beads, they are typically used to filter out interference picked up by (long) cables, so I am not sure they are the right tool against this kind of switching noise, which might saturate them.

    An RC filter as Ronald suggests, or even an LC filter, but with a regular inductor rather than a ferrite bead with low Q factor, might be better.

  4. The VK-200 you tried are not suitable. Those inductors are good for HF RF filtering.

  5. A good portion of the noise from the DC-DC converter can be common-mode, so filtering is needed on the output centre terminal. I have had good success with ‘block’ filters, which are designed like little RF modules, such as the Murata BNX01 series. You need one for each output rail. You need to keep the filtered and unfiltered grounds separated, and the filter block pinout helps with this. If you are using a ground plane, connect it to the output ground of the filter. Treat the unfiltered ground output of the DC-DC converter as a noisy signal and keep its routing short. The filter blocks should go between the DC-DC converter and the linear regulators.

    Remember the DC-DC converter will generate just as much noise on it’s input supply: this is usually less of a problem, but input side filtering may be needed too.

  6. More bypass caps? That’s my noobie solution

Comments are closed.