These SMPS power supplies are all over ebay and picked one up for some projects. However, the output of the unit is rather noisy and it seems to permeate through the input linear regulators and can be seen super-imposed on some analog signal lines that I'm sampling.
Power supply in question:
http://www.ebay.com/itm/Mean-Well-RT-65C-AC-to-DC-Power-Supply-Triple-Output-Enclosed-Switching-65-5W-/111702973357?hash=item1a02041bad:g:WA4AAOSw-vlViwGSThe Noise:
See the "scope_4.png" attachment to see the noise on the power rail AFTER the input linear regulator.
A clean input:
See the "scope_5.png" attachment as a reference for the rail using a benchtop power supply.
Now, it's not something that's absolutely terrible but it's something that I want to learn how to address.
My first plan-of-attack was to add an LC filter between the SMPS and the electronics. It can be seen in the first reply in this thread:
https://www.eevblog.com/forum/beginners/lc-filter-basics-for-smps/Unfortunately, it didn't seem to do anything. The most likely culprit could be the values I used (3.9uH inductors and a 470uF cap). However, the cut-off frequency is 1/(2*pi*sqrt(LC)). This should give me a cut-off of about 3.7kHz. If I'm reading it right, the datasheet link below shows a switching frequency of 60 kHz. It's marked as fosc next to the block diagram.
http://www.meanwell.com/webapp/product/search.aspx?prod=rt-65Taking some measurements on my scope. The most noticeable peaks are roughly at:
244kHz
181kHz
118kHz
116kHz
6.82kHz
Then a really small peak at around the 120Hz mark (8.3ms period) as seen on the scope screenshot.
Considering that all of these are substantially higher (sans the 120Hz) than my input filter I hacked in ( 3.7kHz ), do I just need an inductor with a greater rating? How should I move forward to try to clean up the switching output to mitigate those pulses every 8.3ms?