Nosie generated by an eby puchased SMPS 9V Module

[aht2000]

View attachment Noise generated from ebay 9V SMPS.pdf

I purchased a SMPS 9V module from ebay, and used it to power an STM32 microcontroller circuit and was facing difficulties with the ADC measurements. Using an oscilloscope, I found a lot of noise, and documented my findings in the attached PDF file.
I would expect that the noise be reduced when the inductor in installed on the 9V side. While the actual tests shows that it has no effect, and only was useful when installed on the ground side.

Could be noise generated from the ground side? Is this the right solution to reduce the noise from this SMPS? Can I reduce it more down to below 10mV PP to allow my ADC measurement be done correctly?

Thank you.

 

[betwixt]

The 15MHz will be ringing as the SMPS switch turns off.

I would suggest you try these in order:
1. Add one or more ceramic capacitors across the DC output, suggest 100nF with short wires. That 470uF output capacitor will have high impedance at 15MHz.
2. add the inductor in the + output but also add a 100nF capacitor AFTER it with it other wire going to the negative side.
3. Both 1 and 2 together.

You may also be seeing noise capacitively coupled through the transformer. There will be some impedance in the loop from SMPS AC input to your scope AC input and then to the SMPS output, you might be seeing voltage dropped across that impedance. If that is the case, it probably wont cause problems in your final design.

Brian.

 

[aht2000]

Thank you Brian for your prompt reply. I implemented the first suggestion, but unfortunately did see any improvement. I am attaching a photo of the capacitor I soldered in case you have any comment.



Next step would be implementing your second suggestion.

 

[betwixt]

That is what I meant, SMD capacitors may not be ideal for this purpose but if it was going to work you would have seen a significant, if not perfect improvement.

Try 'plan B' and see if that is enough.

Brian.

 

[jiripolivka]

That is what I meant, SMD capacitors may not be ideal for this purpose but if it was going to work you would have seen a significant, if not perfect improvement.

Try 'plan B' and see if that is enough.

Brian.

Most switcher power supplies are simply bad and not suitable for use with sensitive instruments like yours.
PLEASE buy a LINEAR AC/DC power supply or build yours, with a transformer, rectifier and regulator like 7809 to get clean 9V. Otherwise you will spend a lot of time trying to filter the dirty output from that cheap crap.