Electronic – Mechanical Switch Causes Transients from AC-DC Power Supply

power supplyswitch-mode-power-supplyswitching-transientstransienttransient-suppression

I'm having trouble getting rid of output transients from this Mean Well PSU right after the AC input is switched.

https://www.meanwell.com/Upload/PDF/RPS-500/RPS-500-SPEC.PDF

The PSU is a RPS-500 27VDC with fan. The 120 VAC is coming from an industrial power strip and the switch is a mechanical DPST bat handle style.

The switch transients are showing up on all the outputs: 5Vsb, Power Good (PG), and 27VDC output.

I captured some scope shots from the 5Vsb pin w/out load.

Transient1
Transient2
Duration

Occasionally the transients aren't noticeable.

Would a multi-stage AC/DC EMI filter and perhaps a NTC resistor (to limit inrush) get rid of this?

My guess is that these are due to inrush current or the switch making contact at different phases of the AC input.

Edit 1:

Bench setup schematic

Schematic

RPS-500 block diagram
PSU Block Diagram

Edit 2:

Scope captures from an MSO4054B using TPP0500B probe with spring tip ground. Measured at 2mm double-row connector, no load (for now).

These are showing the 5Vsb ramp and the voltage transients when the switch is toggled.

5VsbRamp
5VsbSpikes
5VsbSpike
5VsbLowSpikes
5VsbLowSpike
5VsbHighSpike

Best Answer

What I think you are observing is the settling of your output unloaded.

You say the "noise" is occurring right after switch on of the AC input but on your oscilloscope we don't see the rise of the output voltage from 0 to 5V.

Thus making it difficult to help you...

But your problems looks a lot like switch bounce combined with the undampened beahviour of a unloaded supply.

What I would suggest you to do:

  • Do not put an EMI filter (this would help if other noisy equipment shared the AC voltage, but not for the switch)
  • A NTC is always a good idea following a switch on an AC line voltage (as you mentionned it will helps with the current inrush if you switch on the highest portion of the sine and if your redressing caps are completely deplated and if your wire has very little resistance)
  • Load your supply at least with a small load do not just put its output on a HiZ such has your oscilloscope, it will mostelikely not behave the same in your final setup (choose your resistor carefully not to set your house on fire, $$P=U^2/R < P_{rating-of-your-resistor} $$)
  • Try a low pass filter at the output of your power supply (a simple capacitor in parrallel with the output might add some dampening of the regulation loop inside you PSU, note that this will worsten the dynamic behaviour of the voltage tracking of the supply, but it might helps with the stability a little bit)
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