I'm designing a ultra low noise smps and I have some questions about ground/earth chassis. The main goal is very low noise on DC output.

  1. Earth (PE) should be connected to one point (only) to the chassis ground?
  2. AC line usually has two Y capacitors after common mode choke connecting earth. This connection should be at chassis or I should connect to single PE point (as in question 1). What's the best value for those Y caps?
  3. Primary side and secondary side transformer have Y capacitors connected across. Should I keep those if I use earth/chassis (I assume I will keep them in place since I want a short path for noise to return)?
  4. DC side ground. In some designs I see it connects to chassis (that's connected to PE) using capacitors. In other designs I see it connects direct. What's the best way keeping in mind original goal?
  5. DC side. I see that even the + is sometimes connected to chassis ground using capacitor. Should I make two connections to chassis ground on the DC side or should I take both (using wire) to single point connection (where earth connects to chassis)?

1 Answer 1


The basic art of the quiet SMPSU (Or pretty much anything else which oscillates!) is to keep the loop areas as small as possible, "Star" earthing is for low speed stuff where the big concern is power frequency noise, not for modern stuff where the concern is high frequency switching edges.

Low noise analogue in a switch mode powered design is actually far more of a system design issue then it is one restricted to the power supply.

I did some VERY expensive audio processing kit for broadcast and the switcher residuals were invisible, but there was no magic involved, we just:

  • Used an LC filter to clean up the ringing (Which is not related to the switching frequency, but rather to the switch and inductor parasitics).
  • Synchronised all of the 4 switchers on the board to harmonics of the sample rate, IIRC the mains down to 18V job ran at 96 kHz, and most of the on board stuff ran at 192 kHz, these were locked to the audio clock generator so that the aliasing products would be mixed down to DC at the output of the ADC...

The result was that the thing apparently had ZERO switcher spurs at the output, they were there but mixed down to DC and removed by the HPF in the DSP.

The PE ALWAYS goes to chassis, it goes directly to chassis, it does not snake around the board, and the safety earth point should be only used for the PE connection (Which should generally be as close to the input connector as possible), incidentally a filtered IEC never hurts in these things.

The class Y stuff generally wants small loop areas, and to be as small as you can get away with while still passing conducted and radiated EMC, making these too large will increase the currents flowing in these loops and potentially both increase radiated fields and increase the current flowing in the PE conductor that can be an issue if you have single ended inputs or things prone to ground loops.

On the output side you generally want the electronic doings reference connection (whatever that may mean to you) connected to chassis at ONE point, the objective is to control the common mode while keeping any currents flowing in the chassis from producing voltage drops across your precision reference plane. Often this means connecting one side of the supply output to chassis, but it depends on what the load is doing, single ended sources and loads for example may argue for something else.

  • \$\begingroup\$ Ok yes PE will go to one point to the chassis . AC line Y caps wil go to the chassis to a different point . DC ground and + DC will be tied together and go the the chassis in a different point. Did I get it right ? (of course + dc will go to the chassis using Y cap ) \$\endgroup\$
    – Johan B.
    May 19, 2018 at 10:06

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