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How does a 2 pin (no earth ground connection) SMPS offer common mode protection? Is it safe to power appliances over it?

Jaacs Pottas
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    show specs , link to device, datasheet – Tony Stewart EE75 Dec 13 '19 at 18:15
  • I am talking about a common scenario. I know that common noise filtering is effectively blocking common mode current flowing from the AC lines to earth. As there is no earth ground connection in a 2 pronged SMPS input and hence no possibility of connecting a common mode choke paired with Y capactiors, wouldn't a common mode surge take the easy path through the SMPS transformer parasitic capacitance, through the secondary circuitry possibly damaging ICs? – Jaacs Pottas Dec 13 '19 at 18:26
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    sorry, that scenario isn't as clear as you think. Even if really simple, please add a schematic. That might just clear up a lot of things, including unifying notation! A somewhat clean napkin drawing does suffice. – Marcus Müller Dec 13 '19 at 18:33

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There are two things to consider. A power supply that is unearthed won’t usually pass output noise regulations without a filter tying the output (via a noise reduction capacitor) to something like ground. Given that the noise on the output is mainly produced by internal switching noise, the choice made by designers is to use an appropriately rated capacitor from one of the output terminals to live, neutral or the DC bus voltage. Not perfect, but for small devices, good enough.

The second thing to consider is that this can produce an open circuit voltage of about 50% of the incoming AC supply voltage on both output terminals and, it’s quite possible to feel a small tingle or see a small arc if handled in such a way so as to promote said tingle or arc.

It’s not dangerous but, it has been known to damage sensitive electronics. This second outcome is because the added capacitor used as a noise reducer on the output inevitably couples low frequency AC from the mains power input to the DC output.

Not perfect I know.

Andy aka
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  • So, do you mean we cannot expect common mode rejection from such an smps? – Jaacs Pottas Dec 13 '19 at 18:44
  • You will get a level of common mode output noise rejection within the limits of the regulations they are designed to meet. – Andy aka Dec 13 '19 at 18:46
  • Fair enough! Thanks Andy – Jaacs Pottas Dec 13 '19 at 18:47
  • Jaacs , When you say appliances and these chokes are only used on SMPS or stepper motors, you can imagine induction motors that draw >600% rated current will saturate coils. – Tony Stewart EE75 Dec 13 '19 at 19:12
  • @Andyaka So do you mean to say that power supply outputs should be connected directly to ground ideally? I see debates going on about that online. But that is something we do as well in our designs and I have personally seen the same practice in many commercial SMPS including laptop chargers from HP etc. – Jaacs Pottas Dec 19 '19 at 10:29
  • Power supply outputs of this type are galvanically isolated but they will have a common-mode voltage related to the AC input. However, the energy that can be delivered from that CM AC voltage is small so won't "hurt" most things connected even if they are earthed. They don't need to be earthed and I am unable to say that they should connect to ground ideally. They are what they are and some badly designed recipients of an output from a wall wart (e.g.) may be damaged. They are not perfect. – Andy aka Dec 19 '19 at 10:36