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Y-capacitors between primary and secondary are often used to reduce common mode noise in flyback SMPS, as explained also here.

I have read that two Y-capacitors in series are needed to be compliant with the safety standards regarding home appliances (60335). This is stated in the application note of some SMPS modules: https://www.cui.com/product/resource/pbo-5c.pdf and https://www.digikey.com/en/articles/how-to-easily-optimize-ac-dc-converters-to-meet-a-wide-range-of-emc-requirements.

Which requirement or clause of the 60335 standard are these application notes referring to? Does it relates to the protective impedance? Specifically, protective impedance shall be implemented with two components, in case one fails? If my device is rated for 4kV impulse voltage (OCIII), can I use two Y2 class capacitors, instead of two Y1 caps across the insulation barrier? My answer would be yes, because two Y2 capacitors in series would fulfill the 3kV dielectric withstand requirement.

Any hints or comments are welcome.

Umberto D.
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    In most hi-volume commercial designs that I have seen, one single Y1 capacitor was always used alone between the primary and the secondary in universal mains applications, e.g. notebook chargers. In cheaper designs, two Y2-caps were connected in series to meet the same maximum peak voltage. See this [document](https://www.ttieurope.com/content/dam/tti-europe/manufacturers/kemet/resources/Kemet%20Safety%20Capacitors%20Overview.pdf) for more information on these safety caps. – Verbal Kint Apr 09 '23 at 20:35

1 Answers1

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Subcl 22.42, meeting 9, agenda item 7.9: A single Y1 capacitor is not allowed as a protective impedance.

Subcl 3.3.6, 22.7 and 22.??, meeting 23 agenda item 7.1: If the safety extra low voltage can be accessible it should be required to comply with the requirements of double insulation (cl. 29) and cl. 22.42, protective impedance (two Y capacitors).

And cl 22.42 says protective impedance must be made of two separate components if one fails, so that accessible parts stay within safe limits.

Justme
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  • That’s the wording but sadly doesn’t explain the reasoning behind it. I’d speculate it’s so that there isn’t a single point of failure, although it’s not obvious that a cap could be guaranteed to fail to open or short circuit. – Frog Apr 09 '23 at 20:24
  • However, I would assume that 2x Y2 capacitors rated for basic insulation in series can be used to maintain reinforced insulation. Am I wrong? – Umberto D. Apr 11 '23 at 06:32