I’ve been looking through a reference designs for an off-line flyback converters (this one by Texas Instruments and this one by Linear Technology). Both of them have a Y-capacitor between primary and secondary for EMI suppression. Like this, for example: enter image description here
from Fig.2 here

So far, so good.

Common UL standards (UL8750, UL1310) require dielectric strength between primary and secondary of 1kVAC plus twice AC line voltage. That adds up-to about 1.5kVAC.
In these reference designs, the Y-capacitors in question are rated for 500VAC (datasheet) and 250VAC (datasheet). Doesn’t this undermine the UL requirement for the dielectric strength between primary and secondary?

One of these capacitors has this paragraph in the datasheet:

Dielectric strength between leads
Component test: 4000 VAC, 50 Hz, 2 s
As repeated test admissible only once with: 3600 VAC, 50 Hz, 2 s
Random sampling test (destructive test): 4000 VAC, 50 Hz, 60 s

Much higher voltage ratings. Does that take care of dielectric strength required by UL standards?

Interestingly, I also couldn’t find Y-capacitors with 1.5kVAC to 2kVAC rating (on DigiKey and Mouser).

  • \$\begingroup\$ Neat schematic design! \$\endgroup\$ Feb 1, 2013 at 9:34

3 Answers 3


Usually the working voltage between primary and secondary is on the order of a few hundred volts when you're dealing with mains-powered (100-240VAC) equipment. A 500VAC-rated UL recognized cap bridging primary-to-secondary is certainly suitable for this application. 250VAC may be a little on the low side.

Don't expect that just because there's a cap in a reference design publication, the design has ever been evaluated for safety compliance. Quite often, they're not, and the onus is on you to make the design 'safe'.

(Much more common configurations that I've seen are two 250VAC Y-caps in series, or a 250VAC Y-cap in series with a 1kV ceramic.)

UL will care about the part not exceeding its working voltage rating under normal conditions in the application. The dielectric strength test is always much higher than the working voltage rating to account for those 'little things' that can happen in the real world like component failures, lightning strikes, surges, etc. - it doesn't mean that you can and should use a 250VAC cap where the working voltage is 350VAC, for example, even if the part will never fail due to its high dielectric withstand rating.

  • \$\begingroup\$ Thanks. I'll add pads for a 1kV ceramic in series with the Y-cap in question. \$\endgroup\$ Feb 7, 2013 at 18:47

Have a look at Table 2 in the link, I believe you can find a Y2 capacitor suitable. (A point of pain is that some times the manufactureres datasheets don't always match the UL ratings for components..)

Reference designs are to get you started, but if your market requirements are different it is up to the designer to find the suitable component.

FOWX2.GuideInfo Across-the-line Capacitors, Antenna-coupling Components, Line-bypass Components and Fixed Capacitors for Use in Electronic Equipment

enter image description here


Following a preference from Dave at www.eevblog.com for Panasonic caps in some video of his... I had a look at Panasonic (E62674) Y2

enter image description here

Edit 2

I believe if the UL site quotes a voltage for a component, this is the working voltage it is rated for. My understanding is the withstand voltage has been taken care of.


If a 2nF capacitor has a working voltage of 500V, that means its voltage rise by roughly one volt for every two nanocoulombs, until it reaches 500 volts. Applying voltages higher than that won't destroy the part, but might not hold as much charge at higher voltages. With some caps, the amount of charge that's let in will decrease with voltage; with others, the amount of charge let in may increase, but that increase would be offset by an increase in the amount of charge/energy lost as heat.

For purposes such as RF suppression, it may not really matter if the capacitor momentarily accepts less charge than it should during a high-voltage spike on the line, if the capacitor resumes normal operation as soon as the spike is gone. Further, RF-suppression caps are often sufficiently small that even if they were to waste a significant fraction of the energy that goes into them, the amount of energy in question is too small to pose any significant danger of overheating.


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