So... Line filter caps can handle huge transients and fail safe (don't shock, catch fire or explode). Class X1 capacitors operate safely up to 4kV spikes. (MPP cap between L and N)
However polypropylene caps are bulky compared to ceramic. :(
Can a SMD ceramic in parallel with a SMD TVS be used instead of an MPP cap? The capacitor handles the noise. The TVS protects the capacitor. Both combined are smaller than a 0.1uF MPP X1 cap. (Canada ~120VAC)
X capacitors are not stressed by hipot, as they are not applied across a safety isolation barrier.
Y1 capacitors may be stressed in this test, as they are suited to the application. There are generally provisions in the test standards for their removal, for the duration of the test, if this is an issue. It shouldn't be necessary if DC hipot methods are used, or if the ratios of input and output capacitive coupling are properly tailored to distribute test stress in a suitable manor during input-to-output stress testing. Consult the standard that you're designing for.
X2 capacitors are generally smaller than X1 types and are generally accepted for use only after a protective element, like an input line fuse. Ceramic capacitors with the X safety rating are difficult to locate above .01uF. Parts carrying the designation are theoretically capable of substitution with other parts with the same designation, provided that the product's 'listed' constructional information indicates that the substitution was anticipated and correctly evaluated/tested in certification.
A capacitor will be an odd fish if it carries both an IEC X1 or X2 designation and a label rating for 120VAC. You'll generate less grey hair in your purchasing department by sticking to generic parts suitable to a realistic world market.
TVS will only function effectively across the line when operating with a suitable source impedance.These are generally ineffective across the line unless isolated by at least one CM/DM filter stage. The test source impedance is practically defined by the relevent surge standards, which specify varying shape/duration test waveforms expected to be applied for varying degrees of demonstrated immunity. See IEC-61000-4-04 and -05.
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