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I read this question and still have some confusion about it.

If Y capacitor suppresses EMI and we know that the \$ X_c=\frac{1}{2 \pi f C}\$ this means to me that for higher frequencies the capacitive reactance becomes lower. That is to say the Xc becomes higher for lower frequencies.

The EMI here is created by the fast switching of transformers.

How can we simulate this and see this EMI creation and capacitor suppression by simple signal generator, a transformer, a capacitor and maybe with some resistors in SPICE? Can you provide a simplistic diagram? A simple circuit where one can see the effect of the Y capacitor's exitance.

Voltage Spike
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user1245
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  • I know for example switching an inductor causes spikes but thats normally handeled by a flyback diode not a cap. So I would love to see what cap really does to the transformer's voltage in a sim. – user1245 Jan 27 '18 at 16:19
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    Simulating differential-mode currents (DM) effects is quite simple if you know the converter input current signature. However, Y-caps are there to offer a circulating path to common-mode currents (CM) which are linked to pcb routing, switching components interaction, transformer inter-winding capacitance and so on. Therefore, it is extremely complicated to simulate and predict how much of CM noise currents will flow in these caps. Nothing replaces hardware for EMI measurements and efficient SPICE setups for CM prediction do not exist I'm afraid. – Verbal Kint Jan 27 '18 at 16:47

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