It turns out I had mistakenly interpreted the role of the capacitor in question here as an EMI filter for line noise. It looks like a capacitor in that position actually serves as a snubber for high-frequency rectifier switching spikes/noise. Although perhaps unlikely to be conducted to later stages of the circuit (being contained by the reverse-biased rectifier diode), without snubbing that high-frequency noise might be radiated.
I'm specifying an EMI bypass cap for placement on the secondary winding of a line transformer. It feeds a simple linear supply that acts as the bias supply for a DC Lab Power Supply I'm designing as a learning project, (C5 in the schematic below):
I'm pretty sure the right answer is a roughly 0.1uF 100V (maybe 250V) film capacitor because that's what I've seen used in other circuits. But I want to understand how to go through the design process myself, even if I end up with the same answer in this case.
Here's what I have so far:
The capacitor shouldn't be bigger than necessary, because it will shunt a significant level of the wanted 60Hz AC current. For example, a 1uF capacitor would shunt about 25mA RMS.
The frequency response of the cap needs to be good so it gets rid of high-frequency noise well into the MHz range.
If the capacitor is too small, it might not bypass lower frequency noise very well as its impedance at a given frequency is inversely related to its capacitance (C).
Here's what I don't know:
The practical range of EMI frequencies we'd be looking to bypass in this sort of situation. I'm not sure what's typically around, and perhaps not all of that gets through the transformer because of its natural inductance or something.
Whether the working voltage of the cap needs to be bumped up to accommodate possible high-ish voltage transients that might get through.
Why a plain-old ceramic cap wouldn't get it done just fine.
Can someone help me understand the design space for this component?