Common mode choke on flyback - Page 1
Forum section: electronics / general

author: uer166
author_rank: Super Contributor
timestamp: March 29, 2020, 04:35:23 pm
content: I'm trying to wrap my head around the usefulness of a CM choke on an isolated mains flyback converter, where the output is referenced back to mains earth. Situation is:Input: L1, L2, GND from mains at 240V, with some X/Y caps (not that the specific voltage matters)L1 and N go through a rectifier, CM choke, etc to generate DC link+ and DC link-Classic fully isolated flyback converterOutput of 48V referenced to mains GNDI feel like I don't fundamentally understand how a CM choke can tone down emissions. An example from a presentation is attached that shows an induced common mode high frequency voltage on an isolated secondary of a flyback, which makes sense that a CM choke will add CM series impedance, reducing that high frequency current. But in my case, that negative terminal is literally tied to GND, so there cannot be any induced voltage by definition. I suppose there can still be high frequency common mode current induced that would create conducted EMI on the mains GND, but that should be shunted by the Y-caps, no?Anyways, in pre-compliance testing it seems it passes conducted emissions by a 20db margin, and I'm trying to understand why, and what components are and aren't necessary.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2020, 09:40:22 pm
content: So, first of all you have this, of course. Think of the switching node as a voltage source in series with the capacitor(s).You can shunt it with a Y-cap, but that's a numbers game, and you can only afford 4.7nF or so before mains leakage is a big problem. Plus what to do with its ESL? It's a big disc on leads, it's not going to do much at 30MHz+ say.The CMC is simply giving you a CLC filter in the common mode. L1 and N, and output +48/0, are effectively shorted together at RF due to the (relatively) large capacitors between them. Reduce them to one node each, and draw a CLC lowpass filter around one or the other (or both).For extra credit, add stray capacitances to heatsinks or the like, if applicable.The goal is to minimize RF voltage between the two ports, mains and output. You can think of one as ground, the other as output, and the internal switching node(s) as input, to the CLC filter. But of course there is no input, output or ground as such, the filter is reciprocal.Here's a basic equivalent circuit:(Huh, I guess R2 should be labeled "rectifier and DC load". Whatever.) Note that the line is treated as characteristic impedance (a LISN is 50 ohms at RF) to ground. Here, the secondary/output is treated as ground, but it could be another LISN as well.There is also capacitance to ground, especially when heatsinks are used, but also just because the board, transformer, etc. take up space, and therefore have capacitance to ground as well. And even if it's not inside a grounded enclosure, there is some capacitance to free space -- which is of course even more dangerous as current to free space tends to radiate in a way that can't be dealt with by merely filtering the connections (if the harmonics go high enough, or the board is large enough).Tim

author: uer166
author_rank: Super Contributor
timestamp: March 30, 2020, 02:38:06 am
content: Quote from: T3sl4co1l on March 29, 2020, 09:40:22 pmYes! I get it now, thank you Tim.