# L1 and C1 value for EMI filter design ?

1. ## L1 and C1 value for EMI filter design ?

Dear Members,
What's the value for C1 and L1 for EMI filter, if I use 220V 50Hz ?
Thanks

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2. ## Re: L1 and C1 value for EMI filter design ?

The capacitor Farad value can not be too high of course, lest it pass too much current and destroy itself.
Probably 1/10 Watt is reasonable. Calculate a Farad value associated with that amount of power at 220V 50 Hz.

3. ## Re: L1 and C1 value for EMI filter design ?

It depends. On the interference frequencies to be filtered, the device power, etc.

C1 47nF up to several ĩF
L1 is sketched as a common mode choke, common mode inductance 100 ĩH - several 10 mH, differential mode inductance one to two orders of magnitude lower.

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4. ## Re: L1 and C1 value for EMI filter design ?

The capacitor Farad value can not be too high of course, lest it pass too much current and destroy itself.
Probably 1/10 Watt is reasonable. Calculate a Farad value associated with that amount of power at 220V 50 Hz.
What's the formula ?
I'm thinking 4.7uF VAC 250VAC X2 for C1, and L1 = ? thanks

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5. ## Re: L1 and C1 value for EMI filter design ?

Why 4.7 uF, why not 47 nF or 470 nF? You didn't yet specify any application parameters.

6. ## Re: L1 and C1 value for EMI filter design ?

Originally Posted by FvM
Why 4.7 uF, why not 47 nF or 470 nF? You didn't yet specify any application parameters.
if it's 470nF what's the formula for L1 ? application parameters ?
do you mean input voltage and frequency?
220V 50Hz

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Originally Posted by FvM
It depends. On the interference frequencies to be filtered, the device power, etc.

C1 47nF up to several ĩF
L1 is sketched as a common mode choke, common mode inductance 100 ĩH - several 10 mH, differential mode inductance one to two orders of magnitude lower.
Device power up to 20W

7. ## Re: L1 and C1 value for EMI filter design ?

L1 is a common-mode choke and used for filter out common-mode noise. It does very effect for the differential mode noise. C1 is used to filter out differential mode noise, while almost no effect for the common-mode noise. So C1 and L1 could be determined separately based on requirement for common-mode/differential noise rejection. Noise rejection is not purely depending on the capacitor/inductor values, it is also a function of the load/source impedance. The engineer's solution is usually try different values until the noise level is below the EMI specification.

Your circuit could be improved by adding small capacitors (100pF-1nF, depending on the safety requirement) from each terminal of L1 to chassis ground. Those capacitors will help to reject both common-mode and differential mode noises.

8. ## Re: L1 and C1 value for EMI filter design ?

As explained by cariban, C1 and L1 aren't directly related, respectively there's no formula linking the values.

For L1, I would use a low current common mode choke like this https://www.we-online.de/katalog/dat.../744862250.pdf

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9. ## Re: L1 and C1 value for EMI filter design ?

There's no simple values to use, EMI is not that easy. Filter theory doesn't always work here, I use a spectrum analyzer and current probe to measure the emissions. The EMI standard being designed to also dictates the filtering needed, as well as the SMPS switching frequency. A 66kHz switcher compared to 250kHz makes a big difference for the LF corner frequency coverage.

The behaviour of the CM choke and X-cap at RF frequencies is unpredictable. I find the choke's core material and the film cap's inductance to sometimes give worse filtering results despite using larger values.
If you line up the EMI filter's resonance with a spur from the SMPS harmonics, that can also make things turn out unexpectedly good or bad.

10. ## Re: L1 and C1 value for EMI filter design ?

if you want a general idea, (theres no real formulae for it), then think of your power level, ...says its a 60w flyback...then check out app notes for 60w flybacks and see what they are using....or do it in the power integrations software and see what they use, after you type in your spec.

Also, if you are using a huge electrolytic cap after the bridge rect, then the Diff mode X2 cap can likely be smaller than otherwise.

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