EMI Filter Design

[iraquois]

Hello.
I am starting to design an EMC filter to filter out noisy components like fans, motors and switch mode power supplies. The EMI filter I design is for general purpose seperate module.

I searched and I found there are almost no resources about common mode filter selection. It seemed to me they choose it by experimentally. Choke values are between 1mH - 10mH.

Where should I start to design or should I first learn low pass filter design etc. ?

[bob91343]

It's a complex subject.  You need to learn about circuit synthesis, Laplace transforms, and a lot more.  There are some cookbook approaches but they are very limited.  There are constant-k and m-derived methods with different sets of parameters.  Lattice and pi and t configurations.  Not to speak of Sallen-Key methods.

What many do is use an empirical method, cut and try.  Pick a configuration and try different values and see what you get.  When you do that you may be missing important issues.

Also the quality of the components comes into play, Q and self resonance and ESL and distributed parameters.

[Terry Bites]

Common mode filters are simple and effective:see https://www.coilcraft.com/getmedia/9c231e30-04a2-4463-b679-c38b99b2669e/doc191_CMFiltDesign.pdf and https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjtmtiX7a3uAhUsXhUIHS8iDs0QFjAAegQIBhAC&url=https%3A%2F
or
%2Fwww.schaffner.com%2Ffileadmin%2Fmedia%2Fdownloads%2Fapplication_note%2FSchaffner_AN_RB_common_chockes.pdf&usg=AOvVaw01QUHCWDUWCG0wdssrDElQ

What currents and voltages are you working with?
AC and DC design requirements are somewhat different.
What are your frequencies of interest
How much leakage current can you allow.
Correct mains rated capacitors will be needed on AC 230V 

You don't really need all the complex maths, just bit of basic algebra. You can get into the nitty gritty if you want, but you don't need to fully understand it all to get the job done. See https://www.coilcraft.com/en-us/tools/common-mode-choke-finder/#/

[awallin]

IIRC Murata has some quite good app notes, try searching around: https://www.murata.com/en-global/products/emc/emifil/knowhow/basic?intcid5=com_xxx_xxx_cmn_bc_xxx

[Kerlin]

Your going about it the wrong way, you'll go broke it costs too much.
Haven't you ever noticed the huge amount of equipment manufactured "over there" that would obviously never meet requirements, ask any radio user or Ham radio operator.
They have their own accredited test labs, if you go about it the right way they can be VERY helpful with compliance, use them.
These stringent EMC requirements are only enforced by local authorities on local companies while non compliant stuff pours in to countries by the billions of items.
It can be in the interests of some local test labs to string it out as long as possible.
I did meet requirements on many items, at huge expense, but later after I lived and worked in the industry "over there" I understood.
I was invited to work there by one of the companies I used to import from, after my company went broke due to this crap, as they thought I was a good engineer who always got things finished.
Then companies from my home land used to come over to buy and import my designs, guess who won and who didn't from my skills obtained in my home country.
The company I worked for over there then also trained me in their neat methods of sales, then used to get me to help in sales by negotiating the trading price and terms with buyers.
Some of whom I knew and had worked for, imagine their face when they came over and found they had to negotiate with me.

[S. Petrukhin]

Your going about it the wrong way, you'll go broke it costs too much.
Haven't you ever noticed the huge amount of equipment manufactured "over there" that would obviously never meet requirements, ask any radio user or Ham radio operator.
They have their own accredited test labs, if you go about it the right way they can be VERY helpful with compliance, use them.
These stringent EMC requirements are only enforced by local authorities on local companies while non compliant stuff pours in to countries by the billions of items.
It can be in the interests of some local test labs to string it out as long as possible.
I did meet requirements on many items, at huge expense, but later after I lived and worked in the industry "over there" I understood.
I was invited to work there by one of the companies I used to import from, after my company went broke due to this crap, as they thought I was a good engineer who always got things finished.
Then companies from my home land used to come over to buy and import my designs, guess who won and who didn't from my skills obtained in my home country.
The company I worked for over there then also trained me in their neat methods of sales, then used to get me to help in sales by negotiating the trading price and terms with buyers.
Some of whom I knew and had worked for, imagine their face when they came over and found they had to negotiate with me.

And what's this about filters? And what's the idea? Don't make filters, but learn how to sell properly? 

[Bud]

S.P. can you try not to quote the entire posts, this is unnecessary. Thanks.

[Kerlin]

The answer to his question is already supplied.
Gets 0% for reading comprehension.

[ocset]

The equipments that you are filtering shoudl have their own internal filters.
So to me, it sounds liek your company just kind of want some gerneral emc filtering, just in case noise issues arise in the future?
So buy up some cheap EMC filter modules, and stick them on your supply, and see how it goes.
https://www.schaffner.com/products/emcemi/
Many offtheshelf emc filter modules even give you the circuit inside them.....so you can build it yourself or modify it.

[iraquois]

Thank you all for answers. Sorry for delay.

Nice story thank you Kerlin. Actually we have a big EMI/EMC lab and I will try my chokes and filters over there. I can acheive good results by trial and error. But this is not my intention. I want to learn how are they calculating or are they calculating anything at all? Or everything is installed on trial and error and simulations?

It seemed to me no one is sure of it . I will look for resources (thank you all) and come back here to update for any developments.

[Terry Bites]

Er, you have access to an EMC lab? There must be some one you can pester there!