# How many filters do I need for a battery charger?

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##### Member
Hello again,

I have some concerns about how many EMI-filters that I should use for my AC/DC charger?
My charger is operating as a low-power charing (950 W).

I have read in in different articles, that some charger have only one input filter, and in some other cases they also have a output filter.
I have studied so much about EMI, since I have not taken regard to that, but for my case. What decides the number of EMI-filter?

https://uk.tdk-lambda.com/faq/question.aspx?id=99
https://www.fairchildsemi.com/techn...gnetic-Interference-EMI-in-Power-Supplies.pdf

Apparently, there is also different kind of EMI filter, in forms of T-filter, Pi-filter and L-filter. I am not sure what to choose here either...

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#### ChrisP58

##### Well-Known Member
There is no set number.

It will be whatever is needed to attenuate the emissions from your actual design to below the acceptable limit of the certification that you're trying to pass.

#### MikeMl

##### Well-Known Member
More than you would be willing to put on it if you were trying to sell it to me. I have a rule: no #$%^%$* switching power supplies of any kind allowed at my place!

#### ronsimpson

##### Well-Known Member
no #$%^%$* switching power supplies of any kind allowed at my place!
How is your computer powered? Steam? Hydro?
You probably have a rotary phone so no switchers there.

##### Member
There is no set number.

It will be whatever is needed to attenuate the emissions from your actual design to below the acceptable limit of the certification that you're trying to pass
I understand, but I need some reference for this..cause I writing a report. I have a THD of 12 %. Should it be enough maybe to have one EMI input filter and one EMI output filter of the charger?

--edit--
The charger have a switching frequency between 150 - 200 kHz.

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#### ChrisP58

##### Well-Known Member
I understand, but I need some reference for this..cause I writing a report. I have a THD of 12 %. Should it be enough maybe to have one EMI input filter and one EMI output filter of the charger?

--edit--
The charger have a switching frequency between 150 - 200 kHz.
I assume that your THD is measured at the AC line frequency (50/60 Hz.) While the EMI frequency of interest starts at about 150KHz and runs up to 30MHz or more. And includes differential mode as well as common mode noise.

Not only are the frequency range very different, but the energy in the in the error massively different. ie. Trying to reduce THD with passive filters would require very large, heavy, iron core inductors. Active PFC circuitry is a much smaller and more efficient way to reduce THD, but generates more EMI.

Your charger is running at 150 - 200KHz. Your PFC is running at ??KHz. Both of those switching frequencies, plus their harmonics, plus the ringing energy from various sources, including PCB trace inductances, will be reflected to the input.

You really won't know how much is going to be there until you build the board and actually measure it. Then you study the noise spectrum and magnitude, and create a custom attenuation system to meet the needs of that charger wit reference to the emission limits of the certification standard.

##### Member
I assume that your THD is measured at the AC line frequency (50/60 Hz.)
Yes, it is true. I measured at the input in the simulation (LTspice).

Your charger is running at 150 - 200KHz. Your PFC is running at ??KHz. Both of those switching frequencies, plus their harmonics, plus the ringing energy from various sources, including PCB trace inductances, will be reflected to the input.
I am using an active PFC, and the PFC is using 150 kHz in the AC/DC converter . and The DC/DC converter is having 200 kHz.

You really won't know how much is going to be there until you build the board and actually measure it. Then you study the noise spectrum and magnitude, and create a custom attenuation system to meet the needs of that charger wit reference to the emission limits of the certification standard.
Okay, since I am not gonna build anything, I guess it is not possible to measure accurate as you said. But do you think it is possible to estimate the number of filters that are needed somehow? I asking cause I need to consider their weight to the charger. Otherwise, if not possible, I can not consider their weight.

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