Good Morning.

Please I need help to know how to design a common mode filter.
I don't know how to proceed.

The mechanical chassis and the ground reference are isolated.
I have DCDC which have a decoupling frequency of 500Khz.
I have previously design a differential mode filter with a cut-off frequency of 12KHz.

I have to respect the current limit constraint of :
- 80 dBµA up to 100KHz
- Decrease of 20dB by decade from 100KHz tto 10MHz
- 40 dBµA from 10MHz and upper

I have read that a two order filter with a commode mode filter and capacitors connected to the mechanical chassis should be enough.
But how to select both component values ? How to simulate thanks to LTspice (attached file) ?

Thanks per advance for your help !

 

A first order filter has a roll off of 20dB per decade.

 

A first order filter has a roll off of 20dB per decade.

Correct @MrChips, a first order should be enough.
But I want to use the classical couple Common Mode Filter + C connected to the mechanical chassis.
Thanks per advance.

 

The most important components in the simulation will be the parasitic inductance of the capacitors, the parasitic capacitance of the inductor, the resistance of the inductor and the leakage inductance. Do you know any of these values? If not, anything above 1MHz is complete guesswork.
At what frequency is the interference that you need to remove?

 

@lan0 : Thanks.
I haven't defined any components (capacitor, inductor) at this moment.
The only thing I have designed is the differential filter (cut off at 12KHz) and the DCDC (work at 500KHz, current consumption 1,3A max).
I don't know how to proceed. I think I must use the simulation for setting the component, correct ?

 

Welcome to AAC.

This sounds like an academic exercise. Are you a student receiving academic credit for this design?

 

Welcome to AAC.

This sounds like an academic exercise. Are you a student receiving academic credit for this design?

Hi Ya'akov.

Not really... Just a designer with no experience.

 

Is this schematic correct to estimate common mode filter ?

Thanks per advance for your help.

 

View attachment 355663

Is this schematic correct to estimate common mode filter ?

Thanks per advance for your help.

In a perfect world, with perfect components it might be, but for real-life components it will be completely hopeless.