[SOLVED] DCDC converter common mode filter

[ctzof]

Hallo,

Maybe this is a simple question but I want to be sure. I want to design a common mode filter for the input stage of a DCDC converter (200w input). I follow this application **broken link removed** where they suggest the filter in figure 1. The input of The converter is a dc voltage which means that VIN- is ground in hat case. Then what I should do with the capacitor C_in4. The upper leg is connected to gnd so for the lower leg I have 3 options

1. Connected to gnd
2. Connected only to the lower right leg of the common mode choke
3. forget about C_in3 and C_in4 and place a capacitor with a value which is double the value of C_in4.

Which of them is the correct?

 

[crutschow]

Not sure what you mean.
The DC-DC converter input ground should not be connected directly to the Vin DC ground, it must go through the common-mode choke as shown.
Otherwise the common-mode choke won't work.

 

[ctzof]

For example this is my schematic. My input is DC 24V and there is a two stage cm filter between the input and the dcdc converter (VCC24V). My question has to do with capacitors C10 and C11, should I put a ground to the pin that is connected to common mode choke or the schematic is already ok?

 

[crutschow]

There can be only one connection to chassis ground (other than for the caps at the choke outputs).
Otherwise the common-mode chokes won't work since the chassis connection bypasses the bottom connection through the chokes.

 

[ctzof]

There can be only one connection to chassis ground (other than for the caps at the choke outputs).
Otherwise the common-mode chokes won't work since the chassis connection bypasses the bottom connection through the chokes.

I don't really understand what you mean. What I really mean by the second and third alternative can be seen in the following pictures. Is any of them correct? Can you propose anything based on this schematic.

 

[crutschow]

It's simple. For a common-mode filter to work, both the top supply and bottom return currents have to go through both filter connections.
If you have ground connections at the common-mode filter outputs then the bottom currents will go through the ground connections and not through the filter, negating the common-mode filtering effect.

So for the common-mode filters to work you have to remove all the ground connections to the right of the left-most filter.

Make sense?

 

[ctzof]

Do you mean something like that

 

[bking]

Like this. The GND connection on the far right is NOT the same as the other grounds. This is your new, filtered ground for all of the other circuitry using the VCC24V.

 

[ctzof]

Like this. The GND connection on the far right is NOT the same as the other grounds. This is your new, filtered ground for all of the other circuitry using the VCC24V.

View attachment 114826

Thanks that is what I was thinking also. I've made some simulations with LTSpice and they work just right.