I've a full bridge dc-dc converter and I'm trying to understand it. It has 48V input and 400V output with 1600W rating, 30kHz switching frequency.

At the input of this SMPS there's a filter additional to SMPS input filter. I gues it's a EMI/EMC filter but I've no idea on them, I don't know much about filters.


simulate this circuit – Schematic created using CircuitLab

How can I design one of these?

I couldn't find a suitable source since I don't know its name and purpose exactly.

  • \$\begingroup\$ Looks like a common mode filter / choke and smoothing/decoupling capacitors electronicdesign.com/power-management/article/21807788/… - Notice the different values and most likely sizes of the caps - check this out micro.rohm.com/en/techweb/knowledge/emc/s-emc/03-s-emc/7669 \$\endgroup\$
    – Sorenp
    Commented Dec 18, 2019 at 10:14
  • \$\begingroup\$ How can I design one of this? start with a specification of what you want in terms of attenuation across the frequency bands of interest to you. However, if you have no idea about those filters and you don't know much about filters then what is driving you to want to design one? \$\endgroup\$
    – Andy aka
    Commented Dec 18, 2019 at 10:18
  • \$\begingroup\$ @Andyaka I'm trying to build a full bridge dc-dc converter also, but I've never designed such a filter like this before. \$\endgroup\$
    – Das D.
    Commented Dec 18, 2019 at 10:21
  • 2
    \$\begingroup\$ Incidentally, there is nothing in the circuit that says this is a common mode filter. I would say it's a differential filter because there is no magnetic coupling indicated between L1 and L2 (they are also called L1 and L2 rather than L1a and L1b) AND importantly, there are no X or Y capacitors to earth indicated. The answer you have selected may well be incorrect. \$\endgroup\$
    – Andy aka
    Commented Dec 18, 2019 at 11:40
  • \$\begingroup\$ Upon further reading I'll agree with @Andyaka Nicely spotted \$\endgroup\$
    – Sorenp
    Commented Dec 18, 2019 at 12:47

1 Answer 1


As others have said, this is a common mode choke, or filter. It's job is to attenuate noise coming back out of the SMPS and prevent it from getting onto your input power lines, 48V in this case.

You need two key pieces of information to design such a filter. First, you need to know your conducted emissions requirement. This comes from your customer or maybe from a government or industry document, and will specify the amount of "noise" allowed as a function of frequency. This is the easy part.

Second, you need to know the amount of noise created by your SMPS and put onto the input lines. It usually takes a detailed analysis of your power system to do that. This is the hard part.

Once you have those two pieces of information, you can determine how much attenuation your filter has to provide as a function of frequency.

A third component to this is the amount of current your supply draws from the 48V source. A 1 amp filter is a lot easier to realize (smaller components, particularly magnetics) than is a 30 A filter.

You've got a pretty sporty filter there, with a 30 KHz switching frequency and 1600 W (33.3 A) of power draw.

To clarify the comments on common mode filters, this is a type of schematic symbol, or some variant thereof, that we use for a common mode choke.

enter image description here

  • \$\begingroup\$ I don't think it's a common mode filter for the reasons I gave in comments under the main question. \$\endgroup\$
    – Andy aka
    Commented Dec 18, 2019 at 11:41
  • \$\begingroup\$ You may be right. Maybe a better term would be "Input Filter". \$\endgroup\$
    – SteveSh
    Commented Dec 18, 2019 at 11:47
  • 1
    \$\begingroup\$ The red note under the schematic states that Box 1 is differential, Box 2 is CM and Box 3 is SMPS input filter. Everyone wins! \$\endgroup\$
    – Stiddily
    Commented Dec 18, 2019 at 12:56
  • 2
    \$\begingroup\$ @Stiddily don't be so hasty in assuming that inductors sharing a common core must make it a common mode choke. This is only the case when dot notation clearly indicates this. If the dots were at opposite ends then it becomes a differential choke. \$\endgroup\$
    – Andy aka
    Commented Dec 18, 2019 at 14:17
  • \$\begingroup\$ Very true, my mistake. \$\endgroup\$
    – Stiddily
    Commented Dec 19, 2019 at 12:32

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