I am using an isolated DC/DC converter to supply a MOSFET's gate drive, in a high frequency and high voltage application, so I am concerned about the EMI in my design.

In the website of the manufacturer, I could find this document that has a suggestion of external EMI filter (page 3 and shown below), but I am having some difficult to understand the suggested topology:

Manufacturer suggested topology for EMI filtering

My analysis

Usually, it is desired a low capacitance between primary and secondary of a isolated power supply. So the use of C2 sound a little bit strange for me in the first time, but I guess that it works like a shunt for the EMI, avoiding it to pass through the power supply. The use of the inductor L1 in series with C2 will create a high impedance for the conducted noise, making noise current flow very weak.

My doubts

Even if my analysis makes sense, it does not seem to enough for EMI suppression. I usually see some sort of dissipative element to “burn” the EMI energy, like resistor or ferrite bead. So, in this circuit, the EMI will only be redirected to the input side, not dissipated. In addition, even if the EMI current has low amplitude (due to the filter attenuation) the non isolated side usually has sensitive devices, so it doesn’t sounds good to redirect the high frequency noise to this side. Would not it be better if it was shunt to another part of circuit and use a bead to dissipate it energy?

Can someone clarify what I am missing in the circuit's analysis?


2 Answers 2


The cap provides a return current path for leakage capacitance from primary to secondary side. L1 is not involved in this. This current will flow one way or another whether the cap is installed or not. Installing the cap allows you to reduce the loop area drastically from what it could potentially be without the cap installed.

Anand ReghunathanKoteshwar Rao,Anant Kamath, EDN article

To quote Rohm semi

"Y-capacitor added between primary and secondary sides; A capacitor called a Y-capacitor is added between ground on the primary side and secondary side. This is one typical method for reducing the common mode noise caused in the secondary side by primary side switching noise via the capacitance across the windings of an isolation transformer. The voltage rating of the Y-capacitor must be equal to the insulation voltage of the transformer"

If you want additional EMI mitigation you can install common mode chokes on the input and output cables. Here is more information in the EDN article that the picture comes from.

How to achieve low radiated emissions with fully integrated data and power isolation

  • \$\begingroup\$ Thanks for your answer and the good reference paper. As far as I understood, the capacitor C2 is there just for reduced the radiated noise generated by the proper power supply. So, it is not there to protect it against the noise created in the rest of the circuit, like in the fast switch of my high voltage MOSFETs, right? My original ideia for the filter was to block any CMM currents coming from the secondary side, avoiding it to cause interfere in the power supply and the circuitry in the primary side... \$\endgroup\$ Apr 17, 2018 at 12:50
  • \$\begingroup\$ Usually some form of damping/clamping network is applied directly at the switch/MOSFET using various combinations of resistors, capacitors and/or diodes. Looking at PoE reference designs provided by manufacturers of PoE IC's is a good place to pick up EMI ideas. I know Texas Instruments has a few App Notes floating around. \$\endgroup\$
    – lucky bot
    Apr 18, 2018 at 4:38

Great question will start with little theory first

Theory First

EMI is of two forms Conducted and Radiated

Conducted interference is further classified into Common mode (CM) and Differential Mode (DM) Noise.

Radiated noise comes usually in telecommunication frequencies which can be suppressed by a EMI shield eg PCB of mobile phones

EMI shield for Radiated noise

Will focus on CM & DM

Common Mode EMI Filter

Common Mode noise flows in same direction on both power conductor (via Positive and Negative) and returns via Earth (Physical earth).

This CM noise can be supressed by adding inductor in series with the line (In our case its same CM Noise.

Further it can be reduced by adding Y-Capacitor, which is connected between Positive to physical earth and negative to Physical earth (PE)

Example topology of CM Noise filter. Note: There are different topologies available

enter image description here

Differential Mode EMI Filter

Differential mode noise is one which flows through one line returns via another line (Flows through positive and return via negative.

This can be eliminated by adding DM choke and X-cap. X- cap connects between two line.

example of Differential mode filter

Differential Mode noise filter

Usually DM noise filter will provide some extend of protection towards CM noise due to leakage inductance of common mode choke (CMC)

The one shown here is CM noise filter which is suggested by manufacturer and usually manufacturer will suggest the values for filter components. If not its little tricky to calculate those values of Inductor, CMC, X,Y-Cap.

Inaddition to that people generally uses different stages of filter. One shown above are single stage filter, it can be cascaded two stage filter, three stage filter. Single stage filter is usually commonly seen.

The concept behind multiple stage filter is to achieve higher noise immunity. Each stage will be tuned to suppress particular band of noise.

Important: Please make sure that you are choosing right voltage rated and type of capacitor.

The capacitors shown above are Film capacitor (usually called as box cap) which will be rated for higher voltage in the range of Kilo Volts, and its also risky to use these if there is any fault it will directly bring line to physical case (PE)


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