Solving Radio Frequency Interference (RFI) Issue in Designs using Isolated DC - DC Converter 2014-01-0241

There are certain products which ask for Isolation requirement to be met. To meet this requirement, we usually make use of Isolated DC - DC converter which is an easy & compact solution. Usually this leads to severe electromagnetic interference issues which badly impact the Certification/Qualification of the product. This is due to the in-built transformer in the Isolated DC - DC converter device, which switches at high frequency, in the range of mega hertz. Considerable amount of time and money is spent in debugging and resolving these issues. This paper is intended to address these issues with solutions, which will help the designer to take care at the Design stage, thereby saving time & money, the important parameters of the project cost.

A set of experiments & analysis were conducted to find the root-cause of the issue. Some of the trials that were experimented to find the root cause is as follows:

1. 1
   Disable the Microcontroller
2. 2
   Disable the forward DC-DC converter
3. 3
   Disable the Isolated DC - DC converter and provide external DC supply.

These experiments clearly indicated that the Isolated DC - DC converter was causing the Radiated emission issue.

The last experiment led to significant result where the emissions were very much below (6 to 10bduV) the limit line. Once the source of emission was identified, various fixes were tried to reduce the emission from the product, so as to qualify the Certification for the product. These fixes will be explained in detail in the paper with emission graph plots too.

As a part of EMI filter circuit, it would be better to have provision for certain ferrite bead and capacitor in the Isolated DC - DC converter circuit on the PCB or the board during the 1^st proto phase. During the test, different values of these components can be tried out and the EMI filtering circuit can be fine tuned to attain best results. This will avoid more revisions of PCB to be built, which in turn reduces the project total cost & increase the product turn over time.