Electromagnetic Compatibility - 4

EMI/EMC tests are always a nightmares to electrical engineers. While there have been lot of design notes available on this topic, none address the requirements at your system level. It is the system designer who himself have to think ahead and integrate things in his design and come up with robust design to face the real world challenges. One among the EMI/EMC tests is the Conducted Emissions test.

Conducted Emission arise in the system due to the common mode noise travelling back to the power supply input from the Equipment under test (EUT) and is radiated from there. The band which is more affected from such radiation is 150KHz to 30MHz. This is the band where the fabrication houses perform Conducted  Emission tests. The EMI/EMC standards define allowable limits for this emissions and for example IEC (Ex: EN 61000-6-3) standard, EN55022 standard and others define Quasi-Peak and Average limit for this tests. The point to remember here is that Conducted emission could be from external interface ports as well. The emission limits defined by these standards are classified into two classes Class A and class B. 

Setup for the Conducted Emissions Testing:

Few points which help reduce the Conducted Emissions:

1. Common Mode filters and Differential Mode filters (called EMI filters) on the power lines. The major problem is that common mode noise generation cannot be stopped. When we say they cannot be stopped, it is mainly due to the operating requirements of the load connected to the power supply. Designer can only include appropriate components like chokes to eliminate the common mode noise.

2. In the PCB design, ensure that the power and ground planes have as low impedance as possible. 

3. Avoid having larger loops on the PCB for ground. This may be re-iteration of the above point but it stresses the need to ensure that there should not be any current lopps on the PCB which can cause radiation and coupling.

4. A good practice is to filter the power supply lines near the respective devices on the PCB.

While our electrical circuit designs have to ensure that the source of conducted emissions are forecast  and appropriate design completed, it is the PCB Layout that plays a crucial rule. PCB Design Engineer and Electrical Engineer have to work hand in hand through out the PCB Layout phase. Post Layout and Pre Layout Simulations are always recommended to ensure that there are no unnecessary coupling paths on the PCB.