Electromagnetic Compatibility - 2

EMI caused on the board can be radiated as well as conducted. The major reasons are power density, faster switching, higher currents. EMI basically occurs because of unwanted coupling of signals that happen in circuits.

In digital circuits, the interference can be on-board. In these cases we provide isolation between each section in the board to eliminate interference. But if the interference is from external source, we have to use mechanism like EMI filter. The noise is dominant in the higher frequencies and hence a low pass filter can also be used to shutout the noise completely. Other than this as discussed, magnetic component like inductor can also be used. To eliminate noise completely and to handle differential and common mode noise, EMI filter is the best. EMI filter is generally used to eliminate low frequency conducted noise in the circuit where as we use techniques like shielding to eliminate radiated high frequency emissions.

We have to understand that the main cause of conducted emission and hence conducted interference comes from transients, ripple that are generated out of common power supplies. The noise couples over the power path. As we talk about the transients here, which meant sudden change, we can come to a conclusion that we have to use magnetic devices that doesn’t allow sudden change in current.

There are various regulations that define the acceptance levels of this noise. The noise here can be classified into natural and artificial. ESD, lightning come under natural noise sources. The noise emitted from electronic devices come under artificial noise. The conductive emission that we are talking here is an artificial noise.

Natural noise sources - Surge voltages from lighting

Artificial noise sources - switching transients, crosstalk in transmission lines, transients in power supply lines

EMI filter is one of the commonly used passive device for protection against the interference that is generated externally as well as within the device. EMI filter protects the device from conducted interference. We always read through power supply specifications especially SMPS and read that it has an inbuilt EMI filter. This EMI filters suppress common mode as well as differential mode noise. Even though we talk only about supply here there are many instances where EMI filter is used.

The different selection criteria for EMI filter are:

• Current carrying capability
•  Impedance
• Maximum DC resistance
• Form factor of the filter
• Capacitance

The various types of EMI filters that can be used for eliminating differential mode noise are:

• Pi filter
• T filter
• L filter

The various types of EMI filters that can be used for eliminating common mode noise are:

• Common mode choke 

EMI filters are used in the following applications:

• USB, Ethernet lines
• AC adaptors
• Power supply input

The various vendors that supply EMI filters are:

• TDK
• MURATA
• PANASONIC
• TE

Some of the EMI/EMC standards:

• CISPR/EN (standard for products used in Europe, Ex: CISPR22, EN55022 which is meant for computing applications)
•  EN61000 (Ex: EN61000-3-2 for applications < 16A)
• FCC (standard for products used in US, Ex: FCC part 15 class B which is meant for unintentional radiators)