ALLPCB
Overview
EMI (electromagnetic interference) is a common issue in electronic circuits and can cause unwanted noise and interference in electrical equipment. EMI filters are designed to reduce or eliminate these unwanted signals. This article explains how to select an appropriate EMI filter.
Basic electrical requirements
The first step is to ensure the EMI filter meets basic electrical requirements, including:
- Rated voltage
- Isolation voltage
- Rated current
- Leakage current
- Ambient temperature
- Operating temperature
- Withstand voltage / dielectric strength
Filter types and stages
There are three basic types of EMI filters: single-stage, multi-stage, and feedthrough filters. Increasing the number of stages can improve filter performance while optimizing the filter package size. It also helps lower the cutoff frequency.
Frequency characteristics
EMI filters are intended to suppress transmission at selected frequencies of a given signal. The most common type is a simple EMI low-pass filter that allows low-frequency signals to pass. The cutoff frequency is typically defined as the point where the signal amplitude is 3 dB below the nominal passband value, and it is influenced by the filter's capacitor and inductor values together with its frequency response.
Selection based on EMC testing
When selecting an EMI filter, ideally the power supply to be filtered should have undergone preliminary EMC testing to establish a baseline for conducted emissions. Test results allow engineers to determine the frequencies at which failures occur. This information can be compared with the EMI filter's insertion loss plot to determine whether sufficient attenuation is provided at the failing frequencies to pass EMC testing.
