EMI Filter Circuit

The so-called electromagnetic interference (EMI) is an electromagnetic phenomenon in which the performance of equipment, transmission channels or systems is reduced due to electromagnetic waves.

EMI Function definition

EMI spreads in two ways: radiation and conduction.
Radiation mode: Energy is coupled through a magnetic field or electric field, or spreads in the form of electromagnetic waves between the interference source and the disturbing device.
Conduction method: Energy is generated or received through power lines, data lines, public ground lines, etc.
There are two types of conducted interference: differential mode (DM symmetric mode) and common mode (CM asymmetric mode).
The current technical measures to suppress EMI include shielding, grounding (floating ground, single-point grounding, and grounding grid), and filtering.
What I am talking about here is the filter circuit, which is mainly used in the input loop of high-frequency switching power supplies and electronic ballasts and the output loop of the power supply. This circuit is used to filter the input and output noise (150kHz～30MHz) of the power supply, and reduce the conduction interference to the DC stabilized power supply.

  China PCB Manufacturer  

EMI Application Scope

A. CISPR standards (radio frequency interference equipment such as motors, household appliances, lighting equipment, etc.)

Category

Frequency range (MHz)

Noise terminal voltage dB(uV)

Household appliances and similar devices

0.15～0.5

B. VDE0871 standard (electromagnetic compatibility standard for purposeful high-frequency wave generator)

C, FCC standards (electromagnetic compatibility standards for industrial, scientific, and medical equipment)

D. VCCI standard (Household appliances and similar devices used in industrial and commercial areas)

EMI Design specification

Circuit schematic and description

This circuit mainly filters the input and weakens the conduction interference to the input of the stabilized power supply or electronic ballast. Among them, C1, C2, and C4, C5 and Lc are used to filter out common-mode noise, and C3 and C6 are used to filter out differential mode noise. The output terminal is generally connected to an electrolytic capacitor. When the load current is large, a high-frequency capacitor is also required to eliminate the noise interference from the load terminal to the input.
C1=C2, C4=C5, C3=C6, Lc=(7~30)mH, ferrite material is used for the magnetic material.

EMI filter has C type (pure capacitor), L type (one inductor and one capacitor), T type (two inductors and one capacitor), π type

Electromagnetic interference (EMI) filter circuit

(One inductor and two capacitors), double π type (two inductors and two capacitors symmetrically wound on the same magnetic core), etc.