What is the mechanism of electromagnetic emission and magnetic field interference?

This article will analyze the generation mechanism of electromagnetic emission and magnetic field interference in electronic products, and introduce various technical measures to effectively suppress and prevent interference.

Electronic and electrical products radiate electromagnetic disturbance to the surrounding space at the same time during normal operation. The radiated disturbance field strength often exceeds the limit in certain frequency bands, which will affect the normal operation of the surrounding electronic equipment and itself. Therefore, understanding the reasons for exceeding the standard and the suppression methods of electromagnetic emission and magnetic field interference is very important for the product electromagnetic compatibility (EMC) design.

1. The mechanism of electromagnetic emission and magnetic field interference

1) Electromagnetic emission

During the operation of various digital circuit chips and high-frequency analog circuit chips, due to the unreasonable design of PCB traces or the connection of various parts of the product, antenna effects are generated and radio frequency interference caused by electromagnetic waves is emitted. When the electromagnetic wave energy reaches a certain value, it will affect the normal operation of the surrounding electronic equipment and itself.

2) Magnetic field interference

The power line inside the product and the magnetic field generated by the high frequency working inductive components interfere with the operation of the product through radiation, resulting in work disorder.

2. Electromagnetic emission and suppression of electronic products

In electronic products, the frequency of the signal transition edge of the digital circuit chip port can reach hundreds of megahertz, and some analog circuit signals have frequencies above megahertz. These digital or analog signals may conduct interference through wires or radiate interference into the air, affecting The electronic device itself does not interfere with other electronic devices. The basic measures to suppress electromagnetic emissions are as follows.

2.1 Reduce the energy of interfering signals

1) Under the premise of not affecting the overall working performance of the product, reduce the transition rate of digital signals or reduce the transmission speed of digital signals;

2) The use of chip components shortens the external pins of the high-frequency working chip and reduces the length of the transmission high-frequency signal trace, which can suppress the antenna effect and reduce the high-frequency signal radiation energy.

2.2 Isolate the propagation path of interfering signals

Grounding in electronic equipment is one of the important methods to suppress electromagnetic noise and prevent electromagnetic interference. The simplest and most effective isolation method is shielding, also known as "shield grounding", which refers to the grounding of the shielding layer (body) used to suppress interference in order to play a good anti-interference effect. There are three common methods of shielding:

1) Enclosed in a magnetically conductive metal shell, the shell is reliably grounded (earth);

2) Local circuits or IC chips that are prone to high-frequency radiation plus a metal shield, the shield is connected to the signal ground;

3) Both sides of the wiring that transmits high-speed digital signals or high-frequency analog signals in the circuit board are covered with copper and connected to the signal ground to achieve isolation from other signal lines.

2.3 Filtering

The filter can suppress the conduction interference from the electronic equipment and the conduction interference from the power grid. EMI (electromagnetic interference) filters are mainly used to suppress interference. The EMI filter consists of a linear component circuit and is installed between the power cord and the electronic equipment. It can pass the power frequency and prevent high-frequency noise from passing, which plays an important role in improving the reliability of the equipment.

1) Directly connect the decoupling capacitor or decoupling resistor capacitor between the power pins of the circuit chip to filter out the high-frequency interference signals that enter the chip through the power supply wiring;

2) Set a power filter at the product AC 220 V power input terminal to prevent the high-frequency interference generated by the product from entering the power grid.

3. The interference mechanism of electromagnetic energy and the sources of interference suppression

When current flows through high-frequency wires (or copper bars) in electronic products, magnetic fields are generated around the wires; high-frequency transformers and all inductive components of switching power supplies must generate leakage flux during operation. The above magnetic flux passes through the chip or sensitive circuit module. The charged particles (electrons and holes) in the semiconductor are subjected to the Lorentz force in the magnetic field and deviate from the original direction of movement, so that the working current waveform of the chip and module is modulated by the change of the magnetic field. And distortion occurs, causing the normal operation of these chips or circuit modules to be disturbed. The signal current always flows in a closed loop. When the external disturbance magnetic flux crosses the area surrounded by the closed loop, current will be induced in the closed loop, which will also cause current waveform distortion. The basic measures to suppress electromagnetic energy interference are as follows.

3.1 Method of shielding interference magnetic field

The most commonly used measure to suppress magnetic field radiation interference is to use conductive or magnetically conductive materials to shield.

1) When the changing interference magnetic flux passes through a conductive material (such as a thin copper skin), an eddy current is generated therein, and a magnetic flux in the opposite direction is generated, which can weaken the interference magnetic flux passing through the conductive shielding layer;

2) The high-frequency transformer magnetic core is covered with a thin copper sheet forming a short-circuit ring, which can effectively suppress the leakage of transformer leakage magnetic flux;

3) Using magnetically permeable materials (iron plate or steel plate) as the case of the equipment is a common method of magnetic shielding of the whole machine. This method can not only prevent external interference magnetic flux from entering the electronic device, but also avoid internal magnetic flux leakage. The better the magnetic permeability of the shielding material and the thicker the board, the less magnetic saturation occurs in the chassis, and the better the shielding effect.

3.2 Reduce the loop area of the signal current

The purpose of reducing the area of the signal current loop is to reduce the disturbing magnetic flux passing through it. Common measures:

1) A twisted pair is used to make the signal wire's outgoing line and return line tightly twisted, which can reduce the area surrounded by the return;

2) Use shielded wires as externally introduced signal wires. When in use, the core wire is used as the signal current line, and the copper wire braided shield layer is used as the signal current return line, and the signal ground must be single-ended. The loop area of this method is smaller than the twisted pair, and the shielding layer can also achieve magnetic field shielding;

3) Under the premise of ensuring insulation safety, the signal wire and the ground wire in the PCB should be as close as possible to reduce the area surrounded by the signal current loop;

4) When selecting the IC chip and circuit module on the PCB, as far as the circuit function is ensured, the package with the power inlet pin and the zero volt line pin should be selected as close as possible;

5) When designing the PCB, keep the power line and zero-volt line close to each other while ensuring insulation safety.

If you want to know more, our website has product specifications for electromagnetic emission and magnetic field interference, you can go to ALLICDATA ELECTRONICS LIMITED to get more information