Do you understand electromagnetic interference analysis and suppression measures in EMC design?

Do you understand electromagnetic interference analysis and suppression measures in EMC design? This article analyzes the generation mechanism of electromagnetic emission and magnetic field interference in electronic products, and introduces various technical measures to effectively suppress and prevent interference. When electronic and electrical products are operating normally, they simultaneously radiate electromagnetic disturbance to the surrounding space. The intensity of the radiated disturbance field often exceeds the limit in certain frequency bands, which will affect the normal operation of surrounding electronic equipment and itself. Therefore, it is very important for product electromagnetic compatibility (EMC) design to understand the reasons for exceeding the standard and the suppression methods of electromagnetic emission and magnetic field interference.

1. The generation 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, the antenna effect occurs due to unreasonable design of PCB traces or connections between various parts of the product, emitting radio frequency interference caused by electromagnetic waves. When the electromagnetic wave energy reaches a certain value, it will affect the normal operation of surrounding electronic equipment and itself.

2) Magnetic field interference

The magnetic field generated by the power cord and high-frequency inductive components inside the product interferes with the operation of the product through radiation, causing work disorder.

2. Electromagnetic emissions from electronic products and their suppression

In electronic products, the frequency of the signal transition edge of the digital circuit chip port can reach hundreds of megahertz, and the signal frequency of some analog circuits reaches above megahertz. These digital or analog signals may conduct interference through wires or radiate interference into the air, affecting electronic devices themselves and interfere with other electronic devices. Basic measures to suppress electromagnetic emissions include the following methods.

2.1 Reduce the energy of interference signals

1) Reduce the jump rate of digital signals or reduce the transmission speed of digital signals without affecting the overall performance of the product;

2) Use patch components to shorten the external pins of high-frequency working chips and reduce the length of high-frequency signal transmission lines, which can suppress the antenna effect and reduce the radiation energy of high-frequency signals.

2.2 Isolate the propagation path of interference 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 "shielding grounding", which refers to the grounding of the shielding layer (body) used to suppress interference to play a good anti-interference effect. There are three commonly used shielding methods:

1) Encapsulated in a magnetically permeable metal shell, and the shell is reliably grounded (earth);

2) Add a metal shield to local circuits or IC chips that are prone to high-frequency radiation, and connect the shield to the signal ground;

3) Both sides of the traces that transmit 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

Filters can suppress both conducted interference from electronic equipment and conductive interference from the power grid. EMI (electromagnetic interference) filters are mainly filters used to suppress interference. EMI filters consist of linear component circuits installed between power lines and electronic equipment. It allows the power frequency to pass through and prevents high-frequency noise from passing through, which plays an important role in improving the reliability of the equipment.

1) Directly connect decoupling capacitors or decoupling resistors and capacitors between the power pins of the circuit chip to filter out high-frequency interference signals entering the chip through the power supply traces;

2) Set up a power filter at the AC 220 V power input end of the product to prevent high-frequency interference generated when the product is working from entering the power grid.

3. Interference mechanism of electromagnetic energy and suppression of interference sources

When a current flows through a high-frequency wire (or copper bar) in an electronic product, a magnetic field is generated around the wire; the high-frequency transformer of the switching power supply and all inductive components inevitably produce leakage flux when they are working. The above-mentioned magnetic flux passes through the chip or sensitive circuit module. The charged particles (electrons and holes) in the semiconductor are subject to the Lorentz force in the magnetic field and deviate from the original direction of movement, causing the operating current waveform of the chip and module to be modulated by changes in the magnetic field. Distortion occurs, causing the normal operation of these chips or circuit modules to be interfered with. Signal current always flows in a closed loop. When external interference magnetic flux passes through the area surrounded by a closed loop, a current will be induced in the closed loop, which will also cause distortion of the current waveform. The basic measures to suppress electromagnetic energy interference include the following methods.

3.1 Methods of shielding interfering magnetic fields

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

1) When the changing interference magnetic flux passes through a conductive material (such as a thin copper sheet), eddy currents will be generated in it and a magnetic flux in the opposite direction will be generated, which can weaken the interference magnetic flux passing through the conductive shielding layer;

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

3) Using magnetically permeable materials (iron plates or steel plates) as the equipment chassis is a common method for magnetic shielding of the entire machine. This method can not only resist external interference magnetic flux from entering the electronic device, but also prevent internal magnetic flux from leaking out. The better the magnetic permeability of the shielding material and the thicker the board, the less likely magnetic saturation will occur in the chassis and the better the shielding effect will be.

3.2 Reduce the loop area of ​​signal current

The purpose of reducing the area of ​​the signal current loop is to reduce the interference magnetic flux passing through it. Commonly used measures:

1) Use twisted pairs to tightly twist the outgoing and return wires of the signal current, which can reduce the enclosed area;

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

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

4) When selecting IC chips and circuit modules on the PCB, under the condition of ensuring the circuit function, you should try to choose a package with the power incoming pin and the zero-volt line pin close to each other;

5) When designing the PCB, on the premise of ensuring insulation safety, arrange the power lines and zero-volt lines close to each other. The above is the electromagnetic interference analysis and suppression measures for EMC design. I hope it can help you.