When the electrical and electronic products are working normally, they radiate electromagnetic disturbance to the surrounding space at the same time, and 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, it is very important to understand the reasons for exceeding the standard and the suppression methods of electromagnetic emission and magnetic field interference for the electromagnetic compatibility (EMC) design of products.
- 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, due to the unreasonable design of PCB traces or connections 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 work of surrounding electronic equipment and itself.
2) Magnetic field interference
The magnetic field generated by the power line inside the product and the inductive components working at high frequency interferes with the operation of the product through radiation, causing the work to be disordered.
- Electromagnetic emission of 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 frequency of some analog circuit signals can reach more than 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. The basic measures to suppress electromagnetic emission are as follows.
2.1 Reduce the energy of interfering signals
1) On the premise of not affecting the overall performance of the product, reduce the hopping rate of digital signals or reduce the transmission speed of digital signals;
2) The use of SMD components shortens the outer pins of the high-frequency working chip and reduces the length of the high-frequency signal transmission line, which can suppress the antenna effect and reduce the radiation energy of the high-frequency signal.
2.2 Isolate the propagation path of the interference signal
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, so as to play a good anti-interference effect. There are three commonly used shielding methods:
1) It is encapsulated by magnetic conductive metal material shell, and the shell is reliably grounded (earth);
2) Add a metal shield to the local circuit or IC chip that is prone to high-frequency radiation, and the shield is connected to the signal ground;
3) The two sides of the traces transmitting high-speed digital signals or high-frequency analog signals in the circuit board are coated with copper and connected to the signal ground to achieve isolation from other signal lines.
2.3 Filtering
The filter can suppress both conducted interference from electronic equipment and conducted interference from the power grid. EMI (Electromagnetic Interference) filters are mainly filters used to suppress interference. EMI filters consist of linear element circuits that are installed between power lines and electronic equipment. It can pass the power frequency and prevent the 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 resistor capacitors between the power pins of the circuit chip to filter out high-frequency interference signals entering the chip through the power traces;
2) Set a power filter at the AC 220 V power input end of the product to prevent the high-frequency interference generated when the product is working from entering the power grid.
- Interference mechanism of electromagnetic energy and its sources of interference suppression
When current flows through the high-frequency wire (or copper bar) in electronic products, the magnetic field generated around the wire; the leakage magnetic flux that must be generated by the high-frequency transformer of the switching power supply and all inductive components during operation. The above-mentioned magnetic flux passes through the chip or sensitive circuit module, and 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 motion, so that the working current waveform of the chip and the module is modulated by the change of the magnetic field Distortion occurs, causing the normal operation of these chips or circuit modules to be disturbed. Signal current always flows in a closed loop. When the external disturbance magnetic flux passes through the area enclosed by the closed loop, a current will be induced in the closed loop, and the current waveform will also be distorted. The basic measures to suppress electromagnetic energy interference are as follows.
3.1 Method of shielding interference magnetic field
The most commonly used measures to suppress the interference of magnetic field radiation are the use of conductive or emi suppressor sheets.
1) When the changing interference magnetic flux passes through the conductive material (such as thin copper sheet), eddy currents will be generated in it, and the 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 covered with a thin copper sheet that forms a short-circuit ring, which can effectively suppress the leakage of the leakage flux of the transformer;
3) Using emi suppressor sheet as the chassis of the equipment is a common method for magnetic shielding of the whole machine. The emi suppressor sheet can not only resist the entry of external interference magnetic flux into electronic equipment, but also avoid the leakage of internal magnetic flux. The better the magnetic conductivity of the emi suppressor sheet, the thicker the board, the less likely the case is to have magnetic saturation, 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 interfering magnetic flux passing through it. Common measures:
1) The use of twisted pair wires makes the outgoing and return wires of the signal current tightly twisted, which can reduce the area enclosed by them;
2) Use shielded wires as externally introduced signal wires. When using, use the core wire as the signal current wire, and the copper wire braided shielding layer is used as the signal current return wire, which must be single-ended to the signal ground. The loop area of this method is smaller than the twisted pair, and the shielding layer can also achieve magnetic field shielding;
3) On 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 IC chips and circuit modules on the PCB, under the condition that the circuit function is guaranteed, the package that is close to the power inlet pin and the zero-volt line pin should be selected as far as possible;
5) When designing the PCB, under the premise of ensuring insulation safety, make the power line and the zero-volt line close to the layout.
PH’s products are mainly divided into absorbing sheets, emi suppressor sheet, rfid absorber, flexible whiteboard, electromagnetic shielding coating, and other electromagnetic functional materials. Our products have the features of absorbing and shielding electromagnetic waves, increasing magnetic flux, anti-metal interference and magnetic field isolation, and are widely used in wireless charging modules, RFID electronic tags, smart cards, NFC modules, EMI, EMC and other fields.
