Switching power adapter is a strong source of electromagnetic interference emission. The primary rectifier bridge of the switching power supply adapter, because it is a nonlinear device, the current is a severely distorted half-sine wave, which contains rich high-order harmonics, so various factors form harmonics, electric fields, magnetic fields, surges, and electric fast transients. A series of continuous, pulsating and transient interferences are conducted and radiated, so electromagnetic compatibility design must be considered in the design of switching power adapters.
The design of EMI is mainly to study the design of noise filter. The use of switching power supply filters can effectively suppress the noise in the power grid from entering the equipment, and can also suppress the noise generated by the equipment from polluting the power grid. There are two ways of noise propagation: one is common mode noise; the other is differential mode noise. Common mode noise is the current flowing into the ground, and differential mode noise is the circuit flowing from line to line.
In the design and application of noise filters, decentralized filtering is the most popular at present. The main components used in decentralized filtering are common-mode inductors and line-to-line capacitors.
According to the propagation path of electromagnetic disturbance, the electromagnetic compatibility design of switching power supply adapter includes: perfect circuit design, grounding design, filter design, shielding design.
1. Perfect circuit design
The circuit topology of the selected switching power supply adapter should not generate excessive voltage and excessive current, so as to avoid high-voltage electric field interference and high-current magnetic field interference. In the case of meeting the requirements, the frequency band of the amplifier “target=_blank> amplifier should be as narrow as possible to make it less susceptible to interference. Increase the buffer circuit appropriately.
The following points should be paid attention to when designing printed circuit boards:
① When high, medium and low speed logic circuits are used at the same time, the high speed should be designed at the entrance of the circuit board;
② Add RC decoupling filter to the signal port to eliminate long-term transmission interference;
③ The current loop in the circuit should be kept to a minimum;
④ The signal line and return line should be as close as possible;
⑤ Use a larger ground plane to reduce ground impedance;
⑥ The power wire and the ground wire should be close to each other;
⑦ In a multi-layer circuit board, the power plane and the ground plane should be separated;
⑧ Arc wiring, no sudden change;
⑨ Keep the connection as short as possible;
⑩ The analog circuit is separated from the digital circuit, and the power circuit is separated from the control circuit.
2. Ground design
A． Grounding is an important method for switching power adapters to suppress electromagnetic noise.
The role of grounding:
① Improve the stability of the system, if it is not connected to the ground, it will be easily interfered by the ground capacitance;
② Discharge the static electricity induced on the chassis to avoid high-voltage discharge;
③ safe operation.
B． Regardless of safety grounding, only from the point of view of the circuit reference point, grounding can be divided into floating grounding, single-point grounding, multi-point grounding and hybrid grounding.
① suspended
The reference ground in the switching power adapter is isolated from the chassis, which can prevent the interference current in the chassis from being directly coupled to the power circuit. When the floating system is close to high voltage, it may accumulate static charges, causing hazards or causing static discharges to form interference currents. In a lightning environment, an arc is generated between the chassis and the unit circuit. Therefore, floating ground should not be used in switching power supplies.
②Single point grounding
Single-point grounding is divided into single-point series grounding and parallel grounding. The advantage of single-point series grounding is that it is relatively simple, and its disadvantage is that each circuit will affect each other through the grounding wire. When using this grounding method, care must be taken to place the highest level circuit closest to ground point A, so that the potential rise at points B and C is minimized. Compared with single-point series grounding, single-point parallel grounding has no common ground impedance interference, but the number of ground wires is large, and the effect is poor at high frequencies (above MHz).
③Multi-point grounding
Each grounding point is grounded nearby, which has the advantages of simple wiring and short leads. High-frequency blocking phenomenon is significantly reduced. Its disadvantage is that the grounding impedance increases with the increase of frequency.
④Hybrid grounding
The actual situation is more complicated, and it is difficult to solve it through a simple grounding method. Instead, a combination of single-point grounding and multi-point grounding is often used to form a hybrid grounding.
filter design
Filtering is a measure often used to eliminate interference. The following issues should be paid attention to when designing and selecting filters:
① Specify the operating frequency and the interference frequency to be suppressed. If the two are very close, you need to apply a filter with very steep frequency characteristics to separate the two frequencies;
②Ensure that the filter can work reliably under high voltage conditions;
③ When the filter continuously passes the maximum rated current, its temperature should be low to ensure that the working performance of the components in the filter will not be damaged when the rated current is continuously working;
④ In order to make the frequency characteristics of the filter at work consistent with the design value, the value of the signal source impedance and load impedance connected to it is required to be equal to the specified value at the time of design;
⑤ The filter must have a shielding structure, the shielding box cover and the body must have good electrical contact, the capacitor lead of the filter should be as short as possible, and it is best to use a feedthrough capacitor with low lead and short inductance;
⑥ It must have high working reliability, because the filter used to protect against electromagnetic interference is often more difficult to find faults than other components.
Pay attention to the following points when installing the filter
①The power line filter should be installed as close as possible to the power supply port of the equipment, and the power line that has not passed the filter should not be detoured in the equipment frame;
②The capacitor leads in the filter should be as short as possible to avoid resonance at low frequencies due to lead inductance and capacitive reactance;
③ There is a large short-circuit current passing through the grounding wire of the filter, which will cause additional electromagnetic radiation, so the filter element itself should be well shielded and grounded;
④ The input and output lines of the filter cannot cross, otherwise crosstalk will be caused by the input-output capacitive coupling path of the filter, thereby reducing the filtering characteristics. The usual method is to add a partition or a frequency shielding layer between the input and output.
shielding design
Shielding serves two purposes, one is to limit the leakage of internally radiated electromagnetic energy out of the internal area, and the other is to prevent external radiation interference from entering the internal area. The principle of electromagnetic shielding is to use the shielding body to reflect, absorb and guide the electromagnetic energy flow, and these effects are closely related to the charge, current and polarization induced on the surface of the shielding structure and in the shielding body.
Shield design principles
① First determine the electromagnetic environment, including the type of electromagnetic field, field strength, frequency, and the distance from the shield to the source, etc.;
② Determine the sensitivity of the receiver and the requirements for shielding;
③ According to the requirements of electromagnetic shielding and the nature of the electromagnetic field, the conductivity, permeability and thickness of the material should be properly selected;
④After determining the shielding material, design the shielding structure. For electric field shielding, high-conductivity materials (such as copper) are mainly selected; for magnetic field shielding, especially low-frequency magnetic field shielding, iron or other high-permeability materials are mainly selected. If the requirements are not met, the method of increasing the thickness can be adopted under permitted conditions;
⑤If a single layer of shielding cannot meet the shielding requirements, more than two layers of shielding can be used to obtain a better shielding effect;
⑥When the shielding room needs to be transparent, metal mesh shielding can be used. The effectiveness of metal mesh shielding is obviously not as good as that of metal solid-wall shielding, so double-layer shielding is generally not used;
⑦ Openings such as ventilation holes, generous shielding shells of detectors, and cable inlet and outlet connectors are designed according to special requirements.
Post time: Mar-31-2023