Electromagnetic compatibility (EMC) means that electronic equipment or systems do not reduce their performance indicators due to electromagnetic interference under the specified electromagnetic environment level, and their own electromagnetic radiation is not greater than the specified limit level, and does not affect other electronic equipment or The normal operation of the system, and achieve the purpose of non-interference between equipment and equipment, system and system, and work together reliably.
Countries in the world have formulated their own EMC standards accordingly. For example, the 1EC61000 and (C1SPR series standards of the International Electrotechnical Commission, the FN series standards of the European Community, the FCC series standards of the U.S. Federal Communications Commission and the current GT3/T13926 series EMC standards of my country. With the increasingly strict international electromagnetic compatibility regulations, products More and more attention is paid to its electromagnetic compatibility performance.
As a kind of power supply equipment, switching power supply is more and more widely used. With the continuous updating of power electronic devices, the switching frequency and switching speed of switching power supplies continue to increase, but the rapid switching of the switches causes rapid changes in voltage and current. These transient voltages and currents, coupled through power lines, parasitic parameters, and stray electromagnetic fields, will generate a lot of electromagnetic interference.
Two, the interference source analysis of the switching power supply
The electromagnetic interference (EMI) generated by the switching power supply can be divided into conduction interference and radiation interference according to the coupling channel; according to the type of noise interference source, it can be divided into spike interference and harmonic interference. The inrush current and peak voltage generated by the switching power supply in the working process form the interference source, the large capacitor charging and discharging used in the power frequency rectification and filtering, the voltage switching during high frequency operation of the switching tube and the reverse recovery current of the output rectifier diode All are such sources of interference.
3. Suppression measures of electromagnetic interference
Electromagnetic interference is generated by the combination of three basic elements: electromagnetic interference source; equipment sensitive to the interference energy; the medium through which electromagnetic interference source is transmitted to sensitive equipment is the transmission channel or coupling path. The suppression measures taken to the electromagnetic interference generated by the switching power supply are mainly considered from two aspects: one is to reduce the interference intensity of the interference source; the other is to cut off the interference propagation path.
Commonly used anti-interference measures include circuit isolation, shielding, grounding, installation of EMI filters, and reasonable layout and wiring of PCB boards.
1. Isolation of the circuit
In the switching power supply, the isolation of the circuit mainly includes: the isolation of the analog circuit, the isolation of the digital circuit, the isolation between the digital circuit and the analog circuit. The main purpose is to cut off the path of noise interference by isolating components, so as to achieve the effect of suppressing noise interference. For the isolation of the analog signal control system of the switching power supply, AC signals are generally isolated by transformers, and DC signals are generally isolated by linear isolators (such as linear optocouplers). The isolation of digital circuits mainly includes: pulse transformer isolation, photoelectric coupler isolation and so on. Among them, the digital input isolation method mainly uses pulse transformer isolation and photoelectric coupler isolation; and the digital output isolation method mainly uses photoelectric coupler isolation and high-frequency transformer isolation.
2. Shield
Shielding is generally divided into two categories, one is electrostatic shielding, which is mainly used to prevent the influence of electrostatic field and constant magnetic field; the other is electromagnetic shielding, which is mainly used to prevent the influence of alternating electric field, alternating magnetic field and alternating electromagnetic field. Shielding is an effective method to suppress the radiation interference of the switching power supply. Materials with good conductivity can be used to shield the electric field, and materials with high magnetic permeability can be used to shield the magnetic field.
3. Ground
In order to prevent various circuits from interfering with each other during operation, so that they can work compatible with each other, according to the nature of the circuit, the working ground is divided into different types. Such as DC ground, AC ground, digital ground, analog ground, signal ground, power ground, power ground, etc. In the design of the circuit, the AC power ground should be separated from the DC power ground, the power ground of the analog circuit and the digital circuit should be separated, and the power ground should be separated from the weak current ground.
4. Install EMI filter
The power filter is installed between the power line and the electronic equipment to suppress the conduction interference induced by the power line, and can also reduce the conduction interference introduced from the power grid, which plays an important role in improving the reliability of the equipment. The electromagnetic interference generated by the switching power supply is mainly conducted interference, and the conducted interference is divided into two types: differential mode disturbance and common mode interference. The basic network that constitutes the switching power supply EMI filter is shown in Figure 1. The filter consists of a common mode choke coil L, a differential mode capacitor Cx and a common mode capacitor Cy. The common mode choke coil L is composed of two windings wound on the same high permeability magnetic core, and its structure makes the magnetic flux generated by the differential mode current cancel each other. This structure obtains a larger inductance value with a smaller volume, and there is no need to worry about saturation due to operating current. Each winding and capacitor Cy form two pairs of independent low-pass filters, LE and NE, to form a common-mode filter network to suppress common-mode interference on the power line. As for the value of common mode choke coil L, differential mode capacitor Cx and common mode capacitor Cy, the resonant frequency of the filter should be lower than the operating frequency of the switching power supply as much as possible, so that the entire frequency band can be filtered.
5. Reasonable layout and wiring of PCB board
The radiation interference of the switching power supply is proportional to the product of the current in the current path, the loop area of the path, and the square of the current frequency, that is, the radiation interference E∝I·A·f 2. The premise of using this relationship is that the channel size is much smaller than the wavelength of the frequency. The above relationship shows that reducing the path area is the key to reducing the radiation disturbance, which means that the components of the switching power supply should be closely arranged with each other. In the primary circuit, the input capacitor, transistor, and transformer are required to be close to each other, and the wiring is compact; in the secondary circuit, the diode, transformer, and output capacitor are required to be close to each other. On the printed board, place the positive load current conductors on both sides of the printed board, and try to keep the two current-carrying conductors parallel to each other, because the external magnetic field generated by the parallel positive load current conductor tends to Cancel each other out.
Four, concluding remarks
To increase the switching frequency and improve the quality of switching power supply products, electromagnetic compatibility is an issue that cannot be ignored. There are still many factors that cause electromagnetic interference in switching power supplies, and there is still a lot of work to suppress electromagnetic interference. Only when the electromagnetic compatibility is fully considered in the design, the switching power supply can be used more generally.