Analysis of Electromagnetic Compatibility Design Method for Switching Power Supply
Due to the advantages of small size and high power factor, switching power supply is widely used in communication, control, computer and other fields. However, due to electromagnetic interference, its further application is limited to a certain extent. This paper will analyze the various mechanisms of electromagnetic interference of switching power supply, and on the basis of it, propose the electromagnetic compatibility design method of switching power supply.
Electromagnetic Interference Analysis of Switching Power Supply
The structure of the switching power supply is shown in Figure 1. First, the power frequency AC is rectified to DC, and then converted to high frequency, and finally output through the rectification and filtering circuit to obtain a stable DC voltage. Unreasonable circuit design and layout, mechanical vibration, poor grounding, etc. will cause internal electromagnetic interference. At the same time, the leakage inductance of the transformer and the peak caused by the reverse recovery current of the output diode are also potential strong interference sources.
1 Internal sources of interference
● switch circuit
The switch circuit is mainly composed of a switch tube and a high frequency transformer. There is a distributed capacitance between the switch tube and its heat sink, the casing and the internal leads of the power supply. The du/dt generated by it has a relatively large pulse, a wide frequency band and rich harmonics. The switching tube load is the primary coil of the high frequency transformer, which is an inductive load. When the switch tube that was originally turned on is turned off, the leakage inductance of the high-frequency transformer generates a counter electromotive force E=-Ldi/dt, and its value is proportional to the current change rate of the collector and proportional to the leakage inductance, superimposed on the off On the cut-off voltage, a turn-off voltage peak is formed, thereby forming a conduction interference.
● Rectifier diodes for rectifier circuits
There is a reverse current when the output rectifier diode is cut off, and the time it returns to zero is related to factors such as junction capacitance. It will produce a large current change di/dt under the influence of transformer leakage inductance and other distribution parameters, and generate strong high-frequency interference, the frequency can reach tens of megahertz.
● Spurious parameters
Due to working at a higher frequency, the characteristics of low-frequency components in the switching power supply will change, resulting in noise. At high frequencies, the stray parameters have a great influence on the characteristics of the coupling channel, and the distributed capacitance becomes the channel of electromagnetic interference.
2 External sources of interference
External interference sources can be divided into power interference and lightning interference, and power interference exists in "common mode" and "differential mode". At the same time, since the AC power grid is directly connected to the rectifier bridge and the filter circuit, in half a cycle, only the peak time of the input voltage has input current, resulting in a very low input power factor of the power supply (about 0.6). Moreover, this current contains a large number of current harmonic components, which will cause harmonic "pollution" to the grid.
EMC Design of Switching Power Supply
There are three necessary conditions for electromagnetic interference: interference source, transmission medium, and sensitive equipment. The purpose of EMC design is to destroy one of these three conditions. For this, the main methods adopted are: circuit measures, EMI filtering, shielding, printed circuit board anti-interference design, etc.
1 Soft switching technology to reduce switching loss and switching noise
Soft switching is an advanced switching technology based on resonance technology or using control technology in zero voltage/current state developed on the basis of hard switching.
The realization method of soft switching is: adding small inductors, capacitors and other resonant components in the original circuit, introducing resonance before and after the switching process, and eliminating the overlap of voltage and current. Figure 2 shows a basic switching unit using soft switching technology.
Use shielding to suppress radiated and induced interference
The interference spectrum of the switching power supply is concentrated in the frequency band below 30MHz, and the diameter r<λ/2π is mainly an electromagnetic field of near-field nature, and it is a low-impedance field. Materials with good electrical conductivity can be used to shield the electric field, while materials with high magnetic permeability can be used to shield the magnetic field. In addition, effective shielding measures should be taken for transformers, inductors, power devices, etc. The ventilation holes on the shielding shell are preferably circular, and the number of holes can be many if ventilation conditions are satisfied, and the size of each hole should be as small as possible. The seams are to be welded to ensure electromagnetic continuity. Filtering measures should be taken at the lead-in and lead-out lines of the shielded enclosure. For electric field shielding, the shielding case must be grounded. For magnetic field shielding, the shielded case does not need to be grounded.
