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EMC design of switching power supply

Source:AdminAuthor:WPOWER Addtime:2019-10-30 Click:
EMC design of switching power supply
Due to its small size and large power factor, switching power supplies are widely used in communications, control, and computer fields. However, due to electromagnetic interference, its further application is limited to a certain extent. This paper will analyze the various generation 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
First, the power frequency AC is rectified to DC, and then inverted to a high frequency, and finally output through a rectification and filtering circuit to obtain a stable DC voltage. Unreasonable circuit design and layout, mechanical vibration, poor grounding, etc. will form internal electromagnetic interference. At the same time, the leakage inductance of the transformer and the spike of the reverse recovery current of the output diode are also potential sources of strong interference.
1 Internal interference source
● Switch circuit
The switching circuit is mainly composed of a switching tube and a high frequency transformer. There is a distributed capacitance between the switch tube and its heat sink and the lead inside the housing and the power supply. The du/dt generated by the switch has a large amplitude pulse, and the frequency band is wide and the harmonics are rich. The switch tube load is the primary coil of the high frequency transformer and is an inductive load. When the originally turned-on switch is turned off, the leakage inductance of the high-frequency transformer produces a back-EMF E=-Ldi/dt, which is proportional to the current change rate of the collector, proportional to the leakage inductance, superimposed on On the breaking voltage, a turn-off voltage spike is formed, thereby forming conducted interference.
● Rectifier diode of rectifier circuit
There is a reverse current when the output rectifier diode is turned off, and the time it returns to zero is related to factors such as junction capacitance. It will generate a large current change di/dt under the influence of transformer leakage inductance and other distributed parameters, resulting in strong high-frequency interference, the frequency can reach several tens of megahertz.
● Stray parameters
Due to the higher frequency, the characteristics of the low frequency components in the switching power supply will change, thereby generating noise. At high frequencies, the spurious parameters have a great influence on the characteristics of the coupled channel, and the distributed capacitance becomes the channel for electromagnetic interference.
2 External interference source
External interference sources can be divided into power supply interference and lightning interference, while power supply interference exists in "common mode" and "differential mode". At the same time, since the AC grid is directly connected to the rectifier bridge and the filter circuit, in the half cycle, only the input voltage peak time has input current, resulting in a low input power factor of the power supply (about 0.6). Moreover, this current contains a large amount of current harmonic components, which can cause harmonic "pollution" to the power grid.
EMC design of switching power supply
There are three necessary conditions for generating electromagnetic interference: interference source, transmission medium, and sensitive equipment. The purpose of EMC design is to destroy one of these three conditions. In response to this, the main methods are: circuit measures, EMI filtering, shielding, printed circuit board anti-interference design.
1 Soft switching technology to reduce switching loss and switching noise
Soft switching is an advanced switching technology based on resonant technology or using control technology developed in the zero voltage/current state based on hard switching.
The implementation of the soft switch is to add a small inductance, capacitance and other resonant components in the original circuit, and introduce resonance before and after the switching process to eliminate the overlap of voltage and current.
2 snubber circuit for reducing interference source interference energy
A buffer circuit is added to the input portion of the switch control power supply, which is composed of a linear impedance stabilization network for eliminating potential interference such as power line interference, electrical fast transients, surges, voltage changes, and power line harmonics. The snubber circuit device parameters are R1=500Ω, C=6nF, L=36mH, and R=150Ω.
3 Cut off the EMI filtering of the interference noise propagation path
The addition of an EMI filter to the switching power supply input and output circuits is a very effective way to suppress conducted emissions. The main parameters are: discharge resistance, insertion loss, Cx capacitance, Cy capacitance and inductance value. Among them, insertion loss is a key parameter of filter performance. Under the premise of considering mechanical properties, environment, cost, etc., the insertion loss should be made as large as possible. The insertion loss IL of the filter can be obtained by using the common mode and differential mode interference measurement results and the standard limits, plus the appropriate margin.
ILCM(dB)=Vcm(dB)-Vlimt(dB)-3(dB)+M(dB) (1)
ILDM(dB)=VDM(dB)-Vlimt(dB)-3(dB)+M(dB) (2)
In the formula, 3dB means that the test result is 3dB larger than the actual value in the test process of separating common mode and differential mode conduction interference; M(dB) means design margin, generally 6dB; Vlimit(dB) is related standard such as CISPR, FCC The specified conduction interference limit.
220V/50Hz AC input switching power supply AC side EMI filter circuit. Cy = 3300pF, L1, L2 = 0.7mH, which form a common mode filter circuit, suppressing common mode interference signals of 0.5 to 30 MHz. Cx=0.1μF, L3, L4=200~500μH, using metal powder magnetic core, and L1/L2, Cx form a low-pass filter between LN ports, used to suppress 0.15~0.5MHz differential mode interference on the power line. signal. R is used to eliminate static buildup that may occur in the filter.
 
It is a DC output side filter circuit of a switching power supply, which is composed of a common mode choke coil L1, L2, a choke coil L3 and capacitors C1 and C2. In order to prevent the magnetic core from being saturated under a large magnetic field strength and causing the choke to lose its effect, the magnetic core must use a constant μ magnetic core with high frequency characteristics and high saturation magnetic field strength.
4 Use shielding to suppress radiation and inductive interference
The switching power supply interference spectrum is concentrated in the frequency band below 30MHz, and the diameter r<λ/2π is mainly the near-field electromagnetic field and belongs to the low impedance field. The electric field can be shielded by a material with good electrical conductivity, and the magnetic field can be shielded with a material with high magnetic permeability. In addition, effective shielding measures should be taken for transformers, inductors, power devices, etc. The venting holes in the shielded outer casing are preferably circular, and the number of holes may be large under the condition of ventilation, and the size of each hole is as small as possible. The joints are to be welded to ensure electromagnetic continuity. Filtering measures should be taken at the introduction of the shielded enclosure and at the lead-out line. For electric field shielding, the shielded enclosure must be grounded. For magnetic field shielding, the shielded enclosure does not need to be grounded.
5 Reasonable PCB layout and wiring
The sensitive line mainly refers to the control circuit and the line directly connected to the interference measuring device. The easiest way to reduce the level of interference is to increase the spacing between the interferer and the sensitive line. However, due to the limitation of the size of the power supply, simply increasing the spacing is not the best way to solve the problem. A more reasonable method is to place the sensitive line in a place with weak interference according to the distribution of the interference electric field.       

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