Electromagnetic interference and electromagnetic c

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Electromagnetic interference and electromagnetic compatibility in switching power supply Abstract: the influence of electromagnetic interference on the efficiency, safety and use of switching power supply has increasingly become a hot spot. We analyze the mechanism of electromagnetic interference produced by switching power supply, and take the three elements of electromagnetic interference as the starting point, based on the analysis and comparison of several effective measures to suppress the electromagnetic interference of switching power supply at present, focus on the electromagnetic compatibility of switching power supply, and put forward reference suggestions for the further study of electromagnetic interference and electromagnetic compatibility of switching power supply

key words: switching power supply electromagnetic interference electromagnetic compatibility

1 Introduction

with the continuous application and development of electronic equipment systems in recent years, the electromagnetic environment is becoming more and more complex. In the complex electromagnetic environment, various equipment or systems to achieve electromagnetic compatibility and normal and stable work, has been paid more and more attention. As an important part of various equipment or systems, switching power supply is both the source of disturbance and the interfered. Therefore, it has become an increasingly concerned problem for electronic product designers to suppress the electromagnetic interference of switching power supply, improve the quality of electronic products, and make it comply with the relevant electromagnetic compatibility (EMC) standards or specifications

2 electromagnetic interference of switching power supply

electromagnetic interference refers to the decline in the performance of equipment, systems or transmission channels caused by electromagnetic interference, which usually exists in a variety of electronic devices. In the switching power supply, we know from its working principle that its electromagnetic interference signal is caused by its internal and external circuits. The specific analysis is as follows:

2.1 electromagnetic interference generated by the peripheral circuit of switching power supply

electromagnetic interference generated by the peripheral circuit of switching power supply can exist in the form of "common mode" or "differential mode". The type of interference can vary from spike interference with a short duration to complete power loss, including voltage change, frequency change, waveform distortion, continuous noise or clutter, and transients. For example, in terms of filter capacitance and some parasitic parameters:

2.1.1 filter capacitance

the output end of switching power supply is connected with filter circuit and load. In order to make the AC component of output voltage very small, large capacity inductance and capacitance are generally used to filter the output voltage and provide working voltage for the load. The voltage on the filter capacitor is constantly changing: when the voltage is less than the reference value, the sampling voltage of the switching power supply becomes smaller, and the switching power supply is closed to charge the capacitor; When the voltage on the capacitor is greater than the reference value, the sampling voltage becomes larger, the switching circuit is disconnected, and the capacitor is discharged. In the process of continuous charging and discharging, the voltage at both ends of the capacitor is changing, and the voltage on the power load is also changing. The voltage change process at both ends of the capacitor is shown in Figure 1

in Figure 1, the vertical axis represents the voltage at both ends of the capacitor, and its allowable variation range is v1~v2. The horizontal axis represents the charge and discharge time of the capacitor. When the load of the switching power supply is constant:

charge time t charge =t2-t1=t4-t3= t6-t5 discharge time t discharge =t3-t2=t5-t4

discharge cycle T cycle =t3-t1=t5-t3... = t charge + T discharge

otherwise, charge time t charge 1=t2-t1, t charge 2=t4-t3, t charge 3=t6-t5... Discharge time t discharge 1=t3-t2, t discharge 2=t 5-t4...

charge and discharge cycle T cycle 1=t3-t1 = t charge 1 + T discharge 1, t cycle 2=t5-t3 = t charge 2 + T discharge 2, ...

at this time, the charging time t charge 1 ≠ t charge 2 ≠ t charge 3 ≠... The discharging time t discharge 1 ≠ t discharge 2 ≠ t discharge 3 ≠...

the charging and discharging cycle T cycle 1 ≠ t cycle 2 ≠ t cycle 3 ≠...

when the load of the switching power supply does not change, the voltage at both ends of the capacitor changes according to a certain cycle. According to Fourier transform, only the signals related to frequency 1/t charge 1 and 1/t discharge 1 and their multiple harmonic signals are generated at both ends of the capacitor. As the working state of the system is constantly changing, the load of switching power supply is also changing. At this time, the charge and discharge cycle of the capacitor will change, and the signals related to frequency 1/t charge 1, 1/t charge 2, 1/t charge 3... And 1/t discharge 1, 1/t discharge 2, 1/t discharge 3... And their multiple harmonic signals will be generated at both ends of the capacitor. In this way, the output spectrum of switching power supply becomes more complex, and the interference of switching power supply to equipment becomes stronger

2.1.2 switching power supply noise caused by distribution and parasitic parameters

the distribution parameters of switching power supply are the internal factors of most interference. The distributed capacitance between switching power supply and radiator, the distributed capacitance between primary stages of transformer, and the leakage inductance of primary and secondary sides are all noise sources. Common mode interference is transmitted through the distributed capacitance between the primary and secondary of the transformer and between the switching power supply and the radiator. The distributed capacitance of transformer winding is related to the winding structure and manufacturing process of high-frequency transformer

2.2 electromagnetic interference generated by the internal circuit of switching power supply

there are many factors causing electromagnetic interference in the internal circuit of switching power supply, among which the current high-order harmonic interference generated by the basic rectifier and the peak voltage interference generated by the transformer type power conversion circuit are the main factors. The reason why they are generated inside the power supply device is that the jump voltage and current generated by the diode and transistor in the switching power supply in the working process are caused by the high-frequency transformer, energy storage inductance coil and wire, system structure, component layout, etc

2.2.1 basic rectifier

the rectification process of basic rectifier is the most common cause of electromagnetic interference. This is because the power frequency AC sine wave is no longer a single frequency current after rectification. According to the Fourier transform formula, the current wave can be decomposed into a DC component and a series of harmonics of different frequencies. The combination of the AC component generated by the internal circuit of the switching power supply and the AC component generated by the external circuit of the switching power supply makes the output frequency component of the switching power supply more complex. In addition, this series of harmonic components with different frequencies, especially high-order harmonics, will produce conducted interference and radiation interference along the transmission line, which will distort the front-end current. On the one hand, it will distort the current waveform connected to the front-end power line, and on the other hand, it will produce RF interference through the power line

2.2.2 power conversion circuit

power conversion circuit is the component to realize voltage transformation, frequency conversion and output voltage adjustment. It is the core of switching regulated power supply, which is mainly composed of switching tubes and high-frequency transformers. The spike voltage generated by it is a narrow pulse with large radiance, which has a wide frequency band and rich harmonics. The main reasons for this pulse interference are:

(1) the load of switching power transistor is high-frequency transformer or energy storage inductor. At the moment when the switch is on, a large current appears in the primary of the transformer, which will cause peak noise when the over excitation of the switch is large. This peak noise is actually a sharp pulse. The light one may cause interference, and the heavy one may puncture the switch tube

(2) high frequency transformer is a transformer in switching power supply, which is used for isolation and voltage transformation. However, due to leakage inductance, it will produce back EMF el= - LDI/DT, which will cause voltage rise between the collector and emitter of the switch tube. This is because when the switch is converted from ton to toff, due to the leakage flux of the transformer, part of the energy is not transmitted from the primary coil to the secondary coil. This part of the energy stored in the leakage inductance will form a peak attenuation oscillation with the capacitance and resistance in the collector circuit, which will be superimposed on the turn-off voltage to form a turn-off voltage peak, which is proportional to the current change rate (di/dt) of the collector and the leakage inductance. This kind of power supply voltage interruption will produce the same magnetization impulse current transient as when the primary of the transformer is connected. It is a kind of conductive electromagnetic interference, which not only affects the primary of the transformer, but also makes the interference transmitted back to the distribution system, causing electrical harmonic electromagnetic interference, affecting the safe and economic operation of other electrical equipment

(3) because there are more carriers accumulated in the PN junction when the output rectifier diode is cut off, there will be a reverse current for a period of time before the carrier disappears, and the time when it recovers to zero is related to factors such as junction capacitance. Among them, the diode that can quickly restore the reverse current to zero is called hard recovery. 2. Parameter setting characteristic diode. This diode will produce strong high-frequency interference under the influence of transformer leakage inductance and other distribution parameters, and its frequency can reach dozens of MHz

as shown in Figure 2, T1 is the primary coil current of the transformer, T2 is the secondary coil current, VDS is the voltage between the drain and source of the switch, and VD is the voltage at both ends of the output diode on the secondary side. When the switch is turned off, the interference with frequency f1 is generated, while the reverse current of the output diode causes the interference with frequency f2

3 electromagnetic compatibility of switching power supply

electromagnetic compatibility (EMC) refers to that electronic equipment or systems do not reduce performance indicators due to electromagnetic interference under the specified electromagnetic environment level, and the electromagnetic radiation generated by them is not greater than the specified limit level, which does not affect the normal operation of other electronic equipment or systems, and achieves the purpose of mutual interference and common and reliable work between equipment and equipment, systems and systems. Switching power supply, like other electronic equipment, to achieve electromagnetic compatibility, safe and stable work, we must first solve the problem of its own electromagnetic interference source. At the same time, we should also consider not being disturbed by other electronic equipment of the system. 4. Hammer handle strength tester: disturbance. From the three elements of electromagnetic interference, we can solve the problem of electromagnetic compatibility of switching power supply from three aspects

3.1 reduce the interference signal generated by the interference source

3.1.1 application of power factor correction (PFC) technology

in order to solve the input current waveform distortion and reduce the current harmonic content, the switching power supply needs to use power factor correction (PFC) technology. PFC technology makes the current waveform follow the voltage waveform, and corrects the current waveform into an approximate sine wave. Thus, the current harmonic content is reduced, the input characteristics of the bridge rectifier capacitor filter circuit are improved, and the power factor of the switching power supply is also improved

3.1.2 application of soft switching technology

soft switching technology is an important method to reduce the loss of switching devices and improve the electromagnetic compatibility characteristics of switching devices. Surge current and peak voltage will be generated when switching devices are turned on and off, which is the main reason for electromagnetic interference and switching loss of switches. Using soft switching technology to make the switch switch switch at zero voltage and zero current can effectively suppress electromagnetic interference. Using a buffer circuit to absorb the peak voltage at both ends of the switch tube or the primary coil of a high-frequency transformer can also effectively improve the electromagnetic compatibility characteristics

3.1.3 principle of series inductance application

the reverse recovery of the output rectifier diode can be suppressed by connecting a saturated inductor in series with the output rectifier tube. As shown in Figure 3, the saturated inductor LS works in series with the diode. The magnetic core of saturated inductance is made of magnetic material with rectangular BH curve. Like the materials used in magnetic amplifiers, the inductance made of this magnetic core has high permeability. This magnetic core has a nearly vertical linear region on the BH curve and is easy to enter saturation. In practice, when the output rectifier diode is turned on, the saturated inductance works in the saturated state, which is equivalent to a section of wire; When the diode is turned off and restored in reverse, the saturated inductor works in the inductance characteristic state, which hinders the large change of the reverse recovery current, and thus suppresses its external interference

Figure 3 application of saturated inductance in reducing diode reverse recovery current

3.2 cut off the propagation path of interference signal

3.2.1 common mode and differential mode power line filter design

power supply

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