Mechanism of EMI generation and its suppression in

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The mechanism of EMI generation in high frequency switching converter and its suppression methods

1 preface

switching power supply has the characteristics of small volume, light weight and high efficiency. It is widely used in communication, automatic control, household appliances, computers and other electronic equipment. However, its disadvantage is that the switching power supply works under high frequency conditions, which produces very strong electromagnetic interference (EMI), which will pollute the surrounding electromagnetic environment and affect the electronic equipment through conduction and radiation. This paper analyzes the circuit structure and devices of switching power supply, and discusses the mechanism of electromagnetic interference and its suppression methods

2 mechanism of electromagnetic interference (EMI) of switching power supply

electromagnetic interference of switching power supply can be divided into conducted interference and radiated interference according to the coupling path. According to the noise interference sources, it can be divided into two categories: one is external noise, such as common mode and differential mode interference transmitted by electricity, and the interference of external electromagnetic radiation on the control circuit of switching power supply; The other is the electromagnetic interference generated by the switching power supply itself, such as the harmonic and electromagnetic radiation interference generated by the current spikes of switching tubes and rectifier tubes. The influence of external noise can be attenuated by power filter. This paper will not discuss it, but only the electromagnetic noise generated by switching power supply itself

the main structure of switching power supply with conventional AC input can be divided into four parts, and its block diagram is shown in Figure 1

among them, the input and rectification filtering part, the high-frequency inverter part, and the output rectification and filtering part are the main sources of electromagnetic interference. In the following, the causes of electromagnetic noise will be clarified by analyzing the voltage and current waveforms of each part

2.1 electromagnetic noise caused by power frequency rectifier

generally, the switching power supply is capacitive filtering. The electromagnetic noise in the input and rectification filtering part is mainly caused by current spikes and voltage fluctuations caused in the rectification process. After the sine wave power supply attenuates the differential mode and common mode signals through the power filter, it is rectified by the rectifier bridge and filtered by the electrolytic capacitor. The voltage obtained is used as the input voltage of the high-frequency inverter. Due to the existence of filter capacitor, the rectifier is not opened for half a cycle as a group of pure rectifiers, but only when the sinusoidal voltage is higher than the capacitor voltage, resulting in very steep current waveform and gentle voltage waveform. The waveforms of current and voltage are shown in Figure 2

according to the Fourier series, the current and voltage waveforms in the figure can be decomposed into the sum of DC components and a series of sinusoidal AC components whose frequency is an integral multiple of the fundamental frequency. The electromagnetic field theory and the test results show that harmonics (especially high-order harmonics) will produce conducted interference and radiated interference. The interference generated through the input and output lines of switching power supply is called conducted interference, and the interference generated by the external radiation of electric field and magnetic field in space is called radiated interference

2.2 electromagnetic noise caused by transformer and switch tube

inverter part is the core of switching regulated power supply, which is used to realize voltage transformation, frequency conversion and complete the adjustment of output voltage. It is mainly composed of switch tube and high-frequency transformer. The electromagnetic noise is mainly caused by the leakage inductance, distributed capacitance of the transformer and the opening and closing of the switch tube. The high-frequency transformer in switching power supply is used for isolation and voltage transformation. In theoretical analysis, the transformer is generally considered as an ideal transformer, but in practical application, the transformer has leakage inductance, and in the case of high frequency, the distributed capacitance between transformer layers should be considered. The equivalent circuit model of high frequency transformer is shown in Figure 3

it can be seen from the figure that the distributed capacitance between transformer layers makes it easy for high-frequency noise in switching power supply to be transmitted between primary stages. Moreover, if the filter capacity of the capacitor is insufficient or the high-frequency characteristic is not good, the high-frequency impedance on the capacitor will cause the high-frequency current to be transmitted to the AC power supply through the parasitic capacitance of the transformer in a differential mode

the fundamental reason for the volume and weight reduction of switching power supply is that the power semiconductor devices work in the high-frequency switching state, but the result is very serious electromagnetic interference. The reason is that high di/dt and dv/dt are produced in the working process, as well as high-frequency oscillation between transformer leakage inductance, circuit parasitic inductance and switch parasitic capacitance. Most of the voltage waveforms in switching power supply are near rectangular periodic waves, such as the driving waveform of switching transistor, MOSFET drain source voltage waveform, etc. The frequency is high, generally above kHz, the rise and fall time is short, and the dv/dt is large. Moreover, after Fourier expansion, the harmonic frequency is very high, which is easy to pollute the surrounding electromagnetic environment

switching tubes (such as MOSFETs) can also cause strong electromagnetic interference when they are turned on and off. Due to the leakage inductance of the primary coil of the transformer and the parasitic inductance of the circuit, part of the energy is not transmitted from the primary side to the secondary side. The energy is stored in the leakage inductance, and the current changes suddenly at the moment of turning off. The di/dt is very high, resulting in back EMF. According to the theory of effective stroke of electromagnetic field theory: 400 mm, e=-ldi/dt. Its value is proportional to the change of current and inductance. Therefore, the leakage inductance will generate very high back EMF superimposed on the turn-off voltage, forming a turn-off voltage spike and generating conductive electromagnetic interference. The parasitic capacitance between the leakage inductor and the switch tube will also vibrate, which will affect the electromagnetic environment in the circuit. It is assumed that the problem can not be solved, resulting in noise. When the switch is turned on, the parasitic capacitor will discharge instantaneously, resulting in peak current. The primary coil will also cause surge current, affecting the electromagnetic environment

2.3 electromagnetic noise caused by reverse recovery of output rectifier diode

when the diode bears the reverse voltage, the charge accumulated in the PN junction will release and form a reverse current. The amplitude, pulse width and shape of the reverse recovery current pulse are related to the characteristics of the diode and circuit parameters, and the time to recover to zero is related to the junction capacitance and other factors. Due to the large amplitude and di/dt of the reverse recovery current pulse of high-frequency rectifier diodes, they will produce high induced voltage in the lead inductance and the circuit connected with them, resulting in strong broadband transient electromagnetic noise. The voltage and current waveforms during diode reverse recovery are shown in Figure 4

fast recovery diodes are used in converter circuits with high repetitive switching frequency, such as high-frequency switching power supply, high-frequency dc/dc resonant converter and power factor correction circuit. Their reverse recovery PCU is more resistant to degradation, and the degradation time is usually in the order of nanoseconds, so the transient electromagnetic noise caused by the lead inductance can not be ignored. Especially in the flyback switching power supply, the diode reverse recovery current peak may also be transferred from the secondary to the primary. When the switch is turned on, a current peak will be formed, which is not only easy to burn the switch tube, but also cause electromagnetic noise

3 suppression measures for electromagnetic interference (EMI) of switching power supply

the three elements of electromagnetic interference are interference source, propagation path and disturbed equipment. Therefore, the suppression of electromagnetic interference should also start from these three aspects. Firstly, the interference source should be suppressed and the interference cause should be eliminated directly; Secondly, eliminate the coupling and radiation between the interference source and the disturbed equipment, and cut off the propagation path of electromagnetic interference; The third is to improve the anti-interference ability of the disturbed equipment and reduce its sensitivity to noise. At present, several measures to suppress interference are basically to cut off the coupling channel between the electromagnetic interference source and the disturbed equipment. The common methods are shielding, grounding and filtering. These methods have been proved to be effective in practice. This paper introduces a feasible technology to improve the circuit and directly control the interference source

the application of soft switching technology has greatly improved the efficiency of power supply and made great contributions to energy saving. However, in some circuit topologies, the application of soft switching technology also greatly reduces electromagnetic interference. The quasi resonant flyback converter is the best example. The circuit structure is shown in Figure 5

compared with the general flyback converter, the quasi resonant only adds a passive component capacitor to the original circuit, which will not produce excessive electromagnetic noise in the circuit. By changing the control mode, the resonance between the primary inductance and the capacitor of the transformer is used to open at the trough of the voltage waveform of the switch; The capacitor is used for buffering when the switch is off in 2012, which can greatly reduce the off voltage spike and on current spike on the switch tube, so as to reduce electromagnetic interference. The quasi resonant flyback switching power supply made by ansenmey's NCP1207 has the voltage waveform on the switch tube as shown in Figure 6:

it can be seen from the figure that the voltage of the switch tube is very low when it is turned on, which is conducive to reducing the current spike. When it is turned off, the voltage spike is small, thus reducing the electromagnetic interference

4 conclusion

with the continuous high frequency of switching power supply, its electromagnetic interference becomes more and more important. In the development and design of switching power supply, how to effectively suppress the electromagnetic interference of switching power supply and improve the anti-interference ability of switching power supply to electromagnetic interference (EMC) is an important topic. Therefore, there is still a lot of work to be done to suppress the electromagnetic interference of switching power supply, which requires the unremitting efforts of all engineering and technical personnel

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