Analysis and Suppression of Electromagnetic Interference of a New Power Converter

Preface:

Power supply is a power conversion system, which is an indispensable component for all electronic equipment. As the source of system energy, the quality of its output voltage and current has a global and decisive impact on the stability, reliability and electromagnetic compatibility (EMC) of the electronic equipment in which it is located! Switching power supply (SMPS) has been widely used in most electronic equipment, but the high-density, wide-spectrum electromagnetic signals generated by the power supply itself have formed serious electromagnetic interference. These interferences are significantly enhanced with the increase of switching frequency and output power, forming a strong electromagnetic interference (EMI) source, which poses a serious threat to the normal operation of electronic equipment. On the contrary, the complex electromagnetic environment requires electronic equipment to have higher electromagnetic compatibility. Electromagnetic compatibility refers to the coexistence of various electrical equipment within a limited space, time and spectrum without causing performance degradation. Electromagnetic compatibility issues have become an issue of increasing concern in various fields and even governments of various countries! Therefore, only by solving the electromagnetic compatibility problem of switching power supply can its output accuracy, efficiency and stability be guaranteed, and its compliance with EMC regulatory indicators can be guaranteed to provide safe, sophisticated and reliable energy for its electronic system. This paper first determines and analyzes the overall EMI situation of the switching power supply, including interference sources, coupling paths and sensitive circuits, in combination with a parallel resonant voltage doubler converter, and at the same time explains the mechanism of EMI generation in the switching power supply. On this basis, the implementation of the EMC design of the switching power supply and the technical links that should be emphasized in the EMC design process are proposed. Finally, methods and ideas for solving the electromagnetic compatibility problems of the switching power supply and the EMI suppression technology of the switching power supply are proposed and summarized.

Figure 1: Basic components of an SMPS

1. The basic structure of SMPS:

As shown in Figure 1, AC power enters the core part of the power supply through the rectifier bridge - the DC/DC converter for power conversion. In addition, there are startup, overcurrent and overvoltage protection, noise filtering and other circuits, which can be collectively referred to as control circuits. The output sampling circuit (R1, R2) detects the output voltage change and compares it with the reference voltage Uf. The error voltage is amplified and pulse width modulation (PWM) circuit, and then controls the duty cycle of the power device through the drive circuit, thereby achieving the purpose of adjusting the output voltage.

2. Analysis of switching power supply EMI:

There are many reasons for the generation of EMI in switching power supplies, and its internal electromagnetic compatibility problems are also relatively complex, including EMC problems of the main circuit and the control circuit, EMI of the main circuit and electromagnetic susceptibility (EMS) of the control circuit, EMC problems between the main circuit and the power grid, the main circuit and the load, and power electronic equipment, 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 reasons. The EMI characteristics of the switching power supply are relatively obvious. Its voltage and current change rate is very high, and the dv/dt and di/dt in the power supply line are large, which generates large surge voltage, surge current and other stray noise, and radiates strong electric and magnetic fields outward; the main forms of interference are conducted interference and near-field interference; the interference sources are mainly concentrated in the power switching devices and the heat sinks and high-frequency transformers connected to them, and the ground current is serious; the extraction of PCB distributed parameters and the estimation of near-field interference are relatively difficult. Now, taking the SMPS structure (Figure 1) and the parallel resonant voltage doubler converter (Figure 2) as examples, the main locations and interference mechanisms of the interference sources of the switching power supply EMI are introduced.

Figure 2: Schematic diagram of EMI for a DC/DC power converter

1. Noise of input rectifier circuit:

As shown in the primary rectification circuit in Figure 2, the rectification process of the basic rectifier is the most common cause of EMI. This is because the sine wave power becomes a unidirectional pulsating current after passing through the rectifier B composed of D1 to D4, which is no longer a single-frequency current. This current wave can be decomposed into a DC component and a sum of a series of AC components with different frequencies. Experiments have shown that harmonics (especially high-order harmonics) will generate conducted interference and radiated interference along the transmission line, which will distort the current waveform connected to the front-end power line on the one hand, and generate radio frequency interference through the power line on the other hand.

2. Noise of the switching circuit:

The transformer type power conversion circuit is used to achieve voltage conversion, frequency conversion and output voltage adjustment. It is the core of the switching voltage stabilizer and is mainly composed of switch tubes Q1, Q2 and high-frequency transformer T. The peak voltage it generates is a narrow pulse with a large amplitude, a wide frequency band and rich harmonics. The main reasons for this pulse interference are:

(1) The inductive load of the switching power transistor is a high-frequency transformer or energy storage inductor. At the moment when the switch is turned off, a large surge current appears in the primary of the transformer T, which will cause spike noise. This spike noise is actually a sharp pulse, which may cause interference in mild cases and may break down the switch in severe cases.

(2) Interference generated by high-frequency transformers: When the originally turned-on switch is turned off, the back electromotive force generated by the leakage inductance of the transformer is: E = -Ldi/dt, which is proportional to the collector current change rate (di/dt) and the leakage inductance. It is superimposed on the turn-off voltage to form a turn-off voltage spike, forming a conductive electromagnetic interference, which not only affects the primary of the transformer, but also transmits to the distribution system, affecting the safe and economical operation of other electrical equipment.

3. Noise of output rectifier circuit:

As shown in Figure 2, the secondary rectification circuit is the interference generated by the output rectifier diode. In Figure 2, when the output rectifier diodes D6 and D7 are cut off, there is a reverse current, and the time it takes to recover to zero is related to factors such as junction capacitance. The diode that can quickly recover the reverse current to zero is called a hard recovery diode. Under the influence of transformer leakage inductance and other distributed parameters, this diode will generate strong high-frequency interference, and its frequency can reach tens of MHz.

4. Non-main circuit noise:

The non-main circuit is the circuit outside the main circuit, including the input and output control circuit, etc., generally referring to the part other than the input and DC/DC converter in Figure 1, among which the pulse control signal of the PWM part is the main noise source. The input circuit is easily affected by the power grid, while the output circuit is easily affected by the load, and both are easy to couple noise into the switching power supply.

5. Noise caused by parasitic distributed parameters of various components and circuits:

As shown in Figure 3, within the frequency range of EMI, commonly used passive components can no longer be considered ideal, and their parasitic parameters seriously affect their high-frequency characteristics. In particular, many parasitic parameters of the transformer, such as leakage inductance, distributed capacitance between the primary and secondary sides, etc., must be considered. In Figure 4, the first is the role of Co. Although there is an insulating gasket between the heat sink k and the collector of the switch tube Q, due to the large contact surface and thin insulating gasket, the distributed capacitance Co between the two cannot be ignored at high frequencies. Therefore, the high-frequency current will flow to the heat sink through Co, then to the chassis ground, and finally to the protective ground wire de of the AC power supply connected to the chassis ground to generate common-mode radiation. The second is the role of C12. The distributed capacitance C12 between the primary and secondary of the pulse transformer may directly couple the high-frequency voltage of the primary side to the secondary side, generating common-mode noise of the same phase on the two power lines used as DC output on the secondary side.

Figure 3: Parasitic capacitance in transformer high frequency circuits Figure 4: Parasitic capacitance in switch circuits

6. Other factors affecting the EMC performance of switching power supply:

In addition to the above components and circuits, the grid conditions, load form and EMI strength of the power supply environment are also important factors affecting the EMC performance of the switching power supply. Taking the load as an example, as the load increases, the |dv/dt| value generated by the switch tube Q turning off increases, while the load change has little effect on the |dv/dt| when turning on. Since the |dv/dt| generated when turning on and off is different, the external interference pulses are also different. In addition, the PCB wiring and component layout are also very critical factors. Practice has proved that the component layout and wiring design of the printed circuit board have a great influence on the EMC performance of the switching power supply. In the high-frequency switching power supply, since there are both low-level small signal control lines and high-voltage power busbars on the printed circuit board, as well as some high-frequency power switches and magnetic components, how to reasonably arrange the component positions within the limited space of the printed circuit board will directly affect the anti-interference performance of each component in the circuit and the reliability of the circuit operation.

7. Sensitive circuit of switching power supply:

Electromagnetic susceptibility (EMS) refers to the ability of a device, equipment or system to avoid performance degradation in the presence of electromagnetic interference. In a switching power supply, each circuit and component is both an interference source and a sensitive circuit. However, in comparison, the input and output terminals and control circuits of the switching power supply are more susceptible to EMI and cause more serious chain reactions. Therefore, these circuits should be protected, while interference sources should be suppressed.

3. Implementation, ideas and methods of switching power supply EMC design:

The mechanism of internal and external interference generation and coupling of switching power supplies is relatively complex, involving many factors. To solve the EMC problem of switching power supplies, it is necessary to analyze the specific circuits and avoid blindly taking measures such as grounding, shielding, and filtering. According to the characteristics of EMI of switching power supplies, the main interference factors (including interference sources, coupling paths, and susceptible circuits) should be determined, and targeted measures should be taken. Specifically, the EMC design of switching power supplies should be carried out in accordance with the idea of ​​suppressing the energy of interference sources, destroying the coupling channels of interference, and focusing on protecting sensitive circuits, keeping a good control on the input and output of the power supply, and specifically considering the following aspects:

1. Suppress interference sources

Suppressing interference sources is the basis for suppressing EMI of switching power supplies and is an effective way to reduce the EMI of switching power supplies below the specified limit.

(1) Reduce the disturbance generated during the on and off process of the power tube

As mentioned above, the main disturbance of the switching power supply comes from the dv/dt of the power switch tube on and off. Therefore, reducing the dv/dt of the power switch tube on and off is an important aspect of reducing the disturbance of the switching power supply. Soft switching technology can reduce the dv/dt of the switch tube on and off to a certain extent. The experimental results of studying the EMI characteristics of various converters and the influence of buffer circuits, clamping circuits, variable frequency and fixed frequency control on the interference level show that the EMI level of the zero voltage fixed frequency switching converter with voltage clamping is the lowest. Therefore, the use of soft switching power supply technology, combined with reasonable component layout and reasonable printed circuit board wiring, can improve the EMI level of the switching power supply to a certain extent.

(2) Switching frequency modulation technology

Change the modulation with constant frequency into random modulation and variable frequency modulation. The interference generated by fixed frequency modulation pulse in the low frequency band is mainly the harmonic interference of the modulation frequency, and the interference in the low frequency band is mainly concentrated at each harmonic point. The basic idea of ​​the method proposed by F. Lin [3] is to disperse the energy concentrated on fc and its harmonics 2fc, 3fc, etc. to the frequency bands around them by modulating the switching frequency fc, so as to reduce the emd amplitude at each frequency point. This method cannot reduce the total interference, but the energy is dispersed to the baseband of the frequency point, so that each frequency point does not exceed the limit specified by emd.

2. Cut off the coupling channel

In order to achieve this goal, the main considerations are to select the appropriate switching power supply circuit topology; adopt correct grounding, shielding, and filtering measures; design reasonable component layout and printed circuit board wiring. For example, adding filters to the input and output ends of the power supply can effectively block the coupling channel of EMI. Installing a power supply filter at the AC input end can suppress differential mode noise and common mode noise. The filter should be grounded because the common mode bypass capacitor of the filter must be grounded to work. The filter can be connected to the metal casing or the filter casing can be connected to the grounding point of the equipment with a thicker wire. The lower the grounding impedance, the better the filtering effect. The filter should be installed as close to the power inlet as possible. The input and output ends of the filter should be as far away as possible to avoid interference signals from being directly coupled from the input end to the output end.

3. Protect sensitive circuits

The interference source and the interfered object are located on the same circuit board. The related components should be placed together as much as possible to avoid interference caused by long printed lines due to the components being too far away. The area of ​​the switch loop should be minimized and the control loop should be placed away from its radiation range. In addition, for the switching power supply, the main thing is to do a good job of housing shielding, high-frequency transformer shielding, switch tube and rectifier diode shielding. This is also a very good way to protect sensitive circuits by shielding interference circuits.

4. Switching power supply EMI suppression technology:

In addition to the commonly used EMI suppression technologies such as shielding, grounding, and filtering, some new technologies are used in view of the characteristics of switching power supplies, such as soft switching technology, power tube optimization drive technology, EMI filter design technology, common mode interference active suppression technology, and the switching frequency modulation technology mentioned above. However, no matter what technology is used, the ultimate goal is to suppress EMI, and the fundamental point is to understand the essence of the EMI problem of switching power supplies. The EMI suppression technology of switching power supplies is still in its infancy. While designing new interference suppression technologies, it is also a good method to comprehensively use existing methods and technologies.

Five: Conclusion:

The EMC design of switching power supplies is a complex subject. It should combine design ideas and methods with specific technologies. On the basis of analyzing the location, mechanism, intensity and other related factors of the EMI generated by the switching power supply, an effective EMI analysis and design model should be established, and various means should be used in combination to solve it. The purpose is to make the power supply product meet the immunity limit value requirements specified in the corresponding EMC standards, and there will be no performance degradation or failure when subjected to certain electromagnetic disturbances; at the same time, it must meet the electromagnetic limit value requirements specified in the relevant EMC standards, not constitute a pollution source for the electromagnetic environment, and achieve electromagnetic compatibility. The EMC design of the switching power supply needs to be taken seriously and given enough attention by the power supply development engineers. This link must be put into the design and development stage of the product, otherwise the price paid will be huge.