Analysis and Suppression of Electromagnetic Interference of Switching Power Supply

　　In recent years, switching power supplies have developed rapidly due to their advantages of high efficiency, small size, and good output stability. However, due to the high frequency, high di /d t and high d v /d t during the operation of the switching power supply, the problem of electromagnetic interference is very prominent. China has replaced CCIB and CCEE certification with the new 3C certification, making the requirements for electromagnetic compatibility of switching power supplies more detailed and strict. Nowadays, how to reduce or even eliminate the EMI problem of switching power supplies has become a matter of great concern to switching power supply designers and electromagnetic compatibility (EMC) designers around the world. This article discusses the causes of electromagnetic interference in switching power supplies and commonly used EMI suppression methods.

1 Analysis of interference sources of switching power supply

The most fundamental reason for the electromagnetic interference generated by the switching power supply is the high di /d t and high d v /d t　　generated during its operation . The surge current and peak voltage they generate form an interference source. The charging and discharging of large capacitors used in power frequency rectification and filtering, the voltage switching of switching tubes during high-frequency operation, and the reverse recovery current of output rectifier diodes are all sources of such interference. Most of the voltage and current waveforms in switching power supplies are close to rectangular periodic waves, such as the driving waveform of the switching tube, the drain-source waveform of the MOSFET, etc. For rectangular waves, the reciprocal of the period determines the fundamental frequency of the waveform; the reciprocal of twice the pulse edge rise time or fall time determines the frequency value of the frequency components caused by these edges. Typical values ​​are in the MHz range, and its harmonics The frequency is higher. These high-frequency signals cause interference to the basic signals of the switching power supply, especially the signals of the control circuit.

　　The electromagnetic noise of switching power supplies can be divided into two major categories in terms of noise sources. One type is external noise, such as common mode and differential mode noise transmitted through the power grid, interference from external electromagnetic radiation to the switching power supply control circuit, etc. The other type is the electromagnetic noise generated by the switching power supply itself, such as the harmonics and electromagnetic radiation interference generated by the current spikes of the switching tube and rectifier tube.

　　As shown in Figure 1, the common mode and differential mode noise contained in the power grid interferes with the switching power supply. While the switching power supply is subject to electromagnetic interference, it also causes electromagnetic interference to other equipment and loads in the power grid (return noise, output noise in the figure) and radiated interference). When designing the EMI/EMC of the switching power supply, on the one hand, it is necessary to prevent the switching power supply from interfering with the power grid and nearby electronic equipment, and on the other hand, it is necessary to strengthen the adaptability of the switching power supply itself to the electromagnetic disturbance environment. The following is a detailed analysis of the causes and ways of switching power supply noise.

Figure 1 Switching power supply noise type diagram

1.1 Electromagnetic noise introduced by power lines

　　Power line noise is caused by electromagnetic disturbance generated by various electrical equipment in the power grid propagating along the power line. Power line noise is divided into two categories: common mode interference and differential mode interference. Common-mode Interference is defined as the undesired potential difference between any current-carrying conductor and the reference ground; Differential-mode Interference is defined as the undesired potential difference between any two current-carrying conductors. There is a potential difference. The equivalent circuits of the two interferences are shown in Figure 2[1]. In the figure , C _P1 is the distributed capacitance between the primary and secondary of the transformer, and C _P2 is the distributed capacitance between the switching power supply and the radiator (that is, the distributed capacitance between the collector of the switching tube and the ground).

(a) Common mode interference

(b) Differential mode interference

Figure 2 Equivalent circuits of two interferences

　　As shown in Figure 2(a), when the switch V1 changes from on to off state, its collector voltage suddenly rises to a high voltage. This voltage will cause the common mode current I _cm2 to charge C _P2 and the common mode current I _cm1 When C _P1 is charged, the charging frequency of the distributed capacitor is the operating frequency of the switching power supply. Then the total common mode current in the line is ( I _cm1 + I _cm2 ). As shown in Figure 2(b), when V ₁ is turned on, the differential mode current I _dm and the signal current _IL flow along the loop composed of the wire, the transformer primary, and the switching tube. It can be seen from the equivalent model that the common-mode interference current does not pass through the ground wire, but is transmitted through the input power line. The differential mode interference current is transmitted through the ground wire and input power line loop. Therefore, when we set up the power line filter, we must consider the difference between differential mode interference and common mode interference, and use differential mode or common mode filter components on its transmission path to suppress their interference to achieve the best filtering effect.

1.2 Noise caused by input current distortion

　　The input of switching power supply generally adopts bridge rectifier and capacitor filter type rectifier power supply. As shown in Figure 3, in the input stage without PFC function, due to the nonlinearity of the rectifier diode and the energy storage effect of the filter capacitor, the conduction angle of the diode becomes smaller, and the input current i becomes a short-term, high-peak value. Periodic spike current. This distorted current actually contains abundant high-order harmonic components in addition to the fundamental component. These high-order harmonic components are injected into the power grid, causing serious harmonic pollution and causing interference to other electrical appliances on the power grid. In order to control the pollution of the switching power supply to the power grid and achieve high power factor, the PFC circuit is an indispensable part.

Figure 3 Input current and voltage waveforms without PFC circuit

1.3 Interference caused by switching tubes and transformers

　　The main switch tube is the core component of the switching power supply and is also a source of interference. Its operating frequency is directly related to the intensity of electromagnetic interference. As the operating frequency of the switching tube increases, the switching speed of the voltage and current of the switching tube accelerates, and its conduction interference and radiation interference also increase. In addition, the anti-parallel clamping diode on the main switch tube has poor reverse recovery characteristics, or the parameters of the voltage spike absorption circuit are improperly selected, which can also cause electromagnetic interference.

　　During the operation of the switching power supply, a high-frequency current loop is formed by the large primary filter capacitor, the primary coil of the high-frequency transformer and the switching tube. This loop will produce large radiated noise. The load of the switching tube in the switching circuit is the primary coil of the high-frequency transformer, which is an inductive load. Therefore, when the switching tube is turned on and off, peak noise will appear at both ends of the primary side of the high-frequency transformer. The light one will cause interference, and the serious one will break down the switch tube. The distributed capacitance and leakage inductance between the main transformer windings are also important factors causing electromagnetic interference.

1.4 Interference caused by output rectifier diode

　　An ideal diode cuts off when subjected to a reverse voltage, and no reverse current flows through it. When the actual diode is forward-conducting, the charge in the PN junction is accumulated. When the diode is subjected to a reverse voltage, the charge accumulated in the PN junction will be released and form a reverse recovery current. The time it takes to return to zero depends on the junction capacitance, etc. factors related. The reverse recovery current will produce strong high-frequency attenuated oscillation under the influence of transformer leakage inductance and other distribution parameters. Therefore, the reverse recovery noise of the output rectifier diode has also become a major source of interference in switching power supplies. The reverse recovery noise can be suppressed by connecting an RC buffer in parallel across the diode .

1.5 Switching power supply noise caused by distribution and parasitic parameters

　　The distribution parameters of the switching power supply are the intrinsic factors of most interferences. The distributed capacitance between the switching power supply and the radiator, the distributed capacitance between the primary and secondary sides of the transformer, and the leakage inductance of the primary and secondary sides are all sources of noise. Common mode interference is transmitted through the distributed capacitance between the primary and secondary of the transformer and the distributed capacitance between the switching power supply and the radiator. The distributed capacitance of the transformer winding is related to the high-frequency transformer winding structure and manufacturing process. The distributed capacitance between windings can be reduced by improving the winding process and structure, increasing the insulation between windings, and using Faraday shielding. The distributed capacitance between the switching power supply and the radiator is related to the structure of the switching tube and the installation method of the switching tube. Using a shielded insulating pad can reduce the distributed capacitance between the switch tube and the heat sink.

　　As shown in Figure 4, components operating at high frequencies have high-frequency parasitic characteristics [2], which affects their working status. When working at high frequency, the wire becomes a emission line, the capacitor becomes an inductor, the inductor becomes a capacitor, and the resistor becomes a resonant circuit. Observing the frequency characteristic curve in Figure 4, we can find that when the frequency is too high, the frequency characteristics of each component undergo considerable changes. In order to ensure the stability of the switching power supply when working at high frequency, the characteristics of the components at high frequency must be fully considered when designing the switching power supply, and components with better high frequency characteristics must be selected. In addition, at high frequencies, the inductive reactance of the wire's parasitic inductance increases significantly, and due to the uncontrollable nature of the inductance, it eventually becomes a emission line. It also becomes a source of radiation interference in the switching power supply.

Figure 4 Component frequency characteristics under high frequency operation

2 Switching power supply EMI suppression measures

　　The three elements of electromagnetic compatibility are interference sources, coupling paths and sensitive bodies. Suppressing any of the above can reduce electromagnetic interference problems. When the switching power supply operates in a high-frequency switching state with high voltage and large current, the electromagnetic compatibility issues caused by it are relatively complex. However, it still conforms to the basic electromagnetic interference model, and we can start from three elements to find ways to suppress electromagnetic interference.

2.1 Suppress various electromagnetic interference sources in switching power supplies

　　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. This reduces the current harmonic content, improves the input characteristics of the bridge rectifier capacitor filter circuit, and also improves the power factor of the switching power supply.

　　Soft switching technology is an important method to reduce the loss of switching devices and improve the electromagnetic compatibility characteristics of switching devices. When switching devices are turned on and off, surge currents and peak voltages will be generated, which are the main causes of electromagnetic interference and switching losses in switching tubes. The use of soft switching technology to enable switching tubes to perform switching transitions at zero voltage and zero current can effectively suppress electromagnetic interference. Using a snubber circuit to absorb the peak voltage across the primary coil of a switching tube or high-frequency transformer can also effectively improve electromagnetic compatibility characteristics.

　　The reverse recovery problem of the output rectifier diode can be suppressed by connecting a saturated inductor in series with the output rectifier diode. As shown in Figure 5, the saturated inductor L _s works in series with the diode. The core of the saturable inductor is made of magnetic material with a rectangular BH curve. Like the materials used in magnetic amplifiers, the inductor made of this type of magnetic core has a high magnetic permeability. This type of magnetic core has a nearly vertical linear region on the BH curve and can easily enter saturation. In actual use, when the output rectifier diode is turned on, the saturated inductor is made to work in a saturated state, which is equivalent to a section of wire; when the diode is turned off and reverse recovery is performed, the saturated inductor is made to work in an inductive characteristic state, hindering reverse recovery. The current changes significantly, thus suppressing its external interference.

Figure 5 Application of saturated inductance in reducing diode reverse recovery current

2.2 Cutting off the transmission path of electromagnetic interference - design of common mode and differential mode power line filters

　　Power line interference can be filtered using a power line filter. The basic circuit of the switching power supply EMI filter is shown in Figure 6. A reasonable and effective switching power supply EMI filter should have a strong suppression effect on differential mode interference and common mode interference on the power line. In Figure 6, C _X1 and C _X2 are called differential mode capacitors, L ₁ is called a common mode inductor, and C _Y1 and C _Y2 are called common mode capacitors. Differential mode filter components and common mode filter components have strong attenuation effects on differential mode and common mode interference respectively.

　　Common mode inductor L ₁ is composed of two windings with opposite winding directions and the same number of turns on the same magnetic ring. Ring cores are usually used, which have small magnetic leakage and high efficiency, but are difficult to wind. When the mains power frequency current flows through the two windings, one in and one out, the generated magnetic field offsets exactly, so that the common mode inductor does not have any hindrance to the mains power frequency current and can be transmitted without loss. If the city network contains common-mode noise currents passing through the common-mode inductor, the common-mode noise currents are in the same direction. When flowing through the two windings, the magnetic fields generated are superimposed in phase, making the common-mode inductor have a greater impact on the interference current. Inductive reactance thus plays a role in suppressing common mode interference. The inductance of L _{1 is related to the rated current} I of the EMI filter . See Table 1 for the specific relationship.

Table 1 Relationship between inductance range and rated current [4]

　　　　　Rated current I /A Inductance L /mH
　　　　　　1 8～23　　　　　　　　　
　　　　　　3 2～4
　　　　　　6 0.4～0.8 　　　　　　　　　　
　　　　　　10 0.2～0.3 　　　　　　　　　　
　　　　　　12 0.1～0.15 　　　　　　　　　　
　　　　　　15 0.0～0.08

　　In actual use, there will be an inductance difference between the two inductor windings of a common mode inductor due to problems with the winding process, but this difference is just used as a differential mode inductor. Therefore, there is no need to set up an independent differential mode inductor in the general circuit. The difference inductance of the common mode inductor and the capacitors C _X1 and C _X2 form a ∏ filter. This filter has better attenuation of differential mode interference.

In addition to the common mode inductor, the capacitors C _Y1 and C _Y2　　in Figure 6 are also used to filter out common mode interference. The attenuation of the common-mode filter is mainly caused by the inductor at low frequencies, and mostly by the capacitors C _Y1 and C _Y2 at high frequencies . The selection of capacitor C _Y should be based on the actual situation. Since the capacitor C _Y is connected between the power line and the ground line and can withstand a relatively high voltage, it needs to have high withstand voltage and low leakage current characteristics. The formula for calculating the leakage current of capacitor C _Y is

　　I _D =2π fC _Y V _cY

In the formula: I _D is the leakage current;

　　f is the grid frequency.

　　Generally, the AC leakage current of a filter installed on movable equipment should be <1mA; if it is a power filter installed on a fixed position and grounded equipment, the AC leakage current should be <3.5mA, as specified for medical equipment. Leakage current is smaller. Due to safety regulations regarding leakage current, the size of the capacitor C _Y is limited, generally 2.2 to 33nF. The capacitor type is generally a ceramic capacitor. When using it, attention should be paid to the resonance effect of the capacitor C _Y and the lead inductance during high-frequency operation .

　　Differential mode interference suppressors are usually composed of low-pass filter components. The simplest one is an input filter circuit formed by connecting a filter capacitor between two power lines (capacitor C _X1 in Figure 6 ). As long as the capacitor is appropriately selected, It can suppress high-frequency interference. This capacitor has very low impedance to high-frequency interference, so the high-frequency interference between the two power lines can pass through it. Its impedance to the power frequency signal is very high, so it has no effect on the transmission of the power frequency signal. The selection of this capacitor mainly considers the withstand voltage value, as long as it meets the withstand voltage level of the power line and can withstand predictable voltage shocks. In order to avoid impact hazards caused by discharge current , _the C The capacitor type is ceramic capacitor or polyester film capacitor.

Figure 6 Switching power supply EMI filter[3]

2.3 Use shielding to reduce the sensitivity of electromagnetic sensitive equipment

　　An effective way to suppress radiated noise is shielding. Materials with good electrical conductivity can be used to shield the electric field, and materials with high magnetic permeability can be used to shield the magnetic field. In order to prevent the magnetic field of the transformer from leaking and ensure good primary and secondary coupling of the transformer, a closed magnetic ring can be used to form a magnetic shield. For example, the leakage flux of the pot-type magnetic core is significantly smaller than that of the E-type. The connecting wires of switching power supplies and power cords should use shielded wires to prevent external interference from coupling into the circuit. Or use EMC components such as magnetic beads and magnetic rings to filter out high-frequency interference from power supply and signal lines. However, it is important to note that the signal frequency cannot be interfered by EMC components, that is, the signal frequency must be within the passband of the filter. The shell of the entire switching power supply also needs to have good shielding properties, and the joints must comply with the shielding requirements specified by EMC. The above measures ensure that the switching power supply is neither interfered by the external electromagnetic environment nor interferes with external electronic equipment.

3 Conclusion

　　Nowadays, the size of switching power supplies is getting smaller and smaller, and the power density is getting larger and larger. EMI/EMC issues have become a key factor in the stability of switching power supplies and are also the most easily overlooked aspect. The EMI suppression technology of switching power supply occupies a very important position in the design of switching power supply. Practice has proved that the earlier EMI problems are considered and resolved, the lower the cost and the better the effect.