Application of EMC Technology in Switching Power Supply

    There are two ways to transmit electromagnetic interference: one is conductive transmission, the other is radiation transmission. Conducted transmission means that there is a complete circuit connection between the interference source and the sensitive equipment, and the interference signal is transmitted to the receiver along the connection circuit to cause electromagnetic interference.

    Radiative transmission is the interference form in which the interference signal propagates outward in the form of electromagnetic wave through the medium. There are three common types of radiation coupling: 1) The electromagnetic wave emitted by one antenna is accidentally received by another antenna, which is called antenna to antenna coupling; 2) The coupling of space electromagnetic field through wire induction is called field to line coupling. 3) The coupling formed by mutual induction of high-frequency signals between two equal conductors is called line to line inductive coupling.

    2. Generation mechanism of electromagnetic interference

    From the perspective of the sensitive equipment being interfered, interference coupling can be divided into conductive coupling and radiation coupling.

    ● Conduction coupling model

    According to its principle, conductive coupling can be divided into resistive coupling, capacitive coupling and inductive coupling.

    ● Radiation coupling model

    Radiation coupling is another way of interference coupling. In addition to the intentional radiation from the interference source, there is also a lot of unintentional radiation. At the same time, the wiring on the PCB, whether it is power line, signal line, clock line, data line or control line, can act as an antenna, radiating interference waves and receiving.

    3 Electromagnetic interference control technology

    ① Transmission channel suppression

    ● Filtering: pay attention to frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding and reliability when designing and selecting filters. Whether the filter is installed correctly or not has a great impact on its insertion loss characteristics. Only when the installation position is correct and the installation method is correct, can the filter play the expected role in filtering interference. When installing the filter, the installation position shall be considered. The wiring at the input and output sides must be shielded and isolated, as well as high-frequency grounding and bonding methods.

    ● Shielding: electromagnetic shielding can be divided into electric field shielding, magnetic field shielding and electromagnetic field shielding according to the principle. Electric field shielding includes electrostatic shielding and alternating electric field shielding; Magnetic field shielding includes low-frequency magnetic field shielding and high-frequency magnetic field shielding. Different types of electromagnetic shielding have different requirements for shielding body. In the actual shielding, the electromagnetic shielding effectiveness depends more on the structure of the shield, that is, the continuity of conductivity. Due to the requirements of manufacturing, assembly, maintenance, heat dissipation, observation and interface connection, the actual shielding body is generally provided with holes with different shapes and sizes. These holes play an important role in the shielding effectiveness of the shielding body. Therefore, measures must be taken to control the electromagnetic leakage of the holes.

    ● Grounding: there are two types of grounding: safety grounding and signal grounding. At the same time, grounding will also introduce grounding impedance and ground loop interference. Grounding technology includes selection of grounding point, circuit combination, grounding design and reasonable application of grounding interference suppression measures.

    ● Lapping: Lapping is a low impedance connection between conductors. Only good lapping can enable the circuit to complete its design function and enable various interference suppression measures to play a role. The lapping method can be divided into permanent lapping and semi permanent lapping, and the lapping type can be divided into direct lapping and indirect lapping.

    ● Wiring: wiring is the key to the electromagnetic compatibility design of printed circuit boards. Reasonable wire width should be selected, and correct wiring strategies should be adopted, such as thickening the ground wire, closing the ground wire into a loop, reducing wire discontinuity, and using multilayer boards.

    ② Spatial separation

    Space separation is an effective method to suppress space radiation disturbance and inductive coupling disturbance. By increasing the space distance between the disturbance source and the sensitive equipment of the receiver, the intensity of the disturbed electromagnetic field reaching the sensitive equipment has been attenuated to less than the sensitivity threshold of the receiving equipment, so as to achieve the purpose of suppressing electromagnetic interference. According to the electromagnetic field theory, the field strength is attenuated by 1/r3 in the near induction field, and the field strength distribution of the far radiation field is reduced by 1/r. Therefore, in order to meet the electromagnetic compatibility requirements of the system, the space distance between various equipment constituting the system shall be increased as much as possible. In equipment and system wiring, limit the minimum spacing of parallel cables to reduce crosstalk. In PCB design, specify the minimum spacing between lead strips. In addition, the space separation also includes the orientation adjustment of the radiation direction of the disturbance source and the control of the spatial orientation of the electric field vector and magnetic field vector of the disturbance source when the space is limited.

    ③ Time separation

    When the disturbance source is very strong and is difficult to be reliably suppressed by other methods, the method of time separation is usually used to make the useful signal transmitted within the time when the disturbance signal stops transmitting, or when the strong disturbance signal is transmitted, make the sensitive equipment vulnerable to disturbance shut down for a short time to avoid damage. There are two forms of time separation control. One is active time separation, which is applicable to the situation where the occurrence time of useful signal and the occurrence time of interference signal have a definite relationship; The other is passive time separation. According to the characteristics of interference signals and useful signals, one of them can be quickly closed, so as to meet the control requirements of non coincidence and non coverage in time.

    ④ Spectrum management

    The planning and division of spectrum is to divide each frequency band into various radio services and formulate frequency bands for specific users. The establishment of national standards and specifications is the basis for preventing interference and, in some cases, ensuring that the communication system achieves the required communication performance. This includes the approval procedures for radio equipment, and the minimum performance standard documents required for the type approval of radio transmitters, receivers and other equipment.

    ⑤ Electrical isolation

    Electrical isolation is a reliable method to avoid conducted interference in the circuit, and it can also enable the normal coupling transmission of useful signals. Common electrical isolation coupling forms include mechanical coupling, electromagnetic coupling, photoelectric coupling, etc. DC/DC converter is a widely used electrical isolation device, which converts one DC voltage into another. In order to prevent the common power source internal resistance interference caused by multiple equipment sharing a power source, DC/DC converter is used to supply power to each circuit separately to ensure that the circuit is not interfered by the signal in the power source.

    Application of EMC Technology in Switching Power Supply

    1. Causes of switching power supply interference

    The switching power supply first rectifies the power frequency AC to DC, then inverts it to high frequency, and finally outputs a stable DC voltage through the rectifier filter circuit, so it contains a lot of harmonic interference. At the same time, due to the leakage inductance of the transformer and the peak caused by the reverse recovery current of the output diode, potential electromagnetic interference is formed. The interference sources in switching power supply mainly focus on the components with large changes in voltage and current, especially on switch tubes, diodes, high-frequency transformers, etc.

    ① Electromagnetic interference generated by switching circuit

    Switching circuit is one of the main interference sources of switching power supply. Switching circuit is the core of switching power supply, which is mainly composed of switch tube and high-frequency transformer. The du/dt produced by this method has large amplitude pulse, wide frequency band and rich harmonics. The main reason for this pulse interference is that the switch tube load is the primary coil of high-frequency transformer, and it is the inductive load. At the moment when the switch is turned on, the primary coil generates a large inrush current, and a high surge peak voltage appears at both ends of the primary coil; At the moment when the switch tube is disconnected, due to the leakage flux of the primary coil, part of the energy is not transmitted from the primary coil to the secondary coil. This part of the energy stored in the inductor will form a attenuation oscillation with a spike with the capacitance and resistance in the collector circuit, which will be superimposed on the switching voltage to form a switching voltage spike. The interruption of power supply voltage will produce the same magnetizing impulse current transient as when the primary coil is connected. This transient is a conductive electromagnetic interference, which not only affects the primary of the transformer, but also returns the conductive interference to the power distribution system, causing harmonic electromagnetic interference in the power grid, thus affecting the safe and economic operation of other equipment.

    ② Electromagnetic interference generated by rectifier circuit

    In the rectifier circuit, there is a reverse current when the output rectifier diode is cut off, and the time when it recovers to zero is related to factors such as junction capacitance. Among them, the diode that can quickly recover the reverse current to zero is called the hard recovery 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 tens of MHz. When the rectifier diode in the high-frequency rectifier circuit is conducting in the forward direction, there is a large forward current flow. When it turns to the cut-off due to reverse bias voltage, because there are more carriers accumulated in the PN junction, the current will flow in the reverse direction for a period of time before the carrier disappears, resulting in a sharp decrease in the reverse recovery current of the carrier disappears and a large current change (di/dt).

    ③ High frequency transformer

    The high frequency switching current loop composed of the primary coil, switch tube and filter capacitor of the high frequency transformer may generate large space radiation and form radiation interference. If the capacitance filtering capacity is insufficient or the high frequency characteristics are poor, the high frequency impedance on the capacitance will cause the high frequency current to be transmitted to the AC power supply in a differential mode to form conducted interference. It should be noted that in the electromagnetic interference generated by diode rectifier circuit, the di/dt of reverse recovery current of rectifier diode is much larger than that of freewheeling diode. As a source of electromagnetic interference, the reverse recovery current of rectifier diode causes strong interference and wide frequency band. However, the voltage jump generated by the rectifier diode is far less than the voltage jump generated when the power switch is turned on and off. Therefore, the effect of │ dv/dt │ generated by rectifier diodes can also be ignored, and the rectifier circuit can be studied as a part of the electromagnetic interference coupling channel.

    ④ Interference caused by distributed capacitance

    Switching power supply operates at high frequency, so its distributed capacitance cannot be ignored. On the one hand, the contact area of the insulating sheet between the radiator and the collector of the switch tube is large and the insulating sheet is thin, so the distributed capacitance between them cannot be ignored at high frequencies. The high-frequency current will flow to the radiator through the distributed capacitance, and then to the chassis ground, generating common mode interference; On the other hand, there is distributed capacitance between the primary stages of the pulse transformer, which can directly couple the primary side voltage to the secondary side and generate common mode interference on the two power lines that make DC output at the secondary side.

    ⑤ Stray parameters affect the characteristics of coupling channels

    In the conducted interference frequency band (30MHz) Most coupling channels of switching power supply interference can be described by circuit network. However, any actual component in switching power supply, such as resistance, capacitance, inductance and even switch tube and diode, contains stray parameters, and the wider the frequency band studied, the higher the order of equivalent circuit. Therefore, the equivalent circuit of switching power supply, including the stray parameters of components and the coupling between components, will be much more complex. At high frequency, stray parameters have a great influence on the characteristics of coupling channel, and the existence of distributed capacitance becomes the channel of electromagnetic interference. In addition, when the power of the switch tube is large, the collector generally needs to be equipped with a heat sink. The distributed capacitance between the heat sink and the switch tube cannot be ignored at high frequencies, which can form space oriented radiated interference and common mode interference conducted by the power line.

    2 Control technology of electromagnetic interference of switching power supply

    To solve the electromagnetic interference problem of switching power supply, we can start from three aspects: 1) reduce the interference signal generated by the interference source; 2) Cut off the transmission path of interference signal; 3) Enhance the anti-interference ability of the interfered body. Therefore, the control technologies for electromagnetic interference of switching power supply mainly include circuit measures, EMI filtering, component selection, shielding and anti-interference design of printed circuit board.

    ① Reduce the interference of switching power supply

    ● Soft switching technology: Inductive and capacitive components are added in the original hard switching circuit, and du/dt and di/dt in the switching process are reduced by using the resonance of inductance and capacitance, so that the voltage drop is prior to the current rise when the switching device is turned on, or the current drop is prior to the voltage rise when the switching device is turned off, to eliminate the overlap of voltage and current.

    ● Switching frequency modulation technology: by modulating the switching frequency fc, the energy concentrated on fc and its harmonics 2fc, 3fc... is distributed to the frequency bands around them to reduce the EMI amplitude at each frequency point. This method cannot reduce the total amount of interference, but the energy is distributed to the baseband of the frequency points, so that each frequency point does not exceed the limit value specified by EMI. In order to reduce the peak value of noise spectrum, there are usually two methods: random frequency method and modulation frequency method.

    ● Active suppression technology of common mode interference: try to take out a compensated EMI noise voltage completely opposite to the main switching voltage waveform causing electromagnetic interference from the main circuit, and use it to balance the original switching voltage.

    ● Buffer circuit for reducing electromagnetic interference: it is composed of linear impedance stabilization network, which is used to eliminate potential interference in power line, including power line interference, electrical fast transient, surge, voltage variation, power line harmonic, etc. These disturbances do not have a great impact on the general regulated power supply, but have a significant impact on the high-frequency switching power supply.

    ● Filtering: One of the main purposes of the EMI filter is to obtain high insertion loss in the frequency band of 150kHz~30MHz, but it does not attenuate the 50Hz power frequency signal, so that the rated voltage and current can pass smoothly, and it must also meet certain size requirements. Conducted interference signals on any power line can be represented by differential mode and common mode signals. In general, the differential mode interference is small in amplitude and frequency, resulting in less interference; Common mode interference has large amplitude, high frequency, and can also generate radiation through the wire, resulting in large interference. Therefore, the most effective way to reduce conducted interference and control EMI signal below the limit level specified in relevant EMC standards is to install EMI filters in the input and output circuits of switching power supply.

    ● PCB design: PCB anti-interference design mainly includes PCB layout, wiring and grounding, which aims to reduce electromagnetic radiation of PCB and crosstalk between circuits on PCB. The best approach to switching power supply layout is similar to its electrical design. After determining the size and shape of PCB, determine the position of special components (such as various generators, crystal oscillators, etc.). Finally, all components of the circuit are arranged according to the functional unit of the circuit.

    ● Selection of components: select components that are not easy to generate noise, conduct and radiate noise. It is generally worth noting the selection of winding components such as diodes and transformers. Fast recovery diode with small reverse recovery current and short recovery time is an ideal device for high-frequency rectifier of switching power supply.

    ② Design of Common Mode and Differential Mode Power Line Filters to Cut off the Propagation Path of Interference Signals

    Power line interference can be filtered by power line filter. A reasonable and effective EMI filter of switching power supply should have a strong suppression effect on both differential mode and common mode interference on the power line.

    ③ Enhance anti-interference ability of sensitive circuit

    This mainly includes shielding and grounding.