Overview of electromagnetic compatibility technology and application of EMC technology in switching power supplies

1 Generation and transmission of electromagnetic interference

There are two ways of electromagnetic interference transmission: one is conduction transmission, and the other is radiation transmission. Conduction 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, causing electromagnetic interference.

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

2. The generation mechanism of electromagnetic interference

From the perspective of sensitive equipment being interfered with, interference coupling can be divided into two categories: conducted coupling and radiated coupling.

• Conductive coupling model

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

• Radiative coupling model

Radiated coupling is another form of interference coupling. In addition to the intentional radiation emitted from the interference source, there is also a large amount of unintentional radiation. At the same time, the traces on the PCB board, whether they are power lines, signal lines, clock lines, data lines or control lines, can act as antennas, radiating interference waves and receiving them.

3 Electromagnetic Interference Control Technology

① Transmission channel suppression

• Filtering: When designing and selecting filters, attention should be paid to frequency characteristics, withstand voltage performance, rated current, impedance characteristics, shielding and reliability. Whether the filter is installed correctly or not has a great influence on its insertion loss characteristics. Only when the installation position is appropriate and the installation method is correct can it play the expected filtering role on interference. When installing the filter, the installation position should be considered, the wiring on the input and output sides must be shielded and isolated, as well as the high-frequency grounding and overlapping methods.
• Shielding: Electromagnetic shielding can be divided into three types according to the principle: electric field shielding, magnetic field shielding and electromagnetic field shielding. 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 the shielding body. In actual shielding, the effectiveness of electromagnetic shielding depends to a greater extent on the structure of the shielding body, that is, the continuity of conductivity. Due to the requirements of manufacturing, assembly, maintenance, heat dissipation, observation and interface connection, the actual shielding body generally has holes of various shapes and sizes. These holes have an important influence on the shielding effectiveness of the shielding body, so measures must be taken to suppress electromagnetic leakage from 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 the selection of grounding points, circuit combination, grounding design and the reasonable application of grounding interference suppression measures.
• Overlap: Overlap refers to the low impedance connection between conductors. Only with good overlap can the circuit complete its designed function and various interference suppression measures work. Overlap methods can be divided into permanent overlap and semi-permanent overlap, and overlap types can be divided into direct overlap and indirect overlap.
• 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 multi-layer boards.

② Spatial separation

Spatial separation is an effective method to suppress spatial radiation disturbance and inductive coupling disturbance. By increasing the spatial distance between the disturbance source and the sensitive device of the receiver, the intensity of the disturbance electromagnetic field is attenuated to below the sensitivity threshold of the receiving device when it reaches the sensitive device, thereby achieving the purpose of suppressing electromagnetic interference. It can be seen from the electromagnetic field theory that the field strength decays in the near-field induction field in a 1/r3 manner, and the field strength distribution of the far-field radiation field decreases in a 1/r manner. Therefore, in order to meet the electromagnetic compatibility requirements of the system, the spatial distance between the various devices that make up the system is increased as much as possible. In the wiring of equipment and systems, the minimum spacing of parallel cables is limited to reduce crosstalk. In PCB design, the minimum spacing between lead wires is specified. In addition, spatial separation also includes the adjustment of the azimuth 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 space is limited.

③Time separation

When the disturbance source is very strong and difficult to suppress reliably by other methods, time separation is usually used to transmit the useful signal during the time when the disturbance signal stops transmitting, or to shut down the sensitive equipment susceptible to disturbance for a short time when a strong disturbance signal is transmitted, so as to avoid damage. There are two forms of time separation control: active time separation, which is applicable to the situation where the appearance time of the useful signal and the appearance time of the interference signal have a certain order; the other is passive time separation, which quickly shuts down one of the signals according to the characteristics of the interference signal and the useful signal, so as to achieve the control requirements of non-overlap and non-coverage in time.

④Spectrum management

The planning and allocation of spectrum is to allocate frequency bands to various radio services and to set frequency bands for specific users. The formulation of national standards and specifications is the basis for preventing interference and, in some cases, ensuring that communication systems achieve the required communication performance. This includes the approval procedures for radio equipment and the minimum performance standard documents required for the approval of radio transmitters, receivers and other equipment models.

⑤Electrical isolation

Electrical isolation is a reliable method to avoid conducted interference in circuits, and it can also enable the normal coupling and 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 that converts one DC voltage into another. In order to prevent multiple devices from sharing a power supply and causing interference from the common power supply internal resistance, a 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 supply.

Application of EMC Technology in Switching Power Supplies

1 Causes of interference caused by switching power supply

The switching power supply first rectifies the industrial frequency AC into DC, then inverts it into high frequency, and finally outputs it through the rectifier and filter circuit to obtain a stable DC voltage. Therefore, it contains a lot of harmonic interference. At the same time, the peaks caused by the leakage inductance of the transformer and the reverse recovery current of the output diode form potential electromagnetic interference. The interference sources in the switching power supply are mainly concentrated on components with large voltage and current changes, especially on the switch tube, diode, high-frequency transformer, etc.

①Electromagnetic interference generated by switching circuits

The switching circuit is one of the main interference sources of the switching power supply. The switching circuit is the core of the switching power supply, mainly composed of a switching tube and a high-frequency transformer. The du/dt it generates has a large pulse amplitude, a wide frequency band and rich harmonics. The main reason for this pulse interference is that the switch tube load is the primary coil of the high-frequency transformer, which is an inductive load. At the moment the switch tube 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 the switch tube is turned off, 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 peaked attenuation oscillation with the capacitor and resistor in the collector circuit, which is superimposed on the turn-off voltage to form a turn-off voltage spike. The interruption of the power supply voltage will generate the same magnetizing impact current transient as when the primary coil is turned on. This transient is a conducted electromagnetic interference, which not only affects the primary of the transformer, but also causes the conducted interference to return to the distribution system, causing harmonic electromagnetic interference in the power grid, thereby affecting the safe and economical operation of other equipment.

②Electromagnetic interference generated by the rectifier circuit

In the rectifier circuit, there is a reverse current when the output rectifier diode is cut off, and the time it takes to recover 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 a hard recovery diode. This diode will produce strong high-frequency interference under the influence of transformer leakage inductance and other distributed parameters, and its frequency can reach tens of MHz. When the rectifier diode in the high-frequency rectifier circuit is forward-conducted, a large forward current flows through it. When it is turned off due to the reverse bias voltage, due to the accumulation of more carriers in the PN junction, the current will flow in the reverse direction for a period of time before the carrier disappears, causing the reverse recovery current of the carrier disappearance to decrease sharply and causing a large current change (di/dt).

③High 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 spatial radiation, forming radiation interference. If the capacitor filter capacity is insufficient or the high-frequency characteristics are not good, the high-frequency impedance on the capacitor will cause the high-frequency current to be conducted to the AC power supply in a differential mode to form conducted interference. It should be noted that in the electromagnetic interference generated by the diode rectifier circuit, the di/dt of the reverse recovery current of the rectifier diode is much larger than the di/dt of the reverse recovery current of the freewheeling diode. As a source of electromagnetic interference, the interference intensity formed by the reverse recovery current of the rectifier diode is large and the bandwidth is wide. However, the voltage jump generated by the rectifier diode is much smaller than the voltage jump generated when the power switch tube is turned on and off. Therefore, the |dv/dt| effect generated by the rectifier diode can also be ignored, and the rectifier circuit can be studied as part of the electromagnetic interference coupling channel.

④Interference caused by distributed capacitance

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

⑤ Characteristics of coupling channels affected by stray parameters

In the conducted interference frequency band (<30MHz), the coupling channels of most switching power supply interference can be described by circuit networks. However, any actual component in the switching power supply, such as resistors, capacitors, inductors, and even switches and diodes, contains stray parameters, and the wider the frequency band studied, the higher the order of the equivalent circuit. Therefore, the equivalent circuit of the switching power supply, including the stray parameters of each component and the coupling between components, will be much more complicated. At high frequencies, stray parameters have a great influence on the characteristics of the coupling channel, and the existence of distributed capacitance becomes a channel for electromagnetic interference. In addition, when the power of the switch tube is large, a heat sink is generally required for the collector. The distributed capacitance between the heat sink and the switch tube cannot be ignored at high frequencies, and it can form radiation interference facing the space 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 propagation path of the interference signal; 3) enhance the anti-interference ability of the interfered object. Therefore, the switching power supply electromagnetic interference control technology mainly includes: circuit measures, EMI filtering, component selection, shielding and printed circuit board anti-interference design.

①Reduce the interference of the switching power supply itself

• Soft switching technology: Add inductor and capacitor elements to the original hard switching circuit, use the resonance of inductor and capacitor to reduce du/dt and di/dt in the switching process, so that the voltage drops before the current rises when the switch device is turned on, or the current drops before the voltage rises when the switch 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 dispersed 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 dispersed to the baseband of the frequency point, so that each frequency point does not exceed the EMI limit specified. In order to achieve the purpose of reducing the peak value of the noise spectrum, there are usually two processing methods: random frequency method and modulation frequency method.
• Active suppression technology for common-mode interference: try to extract a compensating EMI noise voltage from the main circuit that is completely opposite to the main switching voltage waveform that causes electromagnetic interference, and use it to balance the original switching voltage.
• Buffer circuit to reduce electromagnetic interference: It is composed of a linear impedance stabilization network, and its function is to eliminate potential interference in the power supply line, including power line interference, electrical fast transients, surges, voltage high and low changes, and power line harmonics. These interferences do not have a great impact on general voltage-stabilized power supplies, but have a significant impact on high-frequency switching power supplies.
• Filtering: One of the main purposes of EMI filters is to obtain higher insertion loss in the frequency range of 150kHz to 30MHz, but not to attenuate the 50Hz power frequency signal, so that the rated voltage and current can pass smoothly, and at the same time, certain size requirements must be met. Any conducted interference signal on the power line can be represented by differential mode and common mode signals. In general, the differential mode interference has a small amplitude and low frequency, and the interference caused is small; the common mode interference has a large amplitude and high frequency, and can also radiate through the wire, causing greater interference. Therefore, in order to weaken the conducted interference and control the EMI signal below the limit level specified by the relevant EMC standards, the most effective way is to install electromagnetic interference filters in the input and output circuits of the switching power supply.
• PCB design: PCB anti-interference design mainly includes PCB layout, wiring and grounding, and its purpose is to reduce the electromagnetic radiation of the PCB and the crosstalk between the circuits on the PCB. The best method for the layout of the switching power supply is similar to its electrical design. After determining the size and shape of the PCB, determine the location of special components (such as various generators, crystal oscillators, etc.). Finally, all components of the circuit are laid out according to the functional units of the circuit.
• Component selection: Select components that are not prone to noise generation, conduction, or radiation. It is usually especially important to note the selection of winding components such as diodes and transformers. Fast recovery diodes with small reverse recovery current and short recovery time are ideal devices for the high-frequency rectification part of switching power supplies.

②Cut off the propagation path of interference signals—common mode and differential mode power line filter design

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

③ Enhance the anti-interference ability of sensitive circuits

This mainly includes two methods: shielding and grounding.