Switching Power Supply Electromagnetic Interference Mechanism and Suppression Measures

The purpose of the electromagnetic interference suppression of the switching power supply is to enable the product to undergo electromagnetic interference in a certain electromagnetic environment, without performance degradation or failure, and can work normally, and do not pose pollution to the electromagnetic environment at the same time.

First, the switching power supply electromagnetic interference mechanism

The interference generated by the switching power supply is divided according to the type of noise interference source and can be divided into two types: peak interference and harmonic interference. If divided by the coupling path, it can be divided into conduction interference and radiation interference. Now according to the source of noise interference to explain separately;

1, the diode caused by the reverse recovery time interference

When the rectifier diode in the high-frequency rectifier circuit is conducting forward, a large forward current flows. When the rectifier diode is turned to the end due to the reverse bias voltage, more carriers accumulate in the pn junction, so the current flows in the carrier. In the period before the disappearance of the child, the current will flow in the reverse direction, causing the reverse recovery current of the carrier to disappear and a large current change (di/dt) to occur.

2 Harmonic interference caused by switch operation

The power switch tube flows a large pulse current when it is turned on. For example, the input current waveforms of forward-type, push-pull, and bridge converters are approximated as rectangular waves in a resistive load, and they contain rich higher-order harmonic components. When using zero-current, zero-voltage switching, this harmonic interference will be very small. In addition, during the cut-off period of the power switch tube, the sudden change of the current caused by the leakage inductance of the high-frequency transformer windings will also produce spike interference.

3 Interference from the AC input circuit

Switching power input rectifiers without power frequency transformers will cause high-frequency damping oscillations to cause interference during reverse recovery.

The peak interference and harmonic interference energy generated by the switching power supply are transmitted through the input and output lines of the switching power supply and are called conducted interferences. The energy of harmonics and parasitic oscillations are transmitted through the input and output lines. Generate electric and magnetic fields. This kind of interference caused by electromagnetic radiation is called radiation interference.

4, other reasons

The parasitic parameters of the components and the schematic design of the switching power supply are not perfect. The printed wiring board (PCB) traces are usually hand-arranged, with a great deal of randomness, and the pcb's near-field interference is large, and the installation of the device on the printed board, Placement, as well as unreasonable orientation can cause EMI interference.

Second, the relevant theory of electromagnetic interference

1, the main source of electromagnetic interference switching power supply

The main sources of electromagnetic interference in switching power supplies are switching devices, diodes, and non-linear passive components. In the switching power supply, improper wiring of the PCB is also a major factor causing electromagnetic interference.

1.1 Electromagnetic Interference from Switching Circuits

For the switching power supply, electromagnetic interference generated by the switching circuit is one of its main sources of interference. The switching circuit is the core of the switching power supply and is mainly composed of a switch tube and a high-value transformer. The dv/dt he produces has larger pulses, wider frequency bands and more harmonics. The main causes of this kind of impulse interference are:

(1) The load of the switch tube is the primary coil of the high-frequency transformer, which is an inductive load. When the switch is turned on, the primary coil generates a large inrush current and a high surge voltage appears at both ends of the primary coil. At the instant when the switch is disconnected, part of the energy is not transmitted from the primary coil to the secondary coil due to the leakage flux of the primary coil. The energy stored in the inductor will form a sharp peak with the capacitor and resistor in the collector circuit. The attenuating oscillation is superimposed on the turn-off voltage to form a turn-off spike voltage.

This interruption of the power supply voltage will produce the same transient magnetization and inrush current transients as when the primary coil is turned on. This noise will be conducted to the input and output terminals to form a conduction disturbance, which may cause breakdown of the switch tube.

(2) The high-frequency switching current loop formed by the primary coil, switch tube and filter capacitor of the pulse transformer may generate large space radiation and form radiated interference. If the capacitance filtering capacity is insufficient or the high-frequency characteristics are not good, the high frequency on the capacitor The impedance causes high-frequency currents to be conducted into the AC power supply in differential mode to form conductive interference.

1.1.2 Electromagnetic Interference from Diode Rectification Circuits

The 1di/dt1 of the reverse recovery current generated by the rectifier diode in the main circuit is much smaller than the freewheeling diode recovery current Idi/dtl. As a source of electromagnetic interference, the reverse-recovery current of rectifier diodes has a large interference intensity and bandwidth. The voltage jump generated by the rectifier diode is much smaller than the voltage jump generated when the power switch in the power supply is turned on and off. Therefore, it is possible to study the rectifier circuit as part of the electromagnetic interference coupling channel, irrespective of the effects of the rectifier diodes Idv/dtI and Idi/dtl.

2, the switching power supply electromagnetic interference coupling channel

The switching power supply interferes with itself through the coupling channel. Often differential and common mode interference are used for analysis.

"Common mode interference" refers to the size and direction of interference consistent with one another, which exists in any relatively large ground of the power supply, or between the midline and earth. Common mode interference is also called longitudinal mode interference, asymmetrical interference, or grounded interference. It is the interference between the carrier fluid and the earth.

"Differential mode interference" means that the interference is equal in magnitude and opposite in direction. It exists between the power line and the neutral line. Differential mode interference is also called normal mode interference, transverse mode interference, or symmetrical interference. This is interference between the carrier fluids.

Common-mode interference indicates that interference is coupled into the circuit by radiation or crosstalk. The differential mode interference shows that the interference is from the same circuit. Usually these two kinds of interference are existed at the same time, because the impedance of the line is unbalanced, two kinds of interference will change with each other in the transmission. So the situation is very complicated.

Third, several measures to suppress interference

The three elements that form electromagnetic interference are interference sources, propagation routes, and disturbed devices. Therefore, the suppression of electromagnetic interference should also start from these three aspects. Firstly, the interference source should be suppressed, and the cause of interference should be directly eliminated. Secondly, the coupling and radiation between the interference source and the victim device should be eliminated to cut off the transmission path of electromagnetic interference. Third, the immunity of the victim device should be improved and the noise should be reduced. The sensitivity. Currently, several measures to suppress interference are basically used to cut off the coupling channel between the electromagnetic interference source and the victim equipment. They are indeed effective methods. Commonly used methods are shielding, grounding, and filtering.

The use of shielding technology can effectively suppress the electromagnetic interference of the switching power supply. For example, power switch tubes and output diodes usually have a large power loss. In order to dissipate heat, it is often necessary to install a heat sink or mount directly on the power supply base plate. When the device is installed, it requires insulation with good thermal conductivity. This creates a distributed capacitance between the device and the backplane and heat sink. The backplane of the switching power supply is the ground of the AC power supply. Therefore, common-mode interference is generated by coupling the electromagnetic interference to the AC input through the distributed capacitance between the device and the backplane. The solution to this problem is to use a shielding sheet sandwiched between two insulating sheets and to connect the shielding sheet to the DC ground, cutting off the way that radio frequency interference propagates into the input power grid. In order to suppress the radiation generated by the switching power supply and the influence of electromagnetic interference on other electronic devices, the shield can be completely processed in accordance with the method of shielding the magnetic field, and then the whole shield can be connected to the chassis and the ground of the system to integrate the electromagnetic field. Effectively shielded. Some parts of the power supply connected to the earth can play a role in suppressing interference. For example, the grounding of the electrostatic shield can suppress disturbances of varying electric fields.

Electromagnetic shielding conductors may not be grounded in principle, but unshielded shielded conductors often enhance electrostatic coupling and produce a so-called "negative electrostatic shielding" effect. Therefore, it is still better to ground, so that the electromagnetic shielding can also play the role of electrostatic shielding. The common reference point of the circuit is connected to the earth and provides a stable reference potential for the signal loop. Therefore, after the safety ground wire, shielding ground wire, and common ground reference wire in the system each form a grounding busbar, they are all connected to the earth.

It should be followed in the circuit system design. The principle of one-point grounding is that if a multi-point ground is formed, a closed ground loop will occur, and when the magnetic flux passes through the loop, magnetic induction noise will be generated, and it is actually difficult to achieve “one-point grounding.” Therefore, in order to reduce the ground impedance, eliminate The effect of distributed capacitance is to adopt planar or multi-point grounding, using a conductive plane (a conductive plane layer of a backplane or a multilayer printed circuit, etc.) as a reference ground, and the parts that need to be grounded are connected to the reference ground nearby. To reduce the voltage drop of the ground loop, the bypass capacitor can be used to reduce the amplitude of the return current.In a low-frequency and high-frequency coexisting circuit system, the ground of the low-frequency circuit, high-frequency circuit, and power circuit should be connected separately. Connect to a common reference point.

Filtering is a good way to suppress conducted interference. For example, connecting the filter to the input of the power supply can suppress the interference generated by the switching power supply and feed back to the grid. It can also suppress the noise from the grid against the power itself. In the filter circuit, many special filter elements are also used, such as a feedthrough capacitor, a three-terminal capacitor, and a ferrite core, which can improve the filtering characteristics of the circuit. Properly designing or selecting filters and properly installing and using filters is an important part of anti-jamming technology.

There are still many factors that cause electromagnetic interference in switching power supplies. There is still a lot of work to do to suppress electromagnetic interference. The comprehensive suppression of various noises of the switching power supply will make the switching power supply more widely used.

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