Interference and Suppression of Switching Power Supply

As a power supply device for electronic equipment, switching power supply has the advantages of small size, light weight, high efficiency, etc., and has been widely used in digital circuits. However, due to working in a high-frequency switching state, it is a strong interference source, and the interference it generates directly endangers the normal operation of electronic equipment. Therefore, suppressing the electromagnetic noise of the switching power supply itself and improving its immunity to electromagnetic interference to ensure that electronic equipment can work safely and reliably for a long time is an important topic in the development and design of switching power supplies.
1 Generation of switching power supply interference

The interference of switching power supply can be generally divided into two categories: one is the interference caused by the internal components of the switching power supply; the other is the interference caused by the switching power supply due to external factors. Both involve human factors and natural factors.

1.1 Internal interference of switching power supply

The EMI generated by the switching power supply is mainly caused by the high-order harmonic current interference generated by the basic rectifier and the peak voltage interference generated by the power conversion circuit.

1.1.1 Basic Rectifier

The rectification process of the basic rectifier is the most common cause of EMI. This is because the industrial frequency AC sine wave is no longer a single frequency current after rectification, but becomes a DC component and a series of harmonic components with different frequencies. Harmonics (especially high-order harmonics) will generate conducted interference and radiated interference along the transmission line, causing the front-end current to be distorted. On the one hand, it distorts the current waveform connected to its front-end power line, and on the other hand, it generates radio frequency interference through the power line.

1.1.2 Power conversion circuit

The power conversion circuit is the core of the switching power supply, which has a wide bandwidth and rich harmonics. The main components that produce this pulse interference are

1) Switching tube There is distributed capacitance between the switching tube and its heat sink and the casing and the leads inside the power supply. When a large pulse current (roughly a rectangular wave) flows through the switching tube, the waveform contains many high-frequency components. At the same time, the device parameters used in the power supply, such as the storage time of the switching power tube, the large current of the output stage, and the reverse recovery time of the switching rectifier diode, will cause the loop to short-circuit instantly and generate a large short-circuit current. In addition, the load of the switching tube is a high-frequency transformer or energy storage inductor. At the moment the switching tube is turned on, a large inrush current appears in the primary of the transformer, causing spike noise.

2) High-frequency transformer The transformer in the switching power supply is used for isolation and voltage transformation, but due to leakage inductance, it will generate electromagnetic induction noise; at the same time, under high-frequency conditions, the distributed capacitance between the transformer layers will transfer the high-order harmonic noise on the primary side to the secondary, and the distributed capacitance of the transformer to the casing forms another high-frequency path, making the electromagnetic field generated around the transformer more easily coupled to other leads to form noise.

3) When the secondary side rectifier diode is used for high-frequency rectification, due to the reverse recovery time factor, the charge accumulated by the forward current cannot be eliminated immediately when the reverse voltage is applied (due to the existence of carriers, there is still current flowing). Once the slope of the reverse current recovery is too large, the inductance flowing through the coil will produce a spike voltage, which will generate strong high-frequency interference under the influence of the transformer leakage inductance and other distributed parameters, and its frequency can reach tens of MHz.

4) Capacitors, inductors and wires Since switching power supplies operate at higher frequencies, the characteristics of low-frequency components will change, thereby generating noise.

1.2 External interference of switching power supply

External interference of switching power supply can exist in "common mode" or "differential mode". The interference type can vary from short-duration spike interference to complete power failure. It also includes voltage change, frequency change, waveform distortion, continuous noise or clutter, and transients. The types of power supply interference are shown in Table 1.

Among the several types of interference in Table 1, the main ones that can be transmitted through the power supply and cause damage to the equipment or affect its operation are electrical fast transient pulse groups and surge shock waves. As for interference such as electrostatic discharge, it will not cause any impact on the electrical equipment caused by the power supply as long as the power supply equipment itself does not produce vibration stop, output voltage drop and other phenomena.

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2 Switching power supply interference coupling path

There are two ways of switching power supply interference coupling: one is conduction coupling and the other is radiation coupling.

2.1 Conductive coupling

Conductive coupling is one of the main coupling paths between the disturbance source and the sensitive device. Conductive coupling requires a complete circuit connection between the disturbance source and the sensitive device. Electromagnetic disturbance is transmitted from the disturbance source to the sensitive device along this connection circuit, generating electromagnetic interference. According to the coupling mode, it can be divided into circuit coupling, capacitive coupling and inductive coupling. In the switching power supply, these three coupling modes exist simultaneously and are interconnected.

2.1.1 Circuit coupling

Circuit coupling is the most common and simplest conduction coupling method. It has the following types:

1) When the direct conduction coupling wire passes through an environment where interference exists, it picks up the interference energy and conducts it along the wire to the circuit, causing interference to the circuit.

2) Common impedance coupling: Since two or more circuits have a common impedance, when the currents of the two circuits flow through a common impedance, the voltage formed by the current of one circuit on the common impedance will affect the other circuit, which is common impedance coupling. Common impedance coupling interference is caused by the output impedance of the power supply, the common impedance of the ground wire, etc.

2 1.2 Capacitive coupling

Capacitive coupling is also called electrical coupling. Since the peak voltage generated between two circuits is a narrow pulse with a large amplitude, there is parasitic capacitance between the frequencies, so the charge of one circuit affects the other branch through the parasitic capacitance.

2.1.3 Inductive coupling

Inductive coupling is also called magnetic coupling. When there is mutual inductance between two circuits, when the interference source appears in the form of power supply, the magnetic field generated by this current interferes with the adjacent signals through mutual inductance coupling.

2.2 Radiative coupling

The interference coupling caused by the radiation path is called radiation coupling. Radiative coupling is the transmission of electromagnetic energy from the interference source to the receiver through space in the form of electromagnetic field. There are usually four main coupling paths: antenna coupling, wire inductive coupling, closed loop coupling and aperture coupling.

2.2.1 Radiated coupling between antennas

In actual engineering, there are a lot of antenna electromagnetic coupling. For example, the long signal lines, control lines, input and output leads in the switching power supply have antenna effects, which can receive electromagnetic interference and form antenna radiation coupling.

2.2.2 Inductive coupling of electromagnetic field to conductor

The cable of the switching power supply is generally composed of the connection line of the signal loop, the power supply line of the power stage loop and the ground line. Each wire is composed of the input impedance, the output impedance and the return wire to form a loop. Therefore, the cable is the part of the internal circuit exposed outside the chassis, and is most susceptible to the coupling of the radiation field of the interference source and induces interference voltage or interference current, which enters the equipment along the wire to form radiated interference.

2.2.3 Electromagnetic field coupling to closed loop

The coupling of electromagnetic field to closed loop means that the length of the largest part of the loop that is induced is less than 1/4 of the wavelength. When the frequency of the radiated disturbance electromagnetic field is relatively low, the electromagnetic coupling of the radiated disturbance electromagnetic field and the closed loop occurs.

2.2.4 Coupling of electromagnetic fields through apertures

The coupling of electromagnetic fields through holes and gaps refers to the electromagnetic interference caused by the radiated interference electromagnetic field through the holes and gaps on the non-metallic equipment casing, the braided metal shielding of the cable, etc.

3 Some measures to suppress interference

The three elements that form electromagnetic interference are the disturbance source, the propagation path and the disturbed device. Therefore, electromagnetic interference suppression should also start from these three aspects and take appropriate measures. First, the disturbance source should be suppressed to directly eliminate the cause of interference; second, the coupling and radiation between the disturbance source and the disturbed device should be eliminated to cut off the propagation path of electromagnetic interference; third, the anti-interference ability of the disturbed device should be improved to reduce its sensitivity to noise. At present, several measures to suppress interference are basically to cut off the coupling channel between the electromagnetic disturbance source and the disturbed device. Commonly used methods are shielding, grounding and filtering.

1) The use of shielding technology can effectively suppress the electromagnetic radiation interference of the switching power supply, that is, use materials with good conductivity to shield the electric field, and use materials with high magnetic permeability to shield the magnetic field. Shielding has two purposes, one is to limit the leakage of electromagnetic energy radiated from the inside, and the other is to prevent external radiation interference from entering the internal area. The principle is to use the reflection, absorption and guidance of electromagnetic energy by the shielding body. In order to suppress the radiation generated by the switching power supply and the influence of electromagnetic disturbance on other electronic equipment, the shielding cover can be processed completely according to the method of magnetic field shielding, and then the entire shielding cover can be connected to the system casing and ground as a whole, so as to effectively shield the electromagnetic field.

2) Grounding is to establish a conduction path between two points so as to connect electronic equipment or components to some reference points called "ground". Grounding is an important method for switching power supply equipment to suppress electromagnetic interference. Connecting some parts of the power supply to the earth can suppress interference. The principle of "one-point grounding" should be followed in circuit system design. If multiple points are grounded, a closed grounding loop will appear. When the magnetic lines of force pass through the loop, magnetic induction noise will be generated. In fact, it is difficult to achieve "one-point grounding". Therefore, in order to reduce the grounding impedance and eliminate the influence of distributed capacitance, planar or multi-point grounding is adopted. A conductive plane is used as a reference ground, and the parts that need to be grounded are connected to the reference ground nearby. In order to further reduce the voltage drop of the ground loop, bypass capacitors can be used to reduce the amplitude of the return current. In a circuit system where low frequency and high frequency coexist, the ground wires of the low frequency circuit, high frequency circuit, and power circuit should be connected separately and then connected to the common reference point.

3) Filtering is an effective method to suppress conducted interference and plays an extremely important role in the electromagnetic compatibility design of equipment or systems. As an important unit to suppress conducted interference from power lines, EMI filters can suppress the interference from the power grid to the power supply itself, and can also suppress the interference generated by the switching power supply and fed back to the power grid. In the filtering circuit, many special filtering components are also used, such as through-hole capacitors, three-terminal capacitors, and ferrite magnetic rings, which can improve the filtering characteristics of the circuit. Proper design or selection of filters, and correct installation and use of filters are important components of anti-interference technology.

When choosing a filter, keep the following points in mind:

(1) Clearly identify the operating frequency and the interference frequency to be suppressed. If the two are very close, a filter with very steep frequency characteristics is required to separate the two frequencies.

(2) Ensure that the filter can work reliably under high voltage conditions;

(3) When the filter is continuously passing the maximum rated current, its temperature rise should be low to ensure that the working performance of the components in the filter is not damaged when it is continuously operating at the rated current;

(4) In order to make the filter frequency characteristics during operation consistent with the design value, the signal source impedance and load impedance connected to it must be equal to the specified values ​​during design:

(5) The filter must have a shielding structure, the shielding box cover and the body must have good electrical contact, the capacitor leads of the filter should be as short as possible, and it is best to use a through-hole capacitor with short leads and low inductance;

(6) It must have high operating reliability, because the faults of filters used to protect against electromagnetic interference are often more difficult to find than those of other components.

When installing the filter, pay attention to the following points:

(1) The power line filter should be installed as close to the power supply port of the equipment as possible. Do not allow the power line that has not passed through the filter to meander inside the equipment frame.

(2) The capacitor leads in the filter should be as short as possible to avoid resonance at a lower frequency due to the lead inductive reactance and capacitive reactance;

(3) A large short-circuit current passes through the grounding wire of the filter, which will cause additional electromagnetic radiation. Therefore, the filter element itself should be well shielded and grounded;

(4) The input and output lines of the filter cannot cross, otherwise crosstalk will occur due to the input and output capacitance coupling path of the filter, thereby reducing the filtering characteristics. The usual method is to add a partition or shielding layer between the input and output ends.

4 Conclusion

There are many factors that cause switching power supplies to generate electromagnetic interference, and there is still a lot of work to be done to suppress electromagnetic interference. Comprehensively suppressing various noises of switching power supplies will make switching power supplies operate more safely and reliably.