How to suppress power supply noise?

Usually power supplies produce noise, so how to suppress power supply noise? Electromagnetic interference filter is also called EMI filter. It suppresses both series mode and common mode interference. It can effectively suppress power grid noise, improve the anti-interference ability of electronic equipment and the reliability of the system. It can be widely used in electronics. Measuring instruments, computer room equipment, switching power supplies, measurement and control systems and other fields.

1. Basic concepts of power supply noise

Power supply noise is a type of electromagnetic interference, and its conducted noise spectrum is roughly 10kHz to 30MHz, up to 150MHz. Power supply noise, especially transient noise interference, has fast rising speed, short duration, high voltage amplitude and strong randomness, which can easily cause serious interference to microcomputers and digital circuits.

Depending on the direction of propagation, power supply noise can be divided into two major categories:

①. One type is external interference introduced from the power supply line;

②. One type is noise generated by electronic equipment and conducted through power lines.

From the perspective of formation characteristics, noise interference is divided into two types: series mode interference and common mode interference.

①. Series mode interference is the noise between two power lines (referred to as line-to-line).

②. Common mode interference is the noise from two power lines to the ground (referred to as line-to-ground).

2. Interference from switching power supply

Switching power supply is a strong interference source, and the interference it generates directly harms the normal operation of electronic equipment. Therefore, suppressing the electromagnetic noise of the switching power supply itself and improving its immunity to electromagnetic interference require special attention during the design and development process.

The interference of switching power supply is 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 the influence of external factors.

2.1 Interference from internal components

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

①.The rectification process of the basic rectifier is the most common cause of EMI. This is because the power 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. The harmonics (especially high-order harmonics) will flow along the transmission lines. Conducted interference and radiation interference are generated, which distorts the front-end current. On the one hand, it distorts the current waveform connected to the front-end power line, and on the other hand, radio frequency interference is generated through the power line.

②. The power conversion circuit is the core of the switching regulated power supply. The main components that generate this pulse interference are:

a. Switch tube. There is distributed capacitance between the switch tube and its radiator, the casing and the leads inside the power supply. When the switch tube flows through a large pulse current (roughly a rectangular wave), the waveform contains many high-frequency components; at the same time, the switching power supply uses Device parameters 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 an instant short circuit in the loop 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 when the switch tube is turned on, a large inrush current appears in the primary of the transformer, causing peak noise.

b. High frequency transformer. The transformer in the switching power supply is used for isolation and transformation, but due to leakage inductance, electromagnetic induction noise will be generated; at the same time, under high frequency conditions, the distributed capacitance between the transformer layers will cause the primary side high-order harmonic noise to It is passed to the secondary, and the distributed capacitance of the transformer to the shell forms another high-frequency path, making it easier for the electromagnetic field generated around the transformer to couple on other leads to form noise.

c. Rectifier diode. Rectifier diode When the secondary side rectifier diode is used for high-frequency rectification, due to the reverse recovery time factor, the charge accumulated in the forward current cannot be eliminated immediately when the reverse voltage is applied (due to the existence of carriers and the current flow past). Once the slope of the reverse current recovery is too large, the inductance flowing through the coil will generate a peak voltage, which will produce strong high-frequency interference under the influence of transformer leakage inductance and other distribution parameters, with a frequency of up to tens of MHz. .

d. Capacitors, inductors and wires. Since the switching power supply operates at a higher frequency, the characteristics of the low-frequency components will change, thereby generating noise.

2.2 External interference

External interference from switching power supplies can exist in the form of "common mode" or "differential mode". The type of interference can vary from short-duration spikes to complete blackouts. It also includes voltage changes, frequency changes, waveform distortion, continuous noise or clutter, and transients. The types of power interference are shown in the table below.

Table 1-1 Table of external interference types of switching power supply

Typical causes of serial number interference types

1. Drop and lightning strike; heavy load connection; low grid voltage

2. Power loss, bad weather; transformer failure; failure caused by other reasons

3 Frequency offset generator instability; regional power grid failure

4 Electrical Noise Radar; Radio Signals; Flashovers in Electric Utility and Industrial Equipment; Converters and Inverters

5 Surges suddenly reduce the load; the tap of the transformer is inappropriate

6 harmonic distortion rectifier; switching load; switching power supply; speed control drive

7 Transient lightning strikes; switching of power line load equipment; switching of power factor compensation capacitors; disconnection of no-load motors

Among the several types of interference in Table 1-1, 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. Interferences such as electrostatic discharge can be transmitted as long as the power supply equipment itself does not If phenomena such as vibration stop and output voltage drop occur, there will be no impact on electrical equipment caused by the power supply.

3. Some measures to suppress interference

Suppressing electromagnetic interference should focus on the source of interference, the transmission path and the interfered equipment. First, we should suppress the source of harassment and directly eliminate the cause of interference; second, eliminate the coupling and radiation between the source of harassment and the victim equipment, and cut off the propagation path of electromagnetic interference; third, improve the immunity of the victim equipment and reduce its sensitivity to noise. sensitivity. Commonly used methods are shielding, grounding and filtering.

3.1 Shielding

The electromagnetic radiation interference of switching power supplies can be effectively suppressed by using shielding technology, that is, materials with good conductivity are used to shield the electric field, and materials with high magnetic permeability are used to shield the magnetic field.

3.2 Grounding

The so-called grounding is to establish a conductive path between two points to connect electronic equipment or components to some reference point called "ground". Grounding is an important method for suppressing electromagnetic interference in switching power supply equipment. Some parts of the power supply are connected to the earth to suppress interference. The principle of "one point grounding" should be followed in the design of circuit systems. If multiple points of grounding are formed, a closed ground 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 each part that needs to be grounded is connected to the reference ground nearby. . To further reduce the voltage drop in the ground loop, a bypass capacitor 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.

3.3 Filtering

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 for suppressing conducted interference on power lines, EMI filters can suppress interference from the power grid from harming the power supply itself, and can also suppress interference generated by switching power supplies and fed back to the power grid. In the filter circuit, many special filter components are also used, such as feedthrough capacitors, three-terminal capacitors, and ferrite magnetic rings, which can improve the filter characteristics of the circuit. Properly designing or selecting filters, and correctly installing and using filters are important components of anti-interference technology.