Analysis and solution of electronic interference in switching power supply

Switching power supply Due to its small size and high power factor, it is widely used in communication, control, computer and other fields. However, due to the electromagnetic interference, its further application is limited to a certain extent. This article will analyze the various generation mechanisms of electromagnetic interference of switching power supply, and on this basis, propose the electromagnetic compatibility design method of switching power supply.

Analysis of electromagnetic interference of switching power supply

The structure of the switching power supply is shown in Figure 1. First, the industrial frequency AC is rectified into DC, then inverted into high frequency, and finally output through the rectifier and filter circuit to obtain a stable DC voltage. Unreasonable circuit design and layout, mechanical vibration, poor grounding, etc. will all cause internal electromagnetic interference. At the same time, the peaks caused by the leakage inductance of the transformer and the reverse recovery current of the output diode are also potential strong interference sources.

Figure 1 Basic block diagram of AC/DC switching power supply

1 Internal interference sources

● Switching circuit

The switch circuit is mainly composed of a switch tube and a high-frequency transformer. There is a distributed capacitance between the switch tube and its heat sink and the leads inside the casing and the power supply. The du/dt it generates has a large pulse amplitude, a wide frequency band and rich harmonics. The switch tube load is the primary coil of the high-frequency transformer, which is an inductive load. When the originally turned-on switch tube is turned off, the leakage inductance of the high-frequency transformer generates a back electromotive force E=-Ldi/dt, whose value is proportional to the rate of change of the collector current and proportional to the leakage inductance. It is superimposed on the turn-off voltage to form a turn-off voltage spike, thereby forming a conducted interference.

● Rectifier diodes in rectifier circuits

When the output rectifier diode is cut off, there is a reverse current, and the time it takes to recover to zero is related to factors such as junction capacitance. It will produce a large current change di/dt under the influence of transformer leakage inductance and other distributed parameters, and generate strong high-frequency interference, with a frequency of up to tens of megahertz.

● Spurious parameters

Due to the operation at a higher frequency, the characteristics of the low-frequency components in the switching power supply will change, thus generating noise. At high frequencies, the stray parameters have a great influence on the characteristics of the coupling channel, and the distributed capacitance becomes a channel for electromagnetic interference.

2 External interference sources

External interference sources can be divided into power supply interference and lightning interference, and power supply interference exists in "common mode" and "differential mode". At the same time, because the AC power grid is directly connected to the rectifier bridge and filter circuit, in half a cycle, there is input current only at the peak time of the input voltage, resulting in a very low input power factor of the power supply (about 0.6). Moreover, this current contains a large amount of current harmonic components, which will cause harmonic "pollution" to the power grid.

EMC Design of Switching Power Supply

There are three necessary conditions for electromagnetic interference: interference source, transmission medium, and sensitive equipment. The purpose of EMC design is to destroy one of these three conditions. For this purpose, the main methods adopted are: circuit measures, EMI filtering , shielding, printed circuit board anti-interference design, etc.

1 Soft switching technology to reduce switching loss and switching noise

Soft switching is an advanced switching technology developed on the basis of hard switching and is based on resonance technology or control technology under zero voltage/current state.

The implementation method of soft switching is to add small inductors, capacitors and other resonant elements to the original circuit, introduce resonance before and after the switching process, and eliminate the overlap of voltage and current. Figure 2 shows a basic switch unit using soft switching technology.

Figure 2 Basic switching unit in a buck chopper

2 Buffer circuit to reduce interference energy of interference source

A snubber circuit is added to the input of the switch-controlled power supply (see Figure 3), which consists of a linear impedance stabilization network to eliminate potential interference such as power line interference, electrical fast transients, surges, voltage high and low changes, and power line harmonics. The snubber circuit device parameters are D1 is MUR460, R1=500Ω, C=6nF, L=36mH, and R=150Ω.

Figure 3 Buffer circuit

3 EMI filtering to cut off the interference noise propagation path

Installing EMI filters in the input and output circuits of switching power supplies is an effective way to suppress conducted emissions. Its parameters mainly include: discharge resistance, insertion loss, Cx capacitance, Cy capacitance and inductance value. Among them, insertion loss is a key parameter of filter performance. Under the premise of considering mechanical performance, environment, cost, etc., the insertion loss should be as large as possible. The insertion loss IL of the filter can be obtained by using the measurement results of common-mode and differential-mode interference and the standard limit values, plus appropriate margins.

ILCM(dB)=Vcm(dB)-Vlimt(dB)-3(dB)+M(dB) (1)

ILDM(dB)=VDM(dB)-Vlimt(dB)-3(dB)+M(dB) (2)

In the formula, 3dB means that the test result is 3dB larger than the actual value during the test of separating common-mode and differential-mode conducted interference; M (dB) represents the design margin, which is generally 6dB; Vlimit (dB) is the conducted interference limit specified in relevant standards such as CISPR, FCC, etc.

Figure 4 is the circuit of the EMI filter on the AC side of the switching power supply with 220V/50Hz AC input. Cy＝3300PF, L1, L2＝0.7mH, they form a common mode filter circuit to suppress the common mode interference signal of 0.5～30MHz. Cx＝0.1μF, L3, L4＝200～500μH, using metal powder pressed magnetic core, together with L1/L2 and Cx, form a low-pass filter between LN ports to suppress the 0.15～0.5MHz differential mode interference signal on the power line. R is used to eliminate the static electricity accumulation that may appear in the filter.

Figure 4 EMI filter circuit on the AC side of the switching power supply

Figure 5 is the filter circuit on the DC output side of the switching power supply , which consists of common mode chokes L1, L2, choke L3 and capacitors C1, C2. In order to prevent the core from saturating under a large magnetic field strength and making the choke lose its function, the core must use a constant μ core with good high-frequency characteristics and a large saturation magnetic field strength.

Figure 5 Branch side filter circuit

4 Use shielding to suppress radiation and induction interference

The switching power supply interference spectrum is concentrated in the frequency band below 30MHz, with a diameter of r＜λ/2π. It is mainly an electromagnetic field of near-field nature and a low-impedance field. Good conductive materials can be used to shield the electric field, while materials with high magnetic permeability can be used to shield the magnetic field. In addition, effective shielding measures should be taken for transformers, inductors, power devices, etc. The ventilation holes on the shielding shell are preferably circular. Under the condition of satisfying ventilation, the number of holes can be large, and the size of each hole should be as small as possible. The seams should be welded to ensure electromagnetic continuity. Filtering measures should be taken at the lead-in and lead-out wires of the shielding shell. For electric field shielding, the shielding shell must be grounded. For magnetic field shielding, the shielding shell does not need to be grounded.

5 Reasonable PCB layout and wiring

Sensitive lines mainly refer to control circuits and lines directly connected to interference measurement equipment. To reduce the interference level, the simplest way is to increase the distance between the interference source and the sensitive lines. However, due to the limitation of the power supply size, simply increasing the distance is not the best way to solve the problem. A more reasonable way is to place the sensitive lines in places with weaker interference according to the distribution of the interference electric field. The PCB anti-interference layout design process is shown in Figure 6.

Figure 6 PCB anti-interference layout design process