Switching power supplies are widely used in communication, control, computer and other fields due to advantages such as small size and high power factor. However, due to the generation of electromagnetic interference, its further application is limited to a certain extent. In this paper, we will analyse the various generation mechanisms of electromagnetic interference of switching power supply, and on its basis, put forward the electromagnetic compatibility design method of switching power supply.

Switching power supply electromagnetic interference analysis

First of all, the industrial frequency AC rectifier for DC, and then inverter for high frequency, and finally rectifier filter circuit output, to get a stable DC voltage. Circuit design and layout is unreasonable, mechanical vibration, poor grounding, etc. will form internal electromagnetic interference. At the same time, the leakage inductance of the transformer and the spikes caused by the reverse recovery current of the output diode are also potential sources of strong interference.

1 Internal sources of interference

● Switching circuit

The switching circuit is mainly composed of a switching tube and a high-frequency transformer. Switching tube and its heat sink with the shell and the power supply there is a distributed capacitance between the internal leads, it produces du/dt has a large amplitude of the pulse, the bandwidth is wide and rich in harmonics. Switching tube load for high-frequency transformer primary coil, is an inductive load. When the original conduction of the switching tube off, the leakage inductance of the high-frequency transformer generates a counterpotential E = -Ldi/dt, the value of which is proportional to the rate of change of the collector current, proportional to the leakage inductance, iterated on the shutdown voltage, the formation of the shutdown voltage spike, thus forming a conduction interference.

● Rectifier diode of rectifier circuit

The output rectifier diode cuts off with a reverse current whose recovery time to zero is related to factors such as junction capacitance. It produces a large current variation di/dt under the influence of transformer leakage inductance and other distribution parameters, generating strong high-frequency interference with frequencies up to tens of megahertz.

● Spurious parameters

As a result of operating at higher frequencies, the characteristics of the low-frequency components in the switching power supply change, resulting in noise. At high frequencies, the spurious parameters have a great influence on the characteristics of the coupling channel, and the distribution capacitance becomes the channel for electromagnetic interference.

2 External sources of interference

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

EMC Design of Switching Power Supplies

Electromagnetic interference has three necessary conditions: interference source, transmission medium, sensitive equipment, EMC design is to destroy one of these three conditions. For this, the main methods are: circuit measures, EMI filtering, shielding, printed circuit board anti-interference design.

1 soft switching technology to reduce switching losses and switching noise

Soft switching is developed on the basis of hard switching, a resonance-based technology or the use of control technology to achieve in the zero voltage / current state of the advanced switching technology.

Soft switching is achieved by adding resonant components such as small inductors and capacitors to the original circuit, introducing resonance before and after the switching process, and eliminating the overlap of voltage and current.

2 Buffer circuit to reduce the interference energy of the interference source

In the input part of the switching control power supply buffer circuit, which consists of a linear impedance stabilisation network, used to eliminate power line interference, electrical fast transients, surges, high and low voltage variations and power line harmonics and other potential interference. The buffer circuit device parameters are R1=500Ω, C=6nF, L=36mH, R=150Ω.

3 EMI filtering to cut off the propagation path of interference noise

In the switching power supply input and output circuits with EMI filters, is to inhibit the conduction emission of a very effective method. Its parameters are: discharge resistance, insertion loss, Cx capacitance, Cy capacitance and inductance. Among them, insertion loss is a key parameter for filter performance. The insertion loss should be made as large as possible under the consideration of mechanical performance, environment and cost. The insertion loss IL of the filter can be obtained by using the measurements of common mode and differential mode interference with the standard limits, plus an appropriate margin.

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 indicates that the test result is 3dB larger than the actual value during the test of separating common-mode and differential-mode conduction interferences; M(dB) indicates that the design M (dB) represents the design margin, generally take 6dB; Vlimit (dB) for the relevant standards such as CISPR, FCC and other provisions of the conducted interference limit.

Figure 1 shows the circuit of AC side EMI filter for switching power supply with 220V/50Hz AC input. Cy = 3300pF, L1, L2 = 0.7mH, which form a common-mode filtering circuit to suppress common-mode interference signals from 0.5 to 30MHz; Cx = 0.1μF, L3, L4 = 200-500μH, which adopt metal powder compression core, and L1/L2, Cx = 0.1μF, L3, L4 = 200-500μH, which form a L-N inter-port lowpass filter. L3, L4 = 0.1μF, are used to form an L-N inter-port low-pass filter for suppressing 0.15 to 0.5MHz differential mode interference signals present on the power line.R is used to eliminate electrostatic build-up that may occur in the filter.

                                           Figure 1 Switching power supply AC side EMI filter circuit

Figure 2 is the DC output side filter circuit of the switching power supply, which consists of common-mode chokes L1 and L2, choke L3 and capacitors C1 and C2. In order to prevent the core from saturating under a large magnetic field strength and make the choke ineffective, the core must be used with good high-frequency characteristics and saturated magnetic field strength of a large constant μ core.

                                                         Fig. 2 Filter circuit on the tributary side

4 Shielding to suppress radiation and induction interference

Switching power supply interference spectrum is concentrated in the frequency band below 30MHz, diameter r <λ/2π, mainly near-field nature of the electromagnetic field, and is a low-impedance field. Available conductive materials for the electric field shielding, and high permeability materials for magnetic field shielding. In addition, transformers, inductors, power devices and other effective shielding measures. Ventilation holes on the shielded enclosure should preferably be circular, and the number of holes can be more under the condition of satisfying the ventilation, and the size of each hole should be as small as possible. The seams should be welded to ensure electromagnetic continuity. Filtering measures are to be taken at the introduction and exit wires of the shielded enclosure. For electric field shielding, the shielding enclosure must be grounded. For magnetic field shielding, the shielding shell does not need to be grounded.

5 Reasonable PCB layout and wiring

Sensitive lines are mainly control circuits and lines directly connected to the interference measurement equipment. To reduce the level of interference, the simplest way is to increase the spacing between the interference source and the sensitive lines. However, due to the limitations of the size of the power supply, simply increase the spacing is not the best way to solve the problem, a more reasonable method is based on the distribution of interference electric field will be placed in the interference of sensitive lines in a weaker place. PCB anti-interference layout design process shown in Figure 3.

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