【 Weak current College 】 switching power supply for electromagnetic interference suppression and standard 】

I. introduction

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Electromagnetic compatibility (EMC) refers to electronic devices or systems in the electromagnetic environment-level not due to electromagnetic interference and decrease performance indicators, while they themselves produced electromagnetic emissions is not greater than the limit level, without prejudice to other electronic device or system to function properly, and device and equipment, systems and interference between systems and common purpose of working reliably.

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The world has a corresponding established their own EMC standards. For example, the International Electrotechnical Commission (C1SPR 1EC61000 and standards, the European Community of the FN series standard, United States Federal Communications Commission FCC series standard and our present GT3/T13926 series EMC standards, etc. As the international electromagnetic compatibility regulations increasingly stringent product of electromagnetic compatibility is becoming more and more attention.

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Switching power supply as a power device, the application of more and more widely. With the constant upgrading of power electronic devices, switching power supply switching frequency and switch speed continues to increase, but a quick on-off switch, causing the voltage and current of rapid change. These transient voltage and current, through the power lines, parasitic parameters and stray electromagnetic coupling, will produce a lot of electromagnetic interference.

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2. switch on the power of interference analysis

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Switching power supplies produce electromagnetic interference (EMI), the coupling channel, can be divided into conducted interference and radiated interference; press the noise source types to points can be divided into peak interference and harmonic interference. Switching power supply in the course of the surge current and voltage spikes form the source of interference, power frequency rectifier filtering using large capacitor charging, discharging, switching-frequency of the voltage switch and output rectifier diode reverse recovery currents are such interference.

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3. electromagnetic interference suppression measures

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Electromagnetic interference by a combination of three basic elements: electromagnetic interference generated; on the interference source energy sensitive devices; electromagnetic interference sources transfer to sensitive equipment of media is transmission channel or coupled approach. On switching power supplies produce electromagnetic interference suppression measures taken mainly from two parties but consider: first, reducing interference sources of disturbance; one is to cut the noise transmission.

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Commonly used anti-interference measures including circuit isolation, shielding, grounding, retro EMI filters, and PCB layout and routing.

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1. circuit isolation

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In switching power supply, circuit isolation: the isolation analog circuit, digital circuit, digital circuit of quarantine and isolation between analogue circuit. The main aim is the noise isolating components of a path cut off, so as to achieve the effect of suppressing noise interference. For switching power supply of the analog signal control system, AC signal generally use the transformer isolation, DC signal generally use linear isolator (such as linear optoelectronic ouqi) isolation. Digital circuit in isolation are: pulse transformer isolation, photo coupler isolation, etc. One digital input isolation mode mainly uses the pulse transformer isolation, photo coupler isolation; and digital output isolation mode main photo coupler isolated, high-frequency transformer isolation.

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2. masking

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Shielding typically fall into two categories, one category is static, primarily used to prevent electrostatic field and constant magnetic field; another kind is the electromagnetic shielding, primarily used to prevent alternating electric field, alternating magnetic field, as well as the effect of alternating electromagnetic field. Shielding is radiated interference suppression switched mode power supply. You can use conductive good material on electric field shielding, and permeability rate of high magnetic field shielding material.

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3. grounding

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In order to prevent the various circuits generated in work interfere with each other, so that it can work compatible with each other, depending on the nature of the circuit, the Earth is divided into different categories. Such as DC, AC, digital, analog, signal, power, power supply, etc. In circuit design, AC power and DC power source segregation, analog circuits and digital circuits of power to separate, power, and power to separate.

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4. installation of EMI filter

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Power filter installed in the power cable and electronic device for restraining power of conducted noise and reduce the transmission grid, introduced to improve the reliability of the device has an important role. Switching power supplies produce electromagnetic interference to conducted interference, conducted interference and harassment and differential mode EMI. Constitute a switching power supply EMI filter basic network as shown in Figure 1. The filter consists of common mode choke coil L, differential mode capacitor CX and Cy-mode capacitance. Common mode choke coil L consists of two round the same high permeability ferrite core on winding, their structure makes the differential mode current flux offset each other. This structure to the smaller volume for a large inductance value, and not to worry because current result saturation. Each coil and capacitor Cy L-E and N-E respectively composed of two independent ports of low-pass filter, form the common mode filter network, used to restrain the power cord of the common mode interference. As regards the common mode choke coil L, differential mode capacitor CX and Cy-mode capacitance value size, you should try to do filter resonance frequency is below the switching power supply operating frequency, so that you can implement a filter to the entire band.

5.PCB plate reasonable layout and wiring

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Switching power supply's radiation interference and the current path of the current size, access loop area, as well as current frequency squared, etc. are directly proportional to the product of that radiantRadio interference E ¡ø I · A · f2. Use the premise of this relationship is the pathway size much less than frequency wavelength. These relationships that reduce access area is the key to reducing radiation disturbance, it is said that the switching power supply components to each other closely. In the primary circuit, require input capacitance, transistors and transformer closer and compact; the secondary wiring circuit, diodes, transformer and capacitor as close to each other at the output port. In the printed circuit board, will now load flow wires respectively in both sides of the printed circuit board, and to make the two current-carrying conductors remain parallel to each other, because parallel immediately of load current conductor of the external magnetic field is tend to offset each other.

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IV. concluding remarks

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To increase the switching frequency, improved product quality, electromagnetic compatibility is a problem. Produce electromagnetic interference of the switching power supply has a lot of factors, there are a large number of electromagnetic interference suppression. Only at design time to fully consider the problem of electromagnetic compatibility, to make a more universal switching power supply.