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Research on EMC Generic Technology of typical household Appliances

Release time:

2013-06-09 00:00

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1 Introduction

　　Electromagnetic Compatibility (EMC) is defined in the national standard GB/4365-1995 "Electromagnetic Compatibility Terminology" as: "Equipment or system can work normally in its electromagnetic environment and does not constitute intolerable to anything in the environment. Electromagnetic disturbance capability". "Can work normally" refers to "Electromagnetic Susceptibility" (EMS for short), that is: in the presence of electromagnetic disturbance, the ability of a device, equipment or system to avoid performance degradation; "does not constitute intolerable to anything "Electromagnetic Interference Capability" refers to "Electromagnetic Interference" (EMI for short), that is, the degradation of equipment, transmission channels or system performance caused by electromagnetic interference. Therefore, EMC includes two aspects of EMS and EMI.

　　With the development of electronic technology and the improvement of production technology, various household electromagnetics emerge in an endless stream, and their structures are becoming more and more complicated. Many household appliances are not only the receiver of electromagnetic interference, but also the generator of electromagnetic interference. People are constantly exposed to a complex electromagnetic environment, and may be harmed by electromagnetic interference at any time. The State Bureau of Technical Supervision held the "International Symposium on the Implementation of Radio Interference of Household Appliances and Electric Tools" in Shanghai in 1996, emphasizing that at this stage it is necessary to further strengthen the promotion and implementation of EMC mandatory national standards, especially for household appliances, electric tools, information Industries such as technical equipment should carry out this work as soon as possible.

　　Electromagnetic compatibility has become an important indicator to measure the quality of household appliances. At present, typical household appliances mainly include electric fans, washing machines, refrigerators, air conditioners, electronic game consoles, electromagnetic cookers, microwave ovens, etc., which will generate different degrees of electromagnetic interference, especially air conditioners with microprocessors, washing machines, etc., their operation The electromagnetic interference generated at the time is quite serious. Due to the wide variety of household appliances and the complex technical requirements of electromagnetic compatibility, it is particularly important to "research on the common technology of EMC of typical household appliances".

　　2 Standards and inspection items for electromagnetic compatibility (EMC) certification of household appliances

　　GB 4343.1-2009 "Electromagnetic Compatibility Requirements for Electromagnetic Compatibility Household Appliances, Electric Tools and Similar Appliances Part 1: Emission" This standard came into effect on April 1, 2010. This section applies to radio-frequency conducted and radiated disturbances generated by appliances whose main function is performed by motors, switching or regulating devices.

　　GB 4343.2-2009 "Electromagnetic Compatibility Requirements for Electromagnetic Compatibility Household Appliances, Electric Tools and Similar Appliances Part 2: Immunity" This standard came into effect on April 1, 2010. This standard applies to the electromagnetic immunity of household and similar electrical appliances, similar appliances, electric toys and electric tools.

　　Electromagnetic immunity (EMS) inspection items include: ① electrostatic discharge immunity; ② radio frequency electromagnetic field radiation immunity (80MHz ~ 1000MHz); ③ electrical fast transient burst immunity; ④ surge (lightning strike) immunity Disturbance; ⑤Injection current (150kHz～230MHz) immunity; ⑥Voltage sag and short interruption immunity.

　　The inspection items of electromagnetic emission (EMI) are: ① continuous interference voltage (150KHz ~ 30MHz); ② intermittent interference voltage (click) (150kHz, 500kHz, 1.4MHz and 30MHz); ③ disturbance power (30MHz ~ 300MHz); ④Radiation emission (30MHz～1GHz, suitable for toys); ⑤Harmonic current (2～40th harmonic); ⑥Voltage change, voltage fluctuation and flicker.

　　3 Common technology research

　　The so-called "typical household appliances electromagnetic compatibility common technology" refers to "in terms of electromagnetic compatibility, common problems and solutions in household appliances", rather than specific types of household appliances. This paper studies the electromagnetic compatibility technology of switching power supply, motor, and microprocessor of typical household appliances, and provides universally applicable and effective solutions to the interference caused by electrostatic discharge, electrical fast transient bursts, and surges.

　　3.1 Switching power supply

　　The high-speed switching action of power semiconductor devices will inevitably generate a large amount of EMI. As an energy conversion device, its voltage and current change rate is very high, and the interference intensity is very large; the switching frequency is not high, and the main interference forms are conduction interference and near-field interference.

　　Due to frequent actions, when the switching power supply is turned on or closed, a large amount of charge will accumulate on the power line and the internal devices of the electrical appliance, which will inevitably generate electrostatic interference; at the same time, due to the high rate of change of voltage and current, Interference with fast transient pulses will be generated.

　　The interference of switching power supply is mainly conduction interference, and EMI filter is the most effective suppression method at present. In addition, experience shows that the EMC performance of switching power supplies can often be greatly improved by modifying the wiring design of the printed circuit board without adding any components and changing the circuit.

　　3.2 Motor

　　Many household appliances are driven by electric motors. Electromagnetic disturbances are particularly serious for household appliances with commutator-type motors. Because it has a commutator and brushes, when the rotor rotates at high speed, the brushes are constantly connected and disconnected, which is very easy to generate brush arcs and form spark disturbances; the motor is an inductive load, when switching the power supply of the load A high reverse voltage will be generated. Both spark disturbance and high reverse voltage have the characteristics of wide spectrum and rich harmonics, and these frequency ranges cover the normal operating frequencies of many broadcasting, communication and other electronic equipment, which will affect the normal operation of the equipment in severe cases.

　　The effective methods to eliminate the disturbance generated by the motor are filtering, shielding and grounding.

　　3.3 Microprocessor

　　The use of microprocessors makes home appliances more intelligent. However, since the microprocessor's PCB board has a large number of digital circuit boards, the clock, video and data drivers on it, and the current flowing in other oscillators are the main source of disturbance. These currents can also generate conducted interference through wires and radiated interference through space.

　　The various external integrated circuits of the microprocessor circuit in household appliances are connected in the form of a bus, and the peripherals are selected by the address line. When not selected by the microprocessor, the device is in a high-impedance state. Under certain conditions, the charge is very small. Prone to buildup and electrostatic discharge when selected.

　　It is worth mentioning that, in addition to solving the above-mentioned EMI, the microprocessor must also improve its own anti-interference (EMS). Once the high-frequency digital pulse is harassed by the outside world, it is easy to fail to work normally, and even cause the entire system to collapse.

　　4 Anti-interference measures

　　4.1 Electrostatic Discharge (ESD) Immunity

　　Electrostatic interference comes from human body discharge, equipment, device charge accumulation discharge, etc. The anti-static interference design of household appliances lies in good grounding and structural design.

　　There are two functions of grounding: safety ground (provide a common path for damaged circuits), and reference ground (provide a reference for signals entering the system from the outside). Ground wire interference can be divided into ground loop interference and common impedance interference according to the mechanism of generation. Ground loop interference is caused by loop currents consisting of ground wires and cables. Generally, effective ways to reduce ground loop interference include: floating the equipment at one end, using transformers to realize the connection between equipment, using photoelectric isolators, using common mode choke coils, etc.; public impedance interference, as the name implies, is due to two The circuit shares a section of ground wire, and the ground wire itself has impedance. There are two ways to reduce common impedance interference: one is to reduce the impedance of the common ground part, so that the voltage on the common ground is also reduced, thereby controlling the common impedance coupling; the other is to avoid Circuits that are prone to mutual interference share the ground wire. Generally, it is necessary to avoid sharing the ground wire with strong and weak current circuits, and avoid sharing the ground wire with digital and analog circuits.

　　The standard stipulates that all parts of the test product that can be touched by the human body must be subjected to electrostatic discharge tests. It can be seen that the structure of household appliances plays an important role in the anti-static ability. Household appliances are generally placed in metal shells, and the metal shell acts as a channel for ESD current to guide the ESD current around the internal circuit. At this time, the holes and gaps in the shell can act as high-impedance barriers to the current. In order to make it At a minimum, all metal plates must be bonded together and provide a low impedance connection at at least two points.

　　4.2 Electrical fast transient burst immunity

　　When operating switches, relays, and contactors operate, multiple transient pulse groups will be generated on their busbars, which are called group pulse interference. Fast transient bursts generally do not cause damage to devices and systems, but they can have a significant impact on microprocessors in household appliances. When these high-frequency sharp pulses appear on the address bus, data bus or logic gate circuits, it will cause timing errors in the microprocessor and logic circuits, resulting in program confusion and crashes of the microprocessor. Therefore, anti-spike interference is an important link in the EMC design of household appliances with microprocessors.

　　Various components of household appliances, such as relays, contactors, switches, buttons, etc., should be specially treated. Generally, capacitive elements and diodes are connected in parallel or in series at both ends of them to absorb spike interference; secondly, in extremely important circuits Some components that absorb spikes should be added in the circuit.

　　Let's take an electric toy with a motor as an example, and use experimental data to illustrate the effect of the filter.

　　Figure 1 shows the emission test of an electric toy (commutator type motor) for disturbance radiation emission

Figure 1 Radiation harassment of an electric toy Figure 3 Radiation harassment of an improved electric toy

　　It can be seen from the figure that the electromagnetic radiation of modified electric toys is quite serious. In order to reduce radiation and meet the standard of electromagnetic compatibility, we add filters, as shown in Figure 2.

　　In Figure 2(a), two choke coils of 100µH are added to the power line to suppress differential mode noise. The choke coil is installed symmetrically, so that on the one hand, the internal impedance of the choke coil is balanced, and on the other hand, the disturbance voltage between each wire and the ground is balanced. In this case, the disturbance voltages generated by the two brushes are equal in magnitude and opposite in direction, and the disturbance electric fields caused between the wire and the ground cancel each other out. Since the directions of the disturbance currents in the two choke coils are different, the disturbance magnetic fields on the wires also cancel each other out, thus reducing the disturbance.

　　Figure 2 (b) further added a common mode choke, the effect is better. The measured data after using the filter is shown in Figure 3. It can be seen that the filtering effect is quite obvious, and the radiation disturbance intensity is reduced by nearly 30dBµV/m. But if the disturbance intensity is very large, you need to use an absorption filter.

　　Absorptive filters are a common and inexpensive device today. It is composed of lossy devices, and the energy absorbed by the noise in the stop band is converted into heat loss, thereby playing a filtering role. Ferrite absorption filter is a kind of low-pass filter which is developing rapidly at present. It has better high filtering characteristics than conventional L filter.

　　Ferrite material is a mixture of iron oxide and other components such as manganese, zinc-cobalt, and nickel. It is highly sensitive to frequency, and low-frequency signals can pass through without loss. When the frequency increases, its impedance also increases rapidly. , effectively suppressing high-frequency signals. After replacing the inductance in the circuit in Figure 2 with a ferrite absorption filter, the radiation disturbance field strength can be reduced by another 10dBµv/m. The ferrite magnetic ring can be directly put on the wire, the longer the magnetic ring, the greater the impedance. Note that the inner diameter of the ring should be closely attached to the wire to eliminate the magnetic flux generated by the current on the wire and increase the filtering effect.

　　4.3 Surge (lightning strike) immunity

　　Surge refers to the change of power supply voltage and current, which may be caused by the closing of the load switch and natural lightning strikes. It is more harmful and can cause vibration or even burn out the entire system.

　　Household appliances are generally not directly disturbed by lightning, but are mostly disturbed by the induced current or voltage in the conduction line. Good grounding is an effective means to solve this interference.

　　Commonly used devices to prevent surge interference are: gas discharge tube, metal oxide varistor (MOV) and transient voltage suppression diode (TVS).

　　5 Conclusion

　　Electromagnetic compatibility is an important criterion for household appliances. Due to the wide variety of household appliances and their complex structures, the research on their common technologies is extremely important.

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