Electromagnetic compatibility design analysis of household appliances

1. The Influence of Electromagnetic Disturbance of Home Appliances on the Electromagnetic Environment

The impact of electromagnetic disturbances on the electromagnetic environment of household appliances includes electrostatic discharge, pulse bursts, lightning surges, and electromagnetic disturbances. In recent years, with the increasing variety of household appliances, the production scale continues to expand; At the same time, imported products are also flooding the market. On the other hand, with the development of microelectronic control technology, a large number of electronic and household appliances have emerged. The resulting electromagnetic disturbances not only affect and endanger the normal use and reliability of other household appliances or electronic devices, but also have a direct impact on people's physical health. With the establishment and improvement of the electromagnetic compatibility certification system in China, the electromagnetic compatibility of electronic products, including household appliances, will continue to improve, thereby gradually reducing electromagnetic environmental pollution and protecting the vital interests of users.

According to the reasons for the impact of the electromagnetic compatibility design of household appliances on the electromagnetic environment, household appliances can be divided into the following types:

(1) Household appliances that generate electromagnetic interference due to the contact between the brush and the commutator during the operation of a motor with a commutator, such as vacuum cleaners, hair dryers, electric sewing machines, food mixers, and electric shavers; Electromagnetic disturbance refers to any electromagnetic phenomenon that may cause degradation in the performance of other household appliances or electronic devices or have harmful effects on human health;

(2) Household appliances that generate electromagnetic interference due to frequent switching actions, such as rice cookers, electric irons, electric ovens, refrigerators, washing machines, etc;

(3) During startup, various indicators of the low-voltage distribution network may decrease, leading to other household appliances or electronic devices that cannot operate normally. For example, the large current required during startup of an air conditioner may temporarily decrease the grid voltage. When starting an inductive load such as a motor or an inductive ballast, it may cause a decrease in the power factor of the power supply, and the power electronics in a variable frequency adjustable speed air conditioner may also generate power harmonics;

(4) Household appliances with silicon controlled devices, such as electronic dimming lamps, multi-function controllers for electric fans, can generate high order harmonic disturbances;

(5) Various gas discharge lamps, such as fluorescent lamps, generate electromagnetic disturbances due to the plasma generated during gas discharge;

(6) The two-level digital signals used in household appliances with microprocessors, such as fuzzy appliances, appliances with remote control, fully automatic washing machines, DVD players, combined sound systems, home theaters, water heaters, and microwave ovens, can cause electromagnetic interference. Moreover, with the continuous improvement of clock frequency, the interference spectrum can reach as high as hundreds of megahertz;

(7) The leakage of electromagnetic energy generated by microwave ovens, electromagnetic stoves, and home theater power amplifiers not only causes electromagnetic disturbances, but also poses hazards to human health.

If the electromagnetic interference generated by household appliances exceeds the limits specified in national standards, it will cause electromagnetic environmental pollution. For example, harassing the listening and watching of radios and televisions; Misoperation or data loss of computers and their controlled electrical appliances; Causing medical devices such as cardiac pacemakers to malfunction; The impact on human health includes accelerated blood flow, elevated local body temperature, decreased enzyme activity, protein denaturation, heart rate changes, and local tissue damage. In addition, it can also have adverse effects on the human nervous system, blood and immune system, cardiovascular system, and reproductive system.

2. Electromagnetic compatibility of household appliances

The electromagnetic compatibility of household appliances refers to the ability of household appliances to work normally in their electromagnetic environment without causing performance degradation, and without causing unbearable electromagnetic interference to anything in the electromagnetic environment. That is to say, in a complex electromagnetic environment, in addition to being able to withstand external electromagnetic interference and operate normally, each household appliance must not interfere with the normal operation of other household appliances or electronic devices or cause harm to human health. This is the problem that needs to be solved for electromagnetic compatibility of household appliances.

3. Electromagnetic Compatibility Design of Home Appliances

The purpose of electromagnetic compatibility design for household appliances is to make them electromagnetic compatible. This requires starting with analyzing electromagnetic interference sources, interference coupling paths, and sensitive circuits, designing protective measures such as grounding, shielding, and filtering, suppressing electromagnetic interference at the source of interference, cutting off coupling to electromagnetic interference, and enhancing the immunity of sensitive circuits. Experience has shown that in the initial stage of functional design of household appliances, simultaneous EMC design can solve EMC issues such as electromagnetic interference and immunity before the product is finalized; If a problem is discovered and solved after the product has been finalized, even during the production stage or after it reaches the user's hands, it will not only bring great technical difficulties, but also create a huge waste of human and financial resources. Therefore, it is very necessary to solve the electromagnetic compatibility problem as soon as possible. The practice of ignoring electromagnetic compatibility and only designing products in accordance with the convention, then conducting electromagnetic compatibility tests on samples, and then remediating problems is a very risky approach, which is completely undesirable.

3.1 The key to the electromagnetic compatibility design of household appliances is the selection of active devices and the design of printed circuit boards

For example, digital circuits are a common broadband disturbance source. The shorter the rise/fall time, the wider the disturbance spectrum, and the high-frequency component can extend to over megahertz. Therefore, while ensuring circuit functionality, lower pulse repetition rates and slower rise/fall times should be selected. Another example is that power amplifiers in home theaters are important sources of electromagnetic interference. In addition to selecting devices with appropriate power, technical indicators such as harmonic emission and spurious emission should also be selected. On the other hand, high-speed logic circuits, high-speed clock circuits, microprocessors, and video circuits are both potential sources of electromagnetic interference, but they are also vulnerable to interference. If the intensity of pulse interference exceeds the noise tolerance of the circuit, it will cause malfunction. CMOS and HTL devices have high noise tolerance and should be preferably used.

In order to control differential mode radiation, signal lines and return lines should also be placed close together to reduce the loop area formed by the signal path. Because the signal loop functions as a loop antenna that radiates or receives magnetic fields. Due to the impedance of the ground surface, there is a ground potential, known as a common mode voltage, which can excite common mode radiation on the external cable. In order to control common mode radiation, it is necessary to reduce the common mode voltage and select a reasonable grounding point; Onboard filters or filter connectors can also be used to filter common mode currents; Of course, if the shielding layer of the shielded cable and the shielding box form a complete shield, the effect of suppressing common mode radiation can also be achieved. In addition, reducing signal frequency and level is also an important measure to reduce radiation. In order to further reduce the radiation of printed circuit board wires, the design should also meet the 20H and 2W criteria. Here, H is the distance between two layers of printed circuit boards, that is, the component surface should be 20H smaller than the ground surface to avoid radiation caused by edge effects; W is the width of the printed circuit board conductors, that is, the spacing between high-frequency or high-speed circuit conductors is not less than twice the conductor width to reduce crosstalk. The conductor should be short, wide, uniform, and straight. In case of a bend, a 45 ° angle should be used. The conductor width should not change abruptly, and sudden corners should not be allowed.

It should be noted that although single panel manufacturing is simple and easy to assemble, it is only suitable for general circuit requirements and not suitable for occasions with high assembly density or complex circuits; Double sided panels are suitable for applications where only moderate assembly density is required; For high assembly density or complex circuits, multilayer boards should be preferred. At this time, digital circuits and analog circuits can be arranged in different layers, with the power supply layer and ground layer adjacent, and the disturbance sources arranged separately and away from sensitive circuits; High speed and high frequency circuits should be close to edge connectors. After completing the design of the printed circuit board, each circuit on the board should be able to work normally, without mutual interference, and can reduce the interference emission to a low level with good immunity.

3.2 In the electromagnetic compatibility design of household appliances, ground wire design is an important and often difficult design

The safety of household appliances requires independent design to provide reliable safety assurance for the person and household appliances. The signal ground wire is a low impedance path for signal flow back to the source. The types include floating ground, single point ground, multi-point ground, and mixed ground. Floating ground is prone to electrostatic accumulation and discharge, and is rarely used. Since the ground wire is not ideal for zero impedance, the unit circuit should be grounded at a single point; The multi-level circuit grounding point should be selected at the input end of the low-level circuit to reduce the interference of the ground potential on the circuit; The shield of the small signal high gain amplifier should also be grounded at a single point, and the grounding point should be selected on the output terminal ground wire to prevent self-excitation; Complex household appliances often contain multiple electronic circuits, motors, appliances, etc. At this time, the ground wire should be divided into signal ground wire, disturbance source ground wire, shield ground wire, chassis ground wire, etc., and laid in groups, and then concentrated into one ground wire, which can overcome the interference caused by various parts of the ground wire. Multipoint grounding is generally used in frequency ranges above 10 MHz to facilitate nearby grounding. However, due to the existence of ground voltages between two different ground points, when the circuit is grounded at multiple points and there are signal connections between the circuits, it will constitute a ground loop disturbance. The ground voltage will be superimposed on the signal and added to the load terminal, causing differential mode interference. If two conductors are used for signal transmission between two circuits, the ground voltage will be applied to the two conductors. Due to the unequal negative reactance of the two conductors to the ground, the magnitude of the common mode current generated by the ground voltage on the two conductors will also cause differential mode interference at both ends of the load. In order to overcome ground loop interference, the signal ground wire and the chassis ground wire should be laid in groups, insulated from each other, and then combined to increase the ground loop impedance. Most of the ground voltage will be reduced above this impedance, and the portion added to the wire will be greatly reduced. In addition, isolation transformers, common mode chokes, or photocouplers can be inserted between the two circuits to cut off the ground loop, and good results can also be achieved.

3.3 Shielding technology can be used to suppress radiation interference in household appliances

Radiation disturbance propagates in space in the form of electromagnetic fields and waves. Usually, metal or magnetic materials can be used to surround the disturbance source, isolating the "fields" inside and outside the shield. It can be seen that shielding technology can also be used to protect sensitive circuits in household appliances from external radiation interference. The evaluation of shielding effectiveness can be expressed as shielding effectiveness, which is the ratio of the field strength of the point to be measured before shielding and the field strength of the point to be measured after shielding. The shielding effectiveness is the sum of the reflection loss, absorption loss, and multiple reflection correction of the shield.

For interference sources with high voltage and small current, near-field electric field shielding should be used, that is, using a high conductivity metal shield and grounding;

For low voltage and high current disturbance sources, near-field magnetic field shielding should be used, but different measures should be taken at different frequencies. In DC or low frequency applications, shields can be made of high permeability materials such as iron, silicon steel, permalloy, and alloy, and may not be grounded. The higher the magnetic permeability, the higher the shielding effectiveness. Increasing the thickness will also increase the shielding effectiveness, but it is very uneconomical, and multi-layer shielding can be used instead. When the frequency is high, it is no longer suitable due to decreased magnetic permeability and increased magnetic loss. High conductivity metal materials such as copper, aluminum, etc. should be used. At this time, if the shield is well grounded, it can also shield high-frequency electric fields.

When the sensitive circuit is far away from the disturbance source, a far field electromagnetic shield is used, that is, a shield made of highly conductive metal materials. The material of the shielding case or box for household appliances can generally be copper plate, steel plate, aluminum plate, galvanized iron plate, etc., with a thickness of about 0.2-0.8mm. Due to the skin effect of high-frequency current, spraying a layer of conductive paint on the engineering plastic chassis can provide a good shielding effect. However, there are always various sizes of holes, seams, and openings in the actual chassis, which can cause interference coupling. To this end, conductive pads, wire mesh, cut-off waveguide tubes, cut-off waveguide ventilation plates, conductive glass windows, etc. can be used to improve shielding effectiveness.

3.4 Filtering technology can be used to suppress the transmission of conducted disturbances in household appliances, allowing the required frequency components to pass smoothly

The important characteristics of the filters used are characterized by frequency characteristics, namely, the change of insertion loss with frequency. Insertion loss is defined as the ratio of the voltage directly applied to the load by the signal source without a filter to the voltage applied to the load by the signal source after passing through the filter. The frequency range that can pass through the filter without attenuation is called the pass band, while the frequency range that is greatly attenuated is called the stop band. Filters for suppressing conducted disturbances include reflective filters and absorption filters.

Reflective filter is a low-pass filter composed of inductance, capacitance, and other components. It can provide high series impedance and low parallel impedance within the stopband, and seriously mismatches the impedance of the disturbance source. It can reflect high-frequency disturbances back to the disturbance source and be filtered out, while useful low-frequency components are attenuated by a small amount. Commonly used for power line filters and signal line filters.

The absorption filter is composed of lossy devices, which in the stopband absorb the energy of conducted disturbances and convert it into heat loss, thereby playing a filtering role. Ferrite material is a widely used lossy device that can be used to construct low-pass filters. When the low-frequency current in a wire passes through the ferrite, there is almost no loss, but the high-frequency current can suffer significant losses. This is because a ferrite material can be equivalent to a series connection in which both the resistance and inductance vary with frequency. In the low frequency range, inductance plays a major role, while in the high frequency range, resistance plays a major role and increases as frequency increases, while inductance decreases as frequency increases. Therefore, it has a significant attenuation effect on high frequency components, while almost no attenuation effect on DC or low frequency components. According to different applications, ferrite can be made in various forms, such as resistance components that can be directly soldered onto printed circuit boards, magnetic beads that can be strung on low speed signal lines, magnetic rings that can be sheathed on elements, device pins, or wires, cylindrical magnetic rings that can be sheathed on cables, and rectangular magnetic rings.

3.5 Comprehensive use of protective measures such as grounding, shielding, and filtering

Using any protective measure alone will not achieve the desired inhibitory effect, and comprehensive use techniques need to be mastered.

The necessary condition for near-field electric field shielding is to use a high conductivity metal shield and ground it; A well grounded shield can also achieve far field electromagnetic shielding. The shielding effectiveness of the shielding layer of a shielded cable is mainly not obtained by reflection and absorption, but by grounding the shielding layer. This is because after the shield layer is grounded, external disturbances can be short-circuited to ground, avoiding coupling of disturbances to the core due to coupling between the shield layer and the core wire; Similarly, after the interference of the core wire is coupled to the shielding layer, it is also short-circuited to ground, preventing the interference of the core wire from radiating outward through the shielding layer. Visible shielding and grounding are inseparable.

The main ways for electromagnetic interference to invade the shield are the input/output interface and the power line input port. Because electromagnetic interference inside the shield can be coupled to the wires and cables entering and exiting the shield, conducting it outside the shield, causing radiation interference. Similarly, external electromagnetic disturbances can also enter the shield through electromagnetic coupling, conducted by these wires and cables. In order to prevent the inflow or outflow of disturbance current, a simple and feasible method is to install a filter connector and a feedthrough filter at the input/output interface and the power line input port respectively. In addition, the aforementioned cutoff waveguides, cutoff waveguide ventilation plates, and conductive glass windows installed at openings are essentially high pass filters. Visible shielding and filtering are also inseparable.

All types of filters must be grounded, except for filters that are specifically allowed to be ungrounded when used. Because the common mode bypass capacitor in the filter can only function as a filter when grounded. In particular, when the filter is poorly grounded, it is equivalent to parallel connection of capacitors and inductors, completely losing the role of filtering. In addition, when installing filters, shielding should also be used to isolate the input and output terminals to avoid coupling. Therefore, filtering is closely related to grounding and shielding.

In order to reduce ground loop interference, it is necessary to insulate and then centralize the signal ground wire from the chassis ground wire. Therefore, a feedthrough filter must be installed when the signal ground wire passes through the chassis. When household appliances in high-rise buildings need to be installed with ground wires connected to the ground, in order to avoid coupling electromagnetic interference due to excessively long ground wires, shielding sleeves must also be installed outside the ground wires. It can be seen that grounding, filtering, and shielding are also inseparable and must be comprehensively utilized.