Discussion on electromagnetic compatibility design of system based on switching power supply

　　With the rapid development of electronic technology, electronic equipment is also developing towards functional integration and miniaturization, which brings us many conveniences, but electromagnetic coupling between various electronic devices has also become the main problem faced by engineers. The harm of electronic environmental pollution is no less than traditional environmental pollution. And electromagnetic pollution as part of environmental pollution has also been put on the agenda. When electronic equipment is working normally, it will be subjected to various electromagnetic interferences, including mutual interference between its internal components and interference from other electronic equipment around it, and will also generate electromagnetic interference to other electronic equipment around it. The requirements for electronic equipment in different application environments (home, industrial control, power) are very different. In this regard, you can refer to the general standard IEC/EN61000-6 series or the industry requirements of the corresponding products.

　　This kind of electromagnetic interference mainly includes two aspects in terms of transmission path: one is transmission along the wiring harness, which mainly includes transmission along the power port and signal port; the other is mainly transmission along the space.

　　1Electromagnetic interference:

　　The power supply must meet the corresponding minimum emission energy requirements in its application environment, otherwise it will interfere with the surrounding equipment. The standard IEC/EN61000-6 is divided into industrial environment equipment requirements and emission requirements for residential areas, commercial areas and light industrial environments according to general requirements. For general products such as power supplies, electromagnetic interference positioning is carried out in accordance with IEC/EN61000-6-3 or IEC/EN61000-6-4 in the early design stage unless it is a special model.

　　As the size of power supplies continues to shrink and power density continues to increase, the difficulty of electromagnetic interference design for power supplies itself continues to increase. All of MORNSUN's AC-DCs on the market currently not only have built-in filters, but also invest a lot of design costs in transformer shielding and power device noise absorption to meet the promised index requirements; the R2 generation of low-power DC-DC products are all designed with a six-sided shielding structure to meet the industry EN55022/CISPR 22, EN55011/CISPR 11 CLASS A requirements, in line with the grade requirements of basic industries.

　　Although a lot of design costs have been invested in the electromagnetic interference of the power supply itself, and it also meets the promised indicators, it is still inevitable that the electromagnetic interference exceeds the standard in the market application. At this time, many design engineers will think that the root of the problem lies in the power supply. This understanding is actually a misunderstanding, because the electromagnetic interference conducted disturbance test project is mainly aimed at the power port, so the power port becomes its transmission path, and all electromagnetic interference will reach the device under test through the power port. However, the electromagnetic interference tested by the test equipment comes not only from the power supply itself, but also mainly from other parts of the whole machine, as well as the electromagnetic interference inside the device. The electromagnetic interference generated by the resonance of the parasitic parameters of the power supply will be coupled to the test equipment through the power port. The filter inside the power supply cannot filter this part of the electromagnetic interference. The power supply application environment varies greatly. All power supply design filter parts are based on solving their own interference as the primary consideration. At the same time, the filter attenuation characteristics and spectrum characteristics will reserve the maximum margin as much as possible, but it is impossible to be compatible with all applications. Then this requires our whole machine designers to design the front end of the power supply according to the application circuit recommended by the power supply manufacturer. For example: EMI exceeds the standard during the application of LH15 products (see the figure below).

　　The figure above shows the conducted disturbance test result of MORNSUN power supply LH15-10B05. This result complies with the CLASS B requirement of EN55022/CISPR22, and the margin is very sufficient.

　　The above picture shows the conducted interference result of the whole machine after the MORNSUN power supply LH15-10B05 was applied to a certain brand of product. This result cannot meet the CLASS B requirements of EN55022/CISPR22, and even cannot meet the requirements of CLASS A, not to mention the design margin.

　　Therefore, even if the internal electromagnetic interference design level of the power supply is high, the application part must be retained during the application process. For specific parameters, please refer to the specification sheet corresponding to the specific product. MORNSUN's power supply products will have an application circuit column in the specification sheet, which will describe the indicators achieved based on the application circuit in great detail.

　　2Electromagnetic immunity:

　　In addition to meeting the electromagnetic interference requirements mentioned above, the power supply must also meet the anti-interference requirements of the corresponding application environment. If it cannot meet the minimum requirements of this environment, it will be affected by the electromagnetic interference generated by other surrounding equipment, resulting in damage, unstable output and other abnormal phenomena, which will ultimately affect the normal operation of the entire machine.

　　For general products such as power supplies, there are no specific standards requiring the anti-interference performance to reach a certain level. When applied to specific industries, the industry standards are referenced; however, in the early stages of design, there is no specific industry targeted, and only the specific requirements of the general standard IEC/EN61000-6 can be referenced. The standard IEC/EN61000-6-1/2 is divided into the anti-interference requirements for industrial environment equipment and the anti-interference requirements for residential areas, commercial areas and light industrial environments. The AC-DC part of the MORNSUN power supply is designed according to the most stringent level of industrial products, while ensuring that the design margin is very sufficient. At present, this type of power supply promises a four-level indicator of 2KV (differential mode) / 4KV (common mode) protection capability of the product, and the internally designed port protection varistors all use the 14D specification (see the figure below)

It can be clearly seen from the table below that the continuous flow rate of the 14D specification can reach 4.5KA, so the promised index is only 1KA (differential mode) / 333KA (common mode). Through this comparison, it can be seen that the design derating is already very large, but during the long-term use of the product in the market, the varistor will be damaged, and eventually the power supply will burn out. The main reasons are two aspects: on the one hand, it is caused by the aging of the varistor itself. The ZnO varistor, which is very commonly used in the market, has an insulating layer composed of ZnO particles in the middle, and electrodes are formed on both sides by silver plating. When the voltage of the electrodes on both sides exceeds its threshold voltage, the leakage current will increase sharply, and finally form a transient current discharge, which plays a protective role.

　　The varistor discharges current for transient surge pulses. After multiple discharges, the characteristics of the ZnO medium of the varistor will change, so the residual pressure characteristics and discharge capacity of the varistor will be greatly reduced. What is more serious is that the varistor has a two-sided silver-plated structure, and the surface silver plating cannot be 100% uniform. This means that every time a transient surge impact occurs, there must be a point on the surface of the entire varistor that is the first to be turned on. After enduring multiple impacts, the first point that breaks down and turns on will be the first to burn out, eventually causing damage to the varistor (see the figure below).

　　This type of varistor discharges current through the breakdown point, generating a large amount of heat at the discharge point, which will eventually cause the varistor to burn through (see the figure below).

　　On the other hand, the damage factor is due to the failure caused by improper use of the power supply by the end customer. As described above, improper use will cause excessive electromagnetic interference, and the electromagnetic immunity will also be seriously affected. Customer application sites vary greatly, so the non-isolated reference to the MORNSUN isolated AC-DC small power supply module (see the figure below) will cause damage to the power supply. Even if the power supply is lucky enough to be normal in this transient surge impact, the load part at the back end will also produce various abnormal phenomena, which is a very headache for the whole machine design engineer.

　　What kind of problems will the non-isolated application of isolated power supply cause? First, when conducting common-mode surge test, the surge impact between the common-mode line and the ground will become the withstand voltage between the primary and secondary sides of the isolated power supply module. For various industrial, electric power, rail transportation and other application environments with very stringent requirements on product reliability, the line-ground will be tested according to the 4KV surge level. Most industries design the isolation of the power supply according to the requirements of 3KV or lower, so the power supply module will be difficult to avoid damage. Only the isolation of special industries such as medical care will be designed at 4KV, but the isolated power supply at this time needs to sacrifice volume, cost, etc.

　　The second problem is that when there are various noise interference signals such as transient pulses at the input end, the isolated power supply can play a good role in protecting the back-end load. However, after non-isolation application, all interference signals at the input end will be transmitted to the load end intact, which will cause the entire system to become abnormal or even paralyzed.

　　The above application is often questioned. Generally, the whole machine design of a well-known international brand is used as an example to tell us that there are many such applications in the market. Of course, this kind of application does exist. So under what circumstances will this application not cause abnormalities? For areas with very developed power systems, their power grids are already reliable, and the electromagnetic interference of the power grid loads is already very ideal, there is no problem; in addition, this kind of application may have invested a lot of design costs on the load side to avoid transient interference on the input side, and non-isolated applications will not have abnormalities;

　　So, if the isolated power supply must output Vout- and connect it to the PE terminal during the application process, you can connect the two terminals through a capacitor according to the connection idea in the above figure. This design ensures that the customer's special usage requirements are met and the above problems can also be effectively avoided.

　　In short, from the perspective of reliability, this design is highly discouraged.

　　3 Conclusion:

　　The design of electromagnetic compatibility of the whole machine is actually a systematic project, which requires us to fully evaluate the indicators and application environment at the beginning of the design, and fully evaluate the circuit diagram design, raw material selection, PCB drawing, structural design, process installation and other aspects during the design process; any point of design failure may lead to design failure, and even pay a heavy cost. At present, the design failure in this aspect in the industry is limited to the power supply, and it needs to be further sorted out and improved.