Discussion on electromagnetic compatibility issues based on switching power supply system

introduction

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 has also been put on the agenda as part of environmental pollution. 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 of 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.

1 Electromagnetic 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 will be 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 be miniaturized and the power density continues to increase, the difficulty of electromagnetic interference design for the power supply 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 small-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, and meet 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. The electromagnetic interference, 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 disturbance result of the whole machine after the power supply of MORNSUN LH15-10B05 was applied to a certain brand of product. This result cannot meet the CLASS B requirements of EN55022/CISPR22, and even CLASS A cannot meet the requirements, 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 reserved during the application process. For specific parameters, please refer to the specification sheet corresponding to the specific product. MORNSUN 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.

2 Electromagnetic 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. The internally designed port protection varistors all use 14D specifications (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 the two sides exceeds its threshold voltage, the leakage current will increase sharply, and finally form a transient current discharge, which plays a protective role.