summary:
With the rapid development of electronic technology, electronic devices are also evolving towards functional integration and miniaturization, bringing us numerous conveniences. However, electromagnetic coupling between various electronic devices has become a major problem for engineers. The harm of electronic environmental pollution is no less than that of traditional environmental pollution. Electromagnetic pollution, as a part of environmental pollution, has also been put on the agenda. This article briefly introduces the technical issues that need to be considered when designing the electromagnetic compatibility of the entire device, focusing on the power supply itself, as well as the design ideas and methods.
1. Introduction:
With the rapid development of electronic technology, electronic devices are also evolving towards functional integration and miniaturization, bringing us numerous conveniences. However, electromagnetic coupling between various electronic devices has become a major problem for engineers. The harm of electronic environmental pollution is no less than that of traditional environmental pollution. Electromagnetic pollution, as part of environmental pollution, has also been brought to the forefront. During normal operation, electronic devices are subject to various electromagnetic interferences, including mutual interference between their internal components and interference from other surrounding electronic devices, while also generating electromagnetic interference for other surrounding electronic devices. The requirements for electronic devices vary greatly in different application environments (home, industrial control, power). For reference, one can refer to the general standard IEC/EN61000-6 series or the corresponding industry requirements for the product.
This electromagnetic interference mainly has two transmission paths: one is transmission along the wiring harness, which mainly includes transmission along the power port and signal port; the other is transmission along space.
2. Electromagnetic interference:
Power supplies must meet the corresponding minimum emission energy requirements in their application environment; otherwise, they will interfere with equipment in the surrounding environment. Standard IEC/EN61000-6 is divided into requirements for industrial environment equipment and emission requirements for residential, commercial and light industrial environments according to general types. For general products like power supplies, electromagnetic interference positioning in the early design stage is carried out in accordance with IEC/EN61000-6-3 or IEC/EN61000-6-4 unless it is a special model.
As power supplies become increasingly smaller and their power density increases, the design challenges for electromagnetic interference (EMI) protection also increase. MORNSUN's current AC-DC power supplies not only incorporate built-in filters but also incorporate significant design investment in transformer shielding and power device noise absorption to meet promised performance requirements. The R2 generation of low-power DC-DC products all feature a six-sided shielded structure, meeting the CLASS A requirements of industry standards EN55022/CISPR 22 and EN55011/CISPR 11, thus complying with basic industry standards.
Although significant design costs have been invested in the power supply itself to mitigate electromagnetic interference (EMI) and it meets all promised performance requirements, EMI exceeding limits is still unavoidable in market applications. Many design engineers mistakenly believe the power supply is the root cause. While conducted EMI testing primarily targets the power supply port, making it the transmission path for all EMI reaching the device under test, the EMI detected by the testing equipment originates not only from the power supply itself but also from other components within the entire system and from internal sources. Electromagnetic interference (EMI) generated by the resonance of parasitic parameters can couple to the test equipment through the power supply port. The internal filter of the power supply cannot effectively filter this EMI. Power supply application environments vary greatly. All power supply filter designs prioritize solving their own interference while reserving as much margin as possible for filter attenuation and spectral characteristics. However, it is impossible to be compatible with all applications. Therefore, our system designers must design the power supply front-end according to the application circuit recommended by the power supply manufacturer. For example, the LH15 product has EMI exceeding the standard during application (see the figure below).
The image above shows the conducted interference test results of the MORNSUN LH15-10B05 power supply. These results meet the CLASS B requirements of EN55022/CISPR22, and the margin is very sufficient.
The image above shows the conducted interference results of a MORNSUN LH15-10B05 power supply after it was applied to a certain brand's product. The results do not meet the requirements of EN55022/CISPR22 CLASS B, and even fail to meet the requirements of CLASS A, let alone the design margin.
Therefore, even if a power supply has a high internal electromagnetic interference design level, it is essential to include an application circuit in its specifications. Specific parameters can be found in the datasheet of the corresponding product. MORNSUN power supplies typically include an application circuit section in their datasheets, which describes in great detail the specifications achieved based on that application circuit.
3 Electromagnetic immunity:
In addition to meeting the electromagnetic interference requirements mentioned above, power supplies must also meet the immunity requirements of the corresponding application environment. If they cannot meet the minimum requirements of this environment, they will be affected by electromagnetic interference generated by other surrounding devices, resulting in abnormal phenomena such as damage and unstable output, ultimately affecting the normal operation of the entire machine.
For general-purpose products like power supplies, there are no specific standards requiring a certain level of immunity performance. When applying to a specific industry, industry standards should be consulted. However, since no specific industry is defined in the initial design phase, the specific requirements of the general standard IEC/EN61000-6 can be referenced. Standard IEC/EN61000-6-1/2 is divided into immunity requirements for industrial environments and immunity requirements for residential, commercial, and light industrial environments. MORNSUN power supplies' AC-DC sections are designed according to the most stringent standards for industrial products, while ensuring ample design margins. Currently, this type of power supply promises a level 4 protection capability of 2KV (differential mode)/4KV (common mode), and the internal port protection varistors use the 14D specification (see the image below).
The table below clearly shows that the continuous flux of the 14D specification can reach 4.5KA, while the promised performance is only 1KA (differential mode)/333KA (common mode). This comparison demonstrates that the derating in the design is already very significant, but...
During long-term use in the market, products may experience damage to the varistor, eventually leading to power supply burnout. There are two main reasons for this: Firstly, it is due to the aging of the varistor itself. Currently, the ZnO varistor commonly used in the market has an insulating layer composed of ZnO particles in the middle, and silver-plated electrodes on both sides. When the voltage of the two electrodes exceeds its threshold voltage, the leakage current will increase sharply, eventually forming a transient current discharge, which plays a protective role.
Varistors discharge current from transient surge pulses. After multiple discharges, the properties of the ZnO dielectric in the varistor will change, which will greatly reduce the residual voltage characteristics and discharge capacity of the varistor. More seriously, the silver plating on both sides of the varistor cannot be 100% uniform. This means that with each transient surge impact, there will inevitably be a point on the surface of the varistor that conducts first. The point that breaks down first will burn out first after being subjected to multiple impacts, eventually leading to the damage of the varistor (see figure below).
This type of varistor discharges current through a breakdown point, which generates a large amount of heat. This heat eventually causes the varistor to burn out (see figure below).
Another factor contributing to the damage is the failure caused by improper use of the power supply by the end customer. As previously described, improper use can lead to excessive electromagnetic interference, which will also severely affect electromagnetic immunity. Customer application scenarios vary greatly, and non-isolated use of the MORNSUN isolated AC-DC low-power power supply module (see figure below) can lead to power supply damage. Even if the power supply survives such transient surge impacts, various abnormal phenomena will occur in the downstream load section, which is a major headache for system design engineers.
What problems arise from non-isolated applications of isolated power supplies? First, during common-mode surge testing, the surge impact between the common-mode line and ground becomes the withstand voltage between the primary and secondary sides of the isolated power supply module. For various industrial, power, and rail transportation applications with very stringent reliability requirements, the line-to-ground surge test is conducted according to the 4KV surge level. Most industries design power supply isolation according to 3KV requirements or lower, which makes it difficult for the power supply module to escape damage. Only special industries such as medical use 4KV isolation, but in this case, the isolated power supply needs to sacrifice size and cost.
The second problem is that when there are various noise interference signals such as transient pulses at the input, the isolated power supply can play a good role in protecting the downstream load. However, in non-isolated applications, all interference signals at the input will be transmitted to the load without any changes, which will cause the entire system to malfunction or even fail.
The above application is often met with skepticism, usually citing the overall design of a well-known international brand as an example to illustrate the prevalence of such applications on the market, which is true. So, under what circumstances will this application not produce abnormalities? In regions with highly developed power systems, where the power grid is reliable and the electromagnetic interference of the grid load is ideal, there will be no problem. Furthermore, even if significant design costs have been invested in the load side to avoid transient interference at the input, non-isolated applications will not exhibit abnormalities.
If the isolated power supply must be connected to the PE terminal via the output Vout during application, the connection can be made by connecting these two terminals with a capacitor, following the connection method shown in the diagram above. This design ensures that the customer's special usage requirements are met while effectively avoiding the aforementioned problems.
In short, from a reliability perspective, this design is highly discouraged.
4. Conclusion:
Electromagnetic compatibility (EMC) design for a complete system is a systematic project. It requires thorough evaluation of performance indicators and application environment from the initial design phase, and comprehensive review of circuit diagram design, material selection, PCB layout, structural design, and process installation throughout the design process. Any design flaw in any area can lead to design failure and even incur significant costs. Currently, industry-wide design failures in this area are limited to power supply aspects, and further refinement and improvement are needed.
