With the rapid development of electronic technology, electronic devices are also moving toward the integration of functions and miniaturization. This brings us many conveniences, but the electromagnetic coupling between various electronic devices has also become the face of engineers. The main problem. The harm of electronic environmental pollution is no less than traditional environmental pollution. Electromagnetic pollution has also been put on the agenda as part of environmental pollution. This article briefly introduces a technical problem that needs to pay attention to when designing the whole machine electromagnetic compatibility around the power supply itself, as well as the design ideas and methods.
0 Preface:With the rapid development of electronic technology, electronic devices are also moving toward the integration of functions and miniaturization. This brings us many conveniences, but the electromagnetic coupling between various electronic devices has also become the face of engineers. The main problem. The harm of electronic environmental pollution is no less than traditional environmental pollution. Electromagnetic pollution has also been put on the agenda as part of environmental pollution. When the electronic device is working normally, it will suffer various electromagnetic interferences, including mutual interference of its own internal devices and interference of other surrounding electronic devices, as well as electromagnetic interference to other surrounding electronic devices. Electronic equipment in the different application environments (home, industrial control, power) requirements are very different, in this regard can refer to the general standard IEC/EN61000-6 series or the corresponding product industry requirements.
This kind of electromagnetic interference mainly includes two aspects in the transmission route: First, the transmission along the wiring harness, which mainly includes the transmission along the power port and the signal port for transmission; on the other hand, it mainly transmits along the space.
1 electromagnetic interference:The power supply must meet the corresponding minimum emission energy requirement in its application environment, otherwise it will cause interference to the surrounding environment equipment. The standard IEC/EN61000-6 is divided into industrial environment equipment requirements and residential areas, commercials according to the requirements of the common type. The emission requirements for district and light industrial environments; for the general class of power supplies, the electromagnetic interference location at the beginning of the design will be performed in accordance with IEC/EN61000-6-3 or IEC/EN61000-6-4 unless the model is special.
With the continuous miniaturization of the power supply, the increase in power density, and the increasing difficulty in designing electromagnetic interference for the power supply itself, all AC-DCs currently on the market of MORNSUN not only have built-in filters, but also provide transformer shielding. Power device noise absorption has invested a lot of design costs, meet the promised requirements; R2 generation of low-power DC-DC products are all designed with six-sided shielding structure to meet the industry EN55022/CISPR 22, EN55011/CISPR 11 CLASS A Requirements, in line with the level requirements of the basic industry.
Although the power supply's own electromagnetic interference has invested a lot of design costs, and also meet the requirements of the promised indicators, but the power supply is still inevitable in the market application of electromagnetic interference exceeded the standard; at this time, many design engineers will think the root of the problem In terms of power supply, this understanding is misunderstood, because the electromagnetic interference conducted harassment test project, mainly for the power port, then the power port becomes his transmission route, all the electromagnetic interference will be reached through the power port is The test equipment, but the electromagnetic interference tested by the test equipment, in addition to the power supply itself, the main part includes the electromagnetic interference generated by other parts of the machine, and the electromagnetic interference generated by the resonance of the internal parasitic parameters of the equipment. This type of electromagnetic interference Will be coupled to the test equipment through the power port, the filter inside the power supply can not be implemented to filter this part of the electromagnetic interference, power supply application environments vary widely, all power supply design filter part is to address their own interference as the primary considerations at the same time , filter attenuation Sex and spectrum characteristics will be reserved as much as possible, but it is not possible to be compatible with all applications. This requires our entire designer to design the power supply according to the application circuit recommended by the power supply manufacturer. : Excessive EMI in the application process of LH15 products (see the figure below).
The figure above shows the results of the conducted disturbance test of MORNSUN power supply LH15-10B05. This result is in line with the EN 50022/CISPR22 CLASS B requirement and the margin is very sufficient.
The figure above shows the results of the test of the harassment of the MORNSUN power supply LH15-10B05 after it was applied to a certain brand product. This result cannot meet the CLASS B requirements of EN55022/CISPR22. Even the CLASS A cannot meet the requirements, let alone design. margin.
Therefore, even if the power supply has a higher internal electromagnetic interference (EMI) design level, it must be used in the application process. For specific parameters, refer to the specific product specifications. MORNSUN's power supply products will have an application circuit in the specification, and the specifications to be implemented on the basis of the application circuit will be described in great detail.
2 Electromagnetic immunity:In addition to meeting the electromagnetic interference requirements mentioned above, the power supply must also meet the noise immunity requirements of the corresponding application environment. If it fails to meet the minimum requirements of this environment, it will be affected by the electromagnetic interference generated by other equipment around it, resulting in damage. Unusual phenomena such as output instability will eventually affect the normal operation of the complete machine.
For general-purpose products such as power supply, there is no specific standard that requires that the immunity performance reach a certain level. When applied to a specific industry, it refers to the industry standard; but in the early stage of design, there is no specific industry, only reference. The specific requirements of the general class standard IEC/EN61000-6, the standard IEC/EN61000-6-1/2, are divided into immunity requirements for industrial environmental equipment and immunity requirements for residential areas, commercial areas and light industrial environments. MORNSUN power AC-DC parts are designed according to the most stringent grades of industrial type products, and at the same time ensure that the design margin is very sufficient. At present, this type of power supply promises 4 level indicators 2KV (differential touch)/4KV (common mode) The protective capabilities of the product, the internal design of the port protection varistors are all 14D specifications (see the figure below)
Through the following table, it can be clearly seen that the 14D specification continuous flow can reach 4.5KA, then the promised indicator is only 1KA (differential touch)/333KA (common mode). Through this comparison, it can be seen that the derating of the design is already very large. but
model
561KD07
561KD10
561KD14
561KD20
Inrush current
1.75KA
3.5KA
6KA
10KA
Keeping inrush current
1.2KA
2.5KA
4.5KA
6.5KA
When the product is used in the market for a long time, there will be varistor damage, and eventually the phenomenon of power burned out. There are two main reasons for this: On the one hand, due to the aging of the varistor itself, the current market Very commonly used ZnO varistors, in the middle of the insulating layer consisting of ZnO particles, both sides of the electrode formed by silver plating, when the voltage of the two electrodes exceeds its threshold voltage, the leakage current will increase sharply, resulting in transient current leakage Release, play a protective role.
The varistor carries out current discharge for the transient surge pulse. After multiple discharges, the varistor ZnO will undergo characteristic changes, which will greatly reduce the residual pressure characteristics and discharge capability of the varistor; more serious is that Varistors such a silver-plated structure on both sides, the surface of the silver plated and can not achieve 100% uniformity, it shows that each transient surge impact, there will inevitably be a point on the surface of the entire varistor to be the first conduction, the first The point of breakdown is followed by a number of impacts, after which it is first burned and eventually leads to damage to the varistor (see figure below).
This type of varistor draws current through the point of breakdown, creating a large amount of heat at the discharge point, which eventually causes the varistor to burn through (see the figure below).
On the other hand, the damage factor is due to the end user's failure to use the power supply improperly. The use of the current surface has been described as excessive electromagnetic interference will be caused, the same electromagnetic immunity will be seriously affected, the customer application site varies greatly, then the The non-isolated MORNSUN isolated AC-DC low-power power module (see the figure below) will cause damage to the power supply. Even if the power supply survives this transient surge, the back-end load part will also produce various All kinds of abnormal phenomena, this is a headache for the entire design engineer.
What is the problem with non-isolated applications of isolated power supplies? First, when a common-mode surge test is performed, the surge 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, rail transit and other application environments where product reliability is very demanding, line-to-ground will be tested in accordance with the 4KV surge rating. In most industries, the isolation of the power supply is in accordance with 3KV. The design is required to be lower or even lower, so that the power module will not be able to escape damage. Only the isolation of a particular industry such as medical care will be designed at 4KV, but at this time, the isolated power supply needs to sacrifice the volume and cost.
The second problem is that the isolated power supply can play a good role in protecting the back-end load when there are various noise interference signals such as transient pulses at the input. However, after the non-isolated application, all the interference signals at the input will be left intact. The transmission to the load side will cause the entire system to be abnormal or even paralyzed.
The above application is often subject to various questions. Generally speaking, the overall design of an international famous brand is used as an example to tell us that there are many such applications on the market. Of course, this does exist. So under what circumstances will this application not cause anomalies? For areas where the power system is well-developed, his power grid is already reliable, and at the same time, the electromagnetic interference of the load on the power grid is already very satisfactory. There is no problem; in addition, such applications may put a lot of design costs on the load side. Avoid non-isolated applications without exception when input transients are avoided.
Then, if the isolated power supply must be output Vout-connected to the PE terminal during the application process, the two terminals can be connected through capacitors according to the connection diagram in the above figure, so that the design ensures that the customer's special use requirements of colleagues, for The above problems can also be effectively avoided.
In short, from the perspective of reliability, this design is very much less advocating.
3 Conclusion:The electromagnetic compatibility design of the whole machine is actually a systematic project. It requires us to fully evaluate the indicator positioning and application environment in the early stage of design. In the design process, the circuit design, raw material selection, PCB drawing, structural design, process installation, etc. The review is adequate; failure to design a single point can lead to design failures and even a heavy cost. At present, in the industry, the design failure in this area is limited to power supply, and it has yet to be sorted out and improved.
references:
IEC 61000-6-1-2005 General standard immunity standards for residential, commercial and light industrial environments
IEC 61000-6-2-2005 General standard industrial environment immunity standards
IEC 61000-6-3-2007 General Standard Launch Standards for Residential, Commercial, and Light Industrial Environments
IEC 61000-6-4-2007 General Standard Industrial Environment Launch Standard
CISPR 22-2005 Radio disturbance limits and measurement methods for information technology equipment
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