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1、 Electromagnetic compatibility characteristics of UPS

With the continuous development and utilization of electromagnetic waves by human beings, people pay more and more attention to the harmful side of electromagnetic waves while benefiting mankind. Therefore, in recent years, relevant electromagnetic compatibility standards have been stipulated in developed countries and regions, and many internationally famous electromagnetic compatibility certification bodies have emerged. For example, electronic products sold to Europe must pass CE certification, while the United States requires FCC certification, At present, China gradually requires to pass the cemc certification.

Uninterruptible power supply (UPS), as the power protection of IT products in the information age, is a high-power density power supply product integrating automatic control technology and power electronics technology. It is itself a large electromagnetic wave emission source, which may cause electromagnetic interference to the surrounding environment and equipment. At the same time, its own control system may be interfered by the surrounding electromagnetic environment, causing ups work failure or even damage. Therefore, high-quality UPS must have good electromagnetic compatibility characteristics.

2、 Standards and requirements related to electromagnetic compatibility of UPS

For the electromagnetic compatibility characteristics of UPS, the current relevant standards mainly include cispr22-1997 “measurement methods and limits of radio disturbance of information equipment” of the International Special Committee on radio interference, cispr-24-1997 “measurement methods and limits of immunity of information equipment”, and en50091-2 (1995) of Europe “EMC requirements of ups”, China’s CISPR standards are related to gb9254-1998 “measurement methods and limits of radio disturbance of information equipment” and gb/t17618-1998 “immunity limits and measurement methods of information equipment”.

Although the standards are different, they are based on CISPR, so the basic limits of the standards are the same. The main requirements for ups are as follows:

1. Conducted disturbance limit of input power terminal

Table 1 shows the contents of national standards and CISPR standards. The limits do not consider the capacity of UPS, which is obviously unreasonable; In the European standard en59001-2, there are two limits according to the power capacity of UPS, that is, the limits below 25A current and above 25A are different, which is relatively reasonable. The same situation also exists for the limit value of radiation disturbance. The limit value in the following 2 is also the requirement of national standard and CISPR.

2. Radiation disturbance limit (10m) (Table 2)

If the 3M method is used for testing, the above values are all added with 10 according to the relevant formula.

3. Other relevant requirements

Battery end disturbance: the battery end disturbance of UPS can be regarded as an integral part of UPS system. All UPS test indicators are based on UPS system, so it can not be considered separately;

Communication interface disturbance: the communication interface disturbance of UPS is generally not directly related to strong current signals, and it is easy to meet relevant requirements. There is no specific limit here; For detailed requirements, you can find relevant standards.

Output power end disturbance: for the conducted disturbance of the output power terminal of UPS, the current national standard and CISPR standard do not give very clear requirements. The usual practice is to adopt the disturbance limit of the input power terminal, but the test method is somewhat different; In the European standard, it is clearly stipulated that the limit value of the input power terminal plus 14dB is relatively loose.

Immunity requirements: relevant immunity tests must be carried out for the chassis port, signal port, telecommunication port, power input port, etc. of UPS equipment, including electromagnetic field interference test, electrostatic discharge test, surge impact test, electrical fast transient pulse test, etc; Different from the above UPS external disturbance, the immunity of UPS reflects the ability of ups to work normally in various electromagnetic interference environments. When problems occur, they are directly manifested as UPS working faults, which are easy to be found by users. Therefore, UPS manufacturers pay more attention to the reliability problem; From the current UPS market, the vast majority of ups have better compliance in this regard.

3、 EMC compliance design of UPS

According to the research and practical experience of electromagnetic compatibility for many years, assuming that the cost of solving the electromagnetic compatibility problem in the product development stage is 1, the cost of solving it in the model development stage may be 10, the cost of solving it in mass production may be 100, and the cost of solving it in the field installation may be thousands of times or can not be solved. Therefore, the EMC problem of UPS must be solved in the product development stage.

According to the product characteristics of UPS, the electromagnetic compatibility of UPS mainly includes the following parts: input and output conducted interference of power supply; Radiation disturbance of power supply; Anti interference characteristics of ups. The following describes the design methods that meet the requirements of relevant standards one by one.

1. Suppression of input and output conducted interference

For conduction disturbance, three aspects can be considered: interference source, conduction path and direct disturbance suppression.

A. Elimination and reduction of interference sources: in UPS, there are rectifier ac/dc converter, SPWM inverter dc/ac inverter, PFC high-frequency conversion circuit, and dc/dc conversion circuit. These are important disturbance sources in UPS, especially transformers, inductors, and high-frequency current circuits. Therefore, reasonably designing the parameters, processing technology, and layout of corresponding transformers and inductors in the whole machine may greatly reduce their disturbance intensity, Reasonably designing PCB and wiring of high-frequency current can also improve the disturbance of UPS; For the drive circuit in the power converter, the drive resistance can be increased without affecting the efficiency and internal resistance, and the rise and fall time of the switching power supply can be increased, so as to reduce the high-frequency harmonic content of voltage and current.

B. Suppression of conduction path: since all conduction interference can only act on the input and output power terminals of UPS through appropriate space and conductor path, minimizing the transmission path is also an effective method to reduce UPS interference. For example, install all interference sources far away from the input and output terminals, do not route the input and output power lines near the interference sources, strengthen the suppression treatment at the entry and exit positions of the interference sources, spatially isolate the interference sources and other parts by means of shielding, and the input and output of the power sources are located relatively far away from the whole machine.

C. Direct disturbance suppression: for the situation that the above method still cannot meet the standard requirements, directly using the corresponding EMI filter devices in the input and output circuits, such as inductance, high-frequency capacitor, special filter, etc., will again effectively reduce the external conduction interference of the ups. Practice shows that as long as the relevant parameters of the filter and the attenuation DB value are appropriately increased, Generally, the conducted disturbance of UPS can be reduced to within the standard limit. Of course, the filter must be installed closer to the input and output power terminals, because even more than a few centimeters of wiring will increase interference, and the socket filter will be the most ideal choice. In addition, the capacitor in the filter or the additional EMI filter capacitor is preferably non inductive to enhance the filtering effect.

2. Suppression of radiation interference of the whole machine

For the radiation interference of UPS, there are mainly two methods: the intensity suppression of radiation source and the treatment of radiation path.

A. Suppression of radiation source: in UPS, the suppression method of radiation intensity of radiation source is basically the same as that of conduction, because the interference source itself has both conduction disturbance and radiation disturbance; In addition, for radiation disturbance, taking appropriate shielding measures for the radiation source will effectively reduce the level and energy of radiation interference.

B. Treatment of radiation path: equipotential design of the shell of the whole machine: according to the principle of electromagnetic field, there is no mutual interference between the internal and external electromagnetic fields of a well grounded and ideally sealed metal hexahedral shell. Therefore, the shell of UPS should generally be made of metal, and all surfaces should be well connected to ensure that it is an equipotential body, which can effectively reduce the external radiation interference of ups. Generally, for occasions with strict requirements on electromagnetic compatibility, the shell of UPS should not be made of plastic.

Treatment of connecting wires in and out of UPS shell: since UPS must have input and output power terminals, battery expansion terminals and other connecting wires in and out of UPS shell, the anti-interference treatment of these wires will be very important, which will directly affect whether the test results meet the standard requirements. Generally, adding some high-frequency magnetic rings and high-frequency capacitors to these lines will have a good effect.

3. Anti interference design of UPS

The anti-interference of UPS is mainly reflected in the anti-interference of control circuit. From the nature of the circuit, it can be divided into two aspects: the anti-interference of analog circuit and the anti-interference of digital circuit. Good immunity is the condition to ensure the normal operation of ups. Therefore, the immunity of the control circuit must be taken into account at the early stage of the design of UPS control circuit, otherwise, the whole control scheme may be overturned in case of external disturbance.

A. Anti interference of analog circuit:

For the open-loop analog control, generally, a certain RC circuit is properly added to the part where interference may occur to eliminate the interference; For the closed-loop analog control, in addition to using RC, the frequency characteristics of the amplification factor of the closed-loop must also be properly adjusted to ensure that the interference signal will not have adverse consequences on the loop.

For the power part of the circuit, reducing all connections, adding false loads, reducing the power drive circuit can effectively enhance the anti-interference ability of the power circuit.

B. Anti interference of digital circuit:

For the digital control circuit, its immunity is very important to the reliability of UPS, because at present, almost all UPS controls adopt the single chip microcomputer of digital control, and the system with poor immunity may cause UPS shutdown or damage.

The effective filtering of digital circuit power supply is the basic guarantee for digital circuit to be free from interference; All i/o ports shall be properly RC treated; The control circuit should be as far away from the power part as possible; Appropriate electromagnetic shielding measures; Good PCB layout design can effectively avoid external interference to the digital system.

It should be clearly pointed out that for the voltage stabilization and synchronous control of UPS closed-loop, the anti-interference of the control model and the software filtering processing method must be fully and reasonably considered in the system modeling, and a complete experiment must be done during the system debugging.

4、 Concluding remarks

From the above discussion, it can be seen that the electromagnetic compatibility characteristics of ups are extremely important, and only by taking effective preventive measures in the early stage of product design, it is possible to make the electromagnetic compatibility characteristics of UPS meet the requirements of relevant standards at a low cost.