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Solutions for Electromagnetic Compatibility Compliance Issues in UPS Uninterruptible Power Supplies
Release time:
2019-02-20
According to years of research and practical experience in electromagnetic compatibility (EMC), if we assume that addressing EMC issues during the product development phase costs 1 unit, then resolving them during the prototype development phase could cost as much as 10 units. By the time of mass production, the cost could soar to 100 units, and by the time of on-site installation, the cost could be thousands of times higher—or even become impossible to resolve altogether. Therefore, EMC issues for UPS uninterruptible power supplies must be addressed during the product’s development phase. Given the specific characteristics of UPS products, their EMC primarily involves the following aspects: conducted emissions from the power input and output; radiated emissions from the power supply; and the UPS’s immunity to interference. Below, we will discuss in detail the design methods for meeting the relevant standard requirements. 1. Suppression of Conducted Emissions from Input and Output For conducted emissions, we can consider three key aspects: the emission source, the conduction path, and direct suppression of interference. A. Elimination and Reduction of Emission Sources: In a UPS, there are AC/DC rectifiers, DC/AC inverters employing SPWM technology, high-frequency PFC circuits, and DC/DC conversion stages—all of which are significant sources of interference within the UPS. Particularly critical are transformers, inductors, and high-frequency current loops. Therefore, carefully designing the parameters and manufacturing processes of these components and optimizing their placement within the overall system can significantly reduce their interference levels. Additionally, properly designing the PCB layout and wiring for high-frequency currents can further improve the UPS’s emission performance. For the drive circuits in power converters, increasing the drive resistance without compromising efficiency or internal impedance can lengthen the rise and fall times of switching signals, thereby reducing the high-frequency harmonic content of voltage and current. B. Suppression of Conduction Paths: Since all conducted emissions can only affect the UPS’s input and output terminals via appropriate spatial paths and conductive pathways, minimizing these transmission paths is an effective way to reduce the UPS’s interference. For example, placing all emission sources as far away as possible from the input and output terminals, routing power cables away from the vicinity of emission sources, reinforcing suppression measures at the entry and exit points of emission sources, using shielding to spatially isolate emission sources from other components, and positioning the input and output terminals at relatively distant locations within the entire system—these strategies can all help reduce conducted emissions effectively. C. Direct Suppression of Interference: If, after adopting the above methods, the UPS still fails to meet the standard requirements, directly installing appropriate EMI filtering components—such as inductors, high-frequency capacitors, or dedicated filters—in the input and output circuits can once again effectively suppress the UPS’s conducted emissions. Practice has shown that simply increasing the relevant parameters and attenuation dB values of the filters typically allows the UPS’s conducted emissions to be reduced below the standard limits. Of course, it’s crucial to install the filters as close as possible to the input and output terminals, since even a few extra centimeters of wiring can increase interference. Plug-in filters are the most ideal choice in this regard. Moreover, the capacitors used in the filters or externally added EMI filtering capacitors should ideally be non-inductive to enhance their filtering effectiveness. 2. Suppression of Radiated Emissions from the Entire System For radiated emissions from the UPS, there are two primary approaches: suppressing the intensity of the radiation sources and managing the radiation paths. A. Suppression of Radiation Sources: In a UPS, the method for suppressing the radiation intensity of emission sources is essentially the same as that used for conducted emissions, since the emission sources themselves generate both conducted and radiated interference. Additionally, for radiated interference, applying appropriate shielding measures to the radiation sources can effectively reduce the level and energy of the radiated emissions. B. Managing Radiation Paths: Equal-potential design of the entire system’s enclosure: According to electromagnetic field theory, in a well-grounded, ideally sealed metallic enclosure with six closed surfaces, there is no mutual interference between the internal and external electromagnetic fields. Therefore, the UPS enclosure should generally be made of metal, with all surfaces properly connected to ensure they form a single equipotential body. This approach can significantly weaken the UPS’s external radiated emissions. In general, for applications with strict EMC requirements, it’s best not to use plastic for the UPS enclosure. Handling connections entering and exiting the UPS enclosure: Since the UPS must have connections such as input and output power terminals and battery expansion terminals passing through its enclosure, proper handling of these connections to prevent interference is extremely important and directly affects whether the test results meet the standard requirements. Generally, adding suitable high-frequency magnetic rings and high-frequency capacitors to these lines can achieve excellent results. 3. Immunity Design for UPS Systems The immunity of a UPS mainly refers to the anti-interference capability of its control circuitry. From a circuit perspective, this can be divided into two aspects: immunity of analog circuits and immunity of digital circuits. Good anti-interference performance is a prerequisite for ensuring the normal operation of the UPS. Therefore, during the initial design stage of the UPS’s control circuitry, its anti-interference capability must be taken into account. Otherwise, when exposed to external interference, the entire control scheme could be completely compromised. A. Immunity of Analog Circuits: For open-loop analog control systems, it’s common practice to add appropriate RC circuits to parts where interference might occur, thus eliminating the disturbance. For closed-loop analog control systems, in addition to using RC circuits, it’s also essential to appropriately adjust the frequency characteristics of the closed-loop amplifier’s gain to ensure that any interfering signals won’t have adverse effects on the loop. For the power-stage circuits, shortening all wiring lengths, adding dummy loads, and reducing the complexity of the power-drive circuits can all effectively enhance the power circuit’s anti-interference capability. B. Immunity of Digital Circuits: For digital control circuits, their anti-interference capability is crucial to the reliability of the UPS, since nearly all modern UPS control systems now employ microcontrollers with digital control. A system with poor anti-interference performance could lead to UPS shutdowns or damage. Effective filtering of the digital circuit’s power supply is the fundamental guarantee against interference; all I/O ports should undergo appropriate RC treatment; the control circuitry should be kept as far away as possible from the power stage; appropriate electromagnetic shielding measures should be implemented; and a well-designed PCB layout can all effectively prevent digital systems from being affected by external interference. It’s important to emphasize that for closed-loop voltage regulation and synchronization control in UPS systems, the anti-interference capability of the control model and software filtering techniques must be thoroughly and reasonably considered during system modeling and fully tested during system commissioning.
According to years of research and practical experience in electromagnetic compatibility, if we assume that addressing EMC issues during the product development phase costs 1 unit, then addressing them during the prototype development phase could cost as much as 10 units. By the time of mass production, the cost could soar to 100 units, and by the time of on-site installation, the cost could increase by a factor of thousands—or even become impossible to resolve. Therefore, electromagnetic compatibility issues in UPS uninterruptible power supplies must be addressed during the product’s development phase.
With regard to the product characteristics of UPS, its electromagnetic compatibility primarily encompasses the following aspects: conducted emissions from the power input and output; radiated disturbances from the power supply; and the UPS’s immunity performance. The design methods for meeting the relevant standard requirements will be elaborated on item by item below.
1. Suppression of conducted input and output*
For conducted interference, we can consider it from three aspects: *the source, the conduction path, and direct interference suppression.
A. Elimination and Reduction of Noise Sources: In UPS systems, there are AC/DC rectifiers, DC/AC inverters employing SPWM inversion, high-frequency conversion circuits with PFC functionality, and DC/DC conversion stages—all of which are significant sources of electromagnetic interference within the UPS. In particular, transformers, inductors, and high-frequency current loops are major contributors to such interference. Therefore, by carefully designing the parameters and manufacturing processes of these transformers and inductors and optimizing their placement within the overall system, it is possible to substantially reduce their interference levels. Additionally, a well-designed PCB layout and wiring for high-frequency currents can also help mitigate UPS-induced interference. As for the drive circuits in power converters, increasing the drive resistance—without compromising efficiency or internal impedance—can extend the rise and fall times of the switching power supply, thereby reducing the content of high-frequency harmonics in both voltage and current waveforms.
B. Inhibition of Conduction Paths: Since all conducted disturbances can only affect the UPS’s input and output power terminals via appropriate spatial and conductive paths, minimizing the number of transmission paths is an effective way to reduce electromagnetic interference from the UPS uninterruptible power supply. For example, place all disturbance sources as far away as possible from the input and output terminals; avoid routing power cables for input and output near the disturbance sources; enhance suppression measures at the entry and exit points of the disturbance sources; use shielding techniques to spatially isolate the disturbance sources from other components; and position the input and output connections of the power supply at relatively distant locations within the entire unit.
C. Direct Suppression of EMI: For cases where the above-mentioned methods still fail to meet the standard requirements, directly incorporating appropriate EMI filtering components—such as inductors, high-frequency capacitors, or dedicated filters—into the input and output circuits can once again effectively reduce the conducted emissions from the entire UPS system. Practice has shown that by appropriately increasing the relevant parameters and attenuation levels (in dB) of the filters, it is generally possible to bring the UPS’s conducted emissions below the standard limits. Of course, the filters should be installed as close as possible to the input and output power terminals, since even a few centimeters of additional wiring can increase the emissions. Filter units with plug-in connectors would be the most ideal choice. Additionally, the capacitors used within the filters—or any externally added EMI filtering capacitors—should ideally be non-inductive to enhance their filtering performance.
2. Suppression of overall equipment radiation*
Regarding UPS radiation*, there are mainly two approaches: suppressing the intensity of the radiation source and addressing the radiation pathways.
A. Suppression of Radiation Sources: In UPS systems, the methods for suppressing the radiation intensity of radiation sources are essentially the same as those used for handling conducted emissions, since the source itself generates both conducted and radiated disturbances. Additionally, with regard to radiated disturbances, adopting appropriate shielding measures for the radiation source can effectively reduce both the level and energy of the radiated emissions.
B. Handling of Radiation Emissions: Equipotential Design of the Entire Machine’s Enclosure: According to the principles of electromagnetic fields, in an ideally sealed metallic enclosure with good grounding, there is no mutual coupling between the internal and external electromagnetic fields. Therefore, the enclosure of a UPS should generally be made of metal, and all its surfaces should be well-connected to ensure that the entire enclosure forms a single equipotential body. This approach can effectively reduce the UPS’s radiation emissions to the outside world. In general, for applications with stringent electromagnetic compatibility requirements, it is not advisable to use plastic materials for the UPS enclosure.
Handling of wiring connections entering and exiting the UPS enclosure: Since the UPS must have input and output power terminals, battery extension terminals, and other wiring connections that enter and exit the UPS enclosure, it is crucial to properly address electromagnetic interference (EMI) mitigation for these wires. Failure to do so can directly affect whether the test results meet the standard requirements. Generally, adding appropriate high-frequency magnetic beads and high-frequency capacitors to these wires will yield excellent EMI suppression effects.
3. UPS’s anti-* design
The immunity of UPS primarily manifests in the anti-interference capability of its control circuitry. From the perspective of circuit characteristics, this can be divided into two aspects: the anti-interference performance of analog circuits and that of digital circuits. Good anti-interference performance is a prerequisite for ensuring the normal operation of the UPS. Therefore, from the very beginning of the design phase of the UPS’s control circuit, the anti-interference capability of the control circuit must be taken into account; otherwise, when exposed to external disturbances, the entire control scheme could be completely invalidated.
A. Anti-* of analog circuits:
For open-loop analog control systems, it is common practice to appropriately add an RC circuit to sections where interference might occur, thereby eliminating the disturbance. For closed-loop analog control systems, in addition to using RC circuits, it is also essential to properly adjust the frequency characteristics of the closed-loop amplifier's gain, ensuring that when a disturbing signal is introduced, it does not adversely affect the loop performance.
For the power circuit, shortening all wiring, adding dummy loads, and minimizing the power-drive loop can all effectively enhance the power circuit's resistance to interference.
B. Anti-* of digital circuits:
For digital control circuits, immunity to interference is crucial to the reliability of UPS systems, since nearly all modern UPS controllers now employ microcontrollers with digital control. A system with poor immunity could lead to UPS downtime or even damage.
Effective filtering of the power supply in digital circuits is a fundamental guarantee against external interference; all I/O ports should be equipped with appropriate RC filters; the control circuitry should be kept as far away from the power section as possible; proper electromagnetic shielding measures should be implemented; and a well-designed PCB layout can all effectively prevent digital systems from being affected by external interference.
It should be clearly pointed out that, for the closed-loop voltage regulation and synchronization control of UPS uninterruptible power supplies, sufficient and reasonable consideration must be given to the robustness of the control model and software filtering techniques during system modeling, and comprehensive experiments must be conducted during system commissioning.
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