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UPS Design: Challenges and Considerations

Battery-powered uninterruptible power supplies (UPS) are important in protecting sensitive equipment in data centers, medical facilities, factories, telecom hubs, and even homes from short-term grid spikes and outages. In the event of longer outages, they provide the short-term power necessary to enable a prepared outage and prevent data loss.


UPS can generally be divided into “online” (Online) or “offline” (Offline). In an offline UPS, the load is directly connected to the grid, and when the input power fails, the system switches to battery-powered mode—the switchover process typically takes about 10 milliseconds to complete, which limits the use of offline UPS in some applications . In the online UPS, an inverter circuit and a battery charging and discharging circuit are added between the load and the power grid. No matter whether the input power is normal or not, the inverter is always working. Therefore, when there is an input problem, the online UPS can perform “zero interruption” switching and provide emergency power to the load through the battery.


Modular UPS is more favored by designers and users, which can meet greater power demand by paralleling lower power UPS. Modular UPS can quickly and easily expand existing UPS systems and help customers profitably build large-scale systems.


However, as with any power supply design, there are challenges in designing a high-efficiency UPS. Some key factors to consider include size, input and output regulation capabilities, battery management, and topology.


Size matters, especially in applications such as data centers where space is at a premium. In the past, the transformer has been one of the largest components in a UPS, but with the advent of more advanced semiconductor technology, high-frequency switching circuits have replaced the transformer, saving space. A transformerless UPS can provide hundreds of kVA of emergency power in a standard-sized cabinet for a large data center.


Online UPS uses high frequency PWM (Pulse Width Modulation) to perform double conversion (AC-DC then DC-AC), which can solve many input quality problems that offline UPS cannot handle, such as low voltage overvoltage and line noise, etc. At the same time reduce battery usage and prolong battery life.


The inverter determines the output quality of the UPS and also greatly affects the overall efficiency of the UPS. An excellent online UPS can output a high-quality sine wave that is similar to the mains, supplying power to resistive and inductive loads. . This requires high frequency operation of the switching devices ( IGBT / MOSFET ) in the inverter and control algorithms to minimize output noise and EMI problems generated by the switching process.


In a typical UPS, multiple stacked batteries form a complete battery pack, which is managed by the battery management module for charge and discharge. For optimal battery performance and longevity, designs must consider issues such as load balancing, voltage and current protection, charge and discharge control, thermal management, fan control, monitoring, and communications.


One of the most critical decisions in the hardware design of a UPS is choosing the right topology for the application, balancing performance and cost. While two-level topologies, such as three-phase half-bridges, have simple structures and uncomplicated control algorithms, three-level topologies (T-NPC, A-NPC or I-NPC) can provide more advanced UPS High efficiency and lower loss and noise.


Materials for switching devices are also critical, with new wide bandgap (WBG) devices, such as silicon carbide (SiC), capable of operating at higher switching frequencies and lower losses, while reducing the size of passive devices to optimize the UPS’s performance. overall design.


ON Semiconductor has prepared a technical white paper that discusses in detail the issues encountered in the design of UPSs, including an overview of UPS types and key specifications. It considers various existing topologies and describes the trade-offs that drive decisions during the design process. The white paper concludes with an overview of the new SiC devices (and associated drivers) that every power engineer should consider today.

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