26 UNINTERRUPTIBLE POWER SUPPLIES https://bourns.com/ Issue 4 2024 Power Electronics Europe www.power-mag.com Protection Needs for Uninterrupted Power Supply Sub-Circuits Introduction: Essential elements in the latest data center topologies, Uninterruptible Power Supplies (UPS) are indispensable for ensuring continuous network availability and resilience. Because data centers operate as the backbone of today’s digital infrastructure, even milliseconds of downtime can result in service disruptions leading to possible financial loss or data corruption. UPS are not only used to bridge momentary outages, they also shield sensitive equipment from harmful transients, surges, and fluctuations that would otherwise compromise performance or trigger costly downtime. To better understand the level of circuit protection that is needed in these missioncritical environments, this article presents the internal design basics of UPS systems, examining the sub-circuits and component technologies that enable uninterrupted operation. The latest protection component technology and capability advancements are also highlighted giving UPS designers their protection scheme options. Taking a proactive circuit protection approach provides the safeguards needed to achieve the performance, safety, and redundancy required to support the evolving demands of data centers and other high-value applications. UPS System Operational Basics: UPS systems are designed around several common-used topologies. These include standby (offline), line-interactive, and double-conversion online architectures. While standby and line-interactive designs are typically sufficient for less critical load environments, data centers and other highavailability facilities almost exclusively rely on double-conversion architectures due to their superior power conditioning and fault tolerance. In a double-conversion UPS, the input AC is continuously rectified to DC. This configuration is capable of supplying the inverter and maintaining the charge of the energy storage system, which is typically a battery bank. The inverter then reconstructs a regulated AC output, effectively decoupling the load from disturbances on the utility line. Because the inverter is always online, there is no transfer time when the input source fails to help maintain a seamless continuity of power. This topology provides more than just ride-through capability. The rectifier stage filters voltage sags, surges, harmonics, and frequency variations from the incoming AC. The inverter stage delivers a clean sinusoidal waveform within tightly controlled voltage and frequency tolerances, independent of the input condition. Advanced systems also integrate static and manual bypass paths, galvanic isolation (required in some designs), and redundant power modules to support fault tolerance and maintainability without compromising uptime. For data center applications, doubleconversion UPS systems can be further configured in parallel or distributed redundant topologies, enabling 2N+1 resilience. These architectures minimize single points of failure and allow for scalable capacity while meeting Tier III and Tier IV uptime requirements. Power Protection Requirements To be effective, UPS systems need to deliver a stable, clean AC output waveform, regardless of disturbances on the input line. An essential role in maintaining this level of stability is protecting against overcurrent and overvoltage events. These disturbances—whether caused by lightning strikes, switching transients, short circuits, or rapid load changes—can inject thousands of amps or dangerously-high voltages into the system. Unless effective protection is integrated, such events would quickly destroy sensitive power electronics within the UPS. To mitigate these risks, it is advised that UPS designs incorporate multiple layers of circuit protection. Devices such as traditional fuses, metal-oxide varistors (MOVs), and gas discharge tubes (GDTs) each provide targeted defense against specific fault conditions. MOVs and GDTs clamp or crowbar excessive voltage transients to safe levels, ab sorbing or diverting surge energy away from critical components. Because these devices offer reusable operation, they are good solutions to handle recurring transient surges. While MOVs and GDTs safeguard against transient overvoltage events, traditional fuses safeguard against destructive overcurrent conditions. Unlike clamping devices, fuses permanently disconnect the circuit when fault currents exceed rated thresholds, providing the ability to localize the damage to the fuse itself. Adding fuse protection helps prevent widespread Figure 1. Block Diagram of a double-conversion UPS
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