Analysis of the Working Principle and Key Technologies of Uninterruptible Power Supply (UPS) ( Part 1)

In today’s increasingly information-driven and automated world, the threat posed by power quality issues (such as voltage dips, surges, and outages) to critical equipment cannot be ignored. Uninterruptible power supplies (UPS), acting as “power guardians” connecting the mains power and the load, have their core value in switching to backup battery power in milliseconds when the mains power fails, ensuring continuous operation of the load.

I. Basic Operating Modes of UPS

Regardless of product form, all UPS systems are based on three basic operating modes:

Normal AC power mode: The AC power input is converted to DC power by the rectifier . This DC power charges the battery pack and drives the inverter to generate clean AC power to supply the load. In this mode, the load actually receives power regenerated by the UPS, which is isolated from the original AC power.

Battery Inverter Mode: When the mains voltage or frequency exceeds the allowable range or is completely interrupted, the static transfer switch activates quickly, and the battery pack supplies power to the inverter through the boost circuit , continuing to output stable AC power. The switching time is typically 2-4 milliseconds, which is completely imperceptible to most devices.

Bypass mode : When the UPS experiences overload, inverter failure, or maintenance, the system automatically or manually switches to the bypass circuit, receiving direct power from the mains. In this mode, the load is no longer protected, but business operations are guaranteed to continue uninterrupted.

II. Comparison of Three Mainstream Technical Architectures

Understanding the working principles of different types of UPS is the foundation for scientific selection.

Standby (offline) UPS: The simplest in structure. It outputs mains power directly when the mains power is normal, switching to inverter output only when the mains power fails. Its advantages are low cost and high efficiency, but the switching time is relatively long (approximately 10ms), and the output waveform is mostly square wave or stepped wave. Suitable for non-critical loads such as personal computers and home routers.

Online interactive UPS : In addition to backup power, it adds Automatic Voltage Regulation (AVR) functionality. When the mains voltage is too high or too low but not completely interrupted, compensation is provided through transformer taps, reducing the number of battery discharge cycles. Suitable for small servers, network equipment, and other scenarios requiring voltage regulation.