UPS Systems

Uninterruptible Power Supplies provide continuous AC power to critical loads like data centers during grid outages. They rely on power converters and battery banks to ensure seamless operation.

Core Concept

An online UPS continuously supplies power from the inverter stage. The input rectifier converts 230V AC to a stable DC link voltage. This DC link charges the battery and feeds the output inverter.

The inverter uses IGBT devices like the FGA25N120 switching at 20 kHz. High frequency PWM creates a pure sine wave output for the load. Duty cycle variations shape the output voltage profile.

When mains power fails, the battery directly supports the DC link without interruption. Commutation of the switches continues seamlessly to maintain the 230V output.

Key Formulas

Battery Runtime equals battery capacity in Ah multiplied by battery voltage multiplied by efficiency divided by load power. Modulation index m controls the inverter output voltage.

Given: Load = 1000 W, Battery = 48 V, 50 Ah, Efficiency = 0.90
Formula: Runtime = (Ah * V * Eff) / Load
Steps: Runtime = (50 * 48 * 0.90) / 1000 = 2160 / 1000
Final Answer: 2.16 hours

Exam Tip: Transfer time for an online UPS is exactly zero milliseconds. Offline UPS systems have a finite transfer time which is a common GATE trap.

Performance Parameters

• Overall efficiency: typically 85 to 92 percent
• Output voltage regulation: plus or minus 1 percent
• Total Harmonic Distortion: under 3 percent
• Transfer time: 0 ms for online topologies
• DC bus voltage: 48V for medium systems

Quick Revision

• Rectifier converts AC to DC
• Battery stores backup energy
• Inverter regenerates AC
• Static bypass provides fault tolerance
• PWM ensures sinusoidal output
• Exam trap: Assuming battery capacity directly equals runtime