UPS Uninterruptible Power Supply Circuit Working Principle
Under normal conditions, the mains (220V) charges the battery through an adjustable charger, and relay K1 is activated. Resistor R1, along with Zener diodes VZ1 and VZ2, divides and samples the +24V battery voltage. The sampled voltage (Vo) is applied to the base of transistor V1 via R2 and VD3, putting V1 in a linear amplification state. Transistors V2 and V3 are fully saturated, activating the DC control relay (K), allowing the +24V voltage to be sent to the inverter’s V+ terminal through K and K1. The inverter then operates, producing a 220V sine wave output, while self-locking relay K2 is also activated.
When the mains supply is cut off, relay K1 deactivates, but the +24V input voltage remains unchanged, keeping K activated due to the self-locking effect of K2. Initially, +24V continues to be supplied to the inverter. However, as the battery voltage begins to drop, the sampling voltage Vo decreases. This weakens the conduction of V1, causing V2 to increase. When the battery voltage drops to around 22V, V2 exits saturation, entering a linear amplification state, while V3 rapidly decreases. V3 then feeds back to the base of V1 through R7, pushing V2 to rise further, initiating an avalanche process.
As a result, V2 and V3 are quickly cut off, deactivating relay K and cutting off the +24V DC supply from the battery to the inverter, stopping its operation. Relay K2 is also deactivated at the same time. Once the inverter stops, the battery’s electromotive force allows it to quickly recover to 24V. In this state, V2 and V3 are saturated, and K reactivates. However, since K1 and K2 are disconnected, the +24V voltage does not reach the inverter, preventing it from operating, thus protecting the battery. The inverter will only resume operation when the mains supply is restored and relay K1 is activated, while the charger begins recharging the battery.
Adjusting R1 allows for calibrating the lower limit of battery voltage protection when the mains supply is lost and adjusting the maximum UPS working time during power outages.
Component Selection:
- Charger: A commercially available 24-12V/50A adjustable charger.
- Inverter: A 500VA inverter with a 24V DC operating voltage.
- Battery: Two 12.80Ah lead-acid batteries connected in series.
- AC relay: 220V/10A.
- DC relay: 24V/20A.
UPS Circuit Types and Their Structures
Different UPS systems, based on their main circuit structure and power supply mechanisms, are categorized as follows:
1. Backup Type
This is the basic form of static UPS, characterized by its mature technology and widespread use.
- Charger: Charges the battery when mains power is available. Long-delay UPS systems require a charger with high capacity or additional charging devices.
- DC-AC Inverter: Converts DC battery voltage into AC when mains power is lost. The AC output may be square wave, quasi-square wave, or sine wave.
- Output Transfer Switch: Switches between mains and battery power, with a transfer time of around 4ms.
Performance Characteristics:
- High efficiency (98%) when mains power is present.
- Harmonic distortion and power factor depend on the load.
- Switching time of approximately 4ms when mains power is lost.
- Poor voltage stability when mains power is present.
- Simple, low-cost design with high reliability.
- Limited output capacity (typically less than 2KVA) due to transfer switch constraints.
2. Online Interactive Type
In this system, the inverter remains in standby mode and charges the battery. It also reduces switching time and enhances voltage filtering.
- Input Switch: Prevents power feedback to the grid when mains power fails.
- Intelligent Voltage Regulation: Adjusts the stable output voltage.
- DC-AC Inverter: Functions in both directions—charging the battery and converting DC to AC during power outages.
Performance Characteristics:
- High efficiency (>98%) when mains power is available.
- Strong output capability without strict limitations on load surges and overloads.
- Shorter transfer time than backup types.
- Directly connected inverter provides better voltage spike filtering.
- Increased output capacity (up to 5-10KVA) without transfer switch limitations.
3. Dual Inverter Online Type
This structure features two inverters. Inverter I rectifies and charges the battery, while Inverter II powers the load.
- Bypass Switch: Provides direct grid power during faults or overload conditions.
- Performance Characteristics:
- Consistent, high-quality power output (stable voltage and frequency).
- No transfer time during power failures.
- Limited output power due to inverter constraints (e.g., 3:1 current peak factor).
- Efficiency of 80%–92%, depending on power rating.
4. Dual Inverter Voltage Compensation Online Type
Introduced by APC Silcon, this structure applies voltage compensation principles, improving performance while reducing grid disturbances.
- Inverter I: Handles input power factor correction and current harmonic suppression.
- Inverter II: Compensates voltage fluctuations and maintains stable power output during power failures.
Performance Characteristics:
- High output quality with minimal harmonic distortion (<3%).
- Efficient power handling, with 96% efficiency under mains power.
- Enhanced reliability and longevity due to reduced power stress on components.