An IGBT (Insulated-Gate Bipolar Transistor) is a device that combines a MOSFET and a Bipolar Transistor. According to Toshiba, a 1200 V / 100 A IGBT has one-tenth the on-resistance of a power MOSFET with the same withstand voltage and one-tenth the switching time of a GTR with the same specifications. Owing to these characteristics, IGBTs are widely used in UPS (Uninterruptible Power Supply) designs. This article introduces the application of IGBTs in UPSs and precautions for their use.
UPSs primarily come in three configurations: backup, line-interactive, and online. Online UPSs, with their high reliability, stable output voltage, and uninterrupted operation, are widely used in computer rooms in telecommunications, finance, securities, power, railways, civil aviation, and government agencies. These online UPSs, which use IGBTs, offer significant advantages, including high efficiency, excellent surge resistance, and high reliability.
Next, SHYSEMI will use the online configuration as an example to introduce the application of IGBTs in UPSs.
Figure 1: Main Circuit Diagram of an Online Uninterruptible Power Supply
Figure 1 shows the main circuit of an online UPS. An online UPS consists of an independent bypass switch, an AC/DC rectifier, a charger, and a DC/AC inverter. The operating principle is as follows:
When the mains power is normal, the AC/DC rectifier converts the AC power into DC power, simultaneously charging the battery. The DC/AC inverter then inverts the DC power into standard sinusoidal AC power. When the mains power is abnormal, the battery powers the inverter. In the event of a UPS failure, the output switches to the bypass power supply. The voltage and frequency of an online UPS are the most stable, providing users with truly high-quality sinusoidal power.
1.Bypass Switch (AC Bypass Switch)
Bypass switches commonly use relays and thyristors. Relays are widely used in low and medium-power UPSs. Their advantages are simple control and low cost. However, their disadvantages include the switching time required for relays and the limited lifespan of the electromechanical components.
Thyristors are commonly found in medium and large-power UPSs. Their advantages include high current handling capability and minimal switching time. The main disadvantage is their complex control. Due to the triggering characteristics of thyristors, they must be reverse-biased to turn off after being triggered. This can generate a circulating current for up to 10ms, as shown in Figure 2.
Figure 2: Diagram of SCR Delayed Turn-Off
Using IGBTs, as shown in Figure 3, can avoid this problem. While IGBTs offer the advantage of simpler control, they come at a higher cost. Their operating principle is as follows: during the positive half-cycle of the input, current flows through Q1 and D2; during the negative half-cycle, current flows through D1 and Q2.
Figure 3: Bypass Switch Using IGBTs
2.AC/DC Rectifier
UPS rectifier circuits are categorized into conventional bridge rectifiers, SCR phase-controlled rectifiers, and PFC high-frequency power factor correction rectifiers. Traditional rectifiers have a 50 Hz base frequency, and the filters are relatively large and heavy. With the advancement of UPS technology and increasing requirements for input power factor in various countries, UPSs using PFC (Power Factor Correction) are becoming increasingly popular.
PFC circuits operate at a base frequency of at least 20 kHz, significantly reducing the size and weight of the filter inductors and capacitors. This allows for an input power factor of 0.99 without the need for harmonic filters. IGBTs are commonly used as power switching devices in PFC circuits. PFC rectifiers using IGBTs offer high efficiency, high power capacity, and environmental friendliness.
3.Charger
Common UPS chargers include flyback, boost, and half-bridge types. High-current chargers can use IGBT discretes for power control, achieving high efficiency and high charging current.
4. DC/AC Inverter
Online UPSs with a power rating of 3 kVA or above almost all use IGBTs as the power switching devices in the inverter section, often employing full-bridge and half-bridge circuits, as shown in Figure 4 below.
Figure 4: Inverter Circuit Topologies
From the above content, we can see that the power devices used in UPSs include bipolar power transistors, power MOSFETs, thyristors, and IGBTs. IGBTs combine the advantages of power MOSFETs, such as easy driving, simple control, and high switching frequency, with the advantages of power transistors, such as low on-state voltage and high current-carrying capacity. The use of IGBTs has become the preferred choice for UPS power design. Only by fully understanding the characteristics of IGBTs and designing the circuit for reliability can the advantages of IGBTs be fully realized.