Summary of IGBT Application in Uninterruptible Power Supply

The power devices used in UPS include bipolar power transistors, power MOSFETs, thyristors and IGBTs. IGBTs have the advantages of power MOSFETs, such as easy driving, simple control and high switching frequency, as well as the advantages of power transistors, such as low on-state voltage and large on-state current. Therefore, IGBTs have become the first choice for UPS power design.

　　1. Introduction

　　The power devices used in UPS include bipolar power transistors, power MOSFETs, thyristors and IGBTs. IGBTs have the advantages of easy driving, simple control and high switching frequency of power MOSFETs, as well as the advantages of low on-state voltage and large on-state current of power transistors. The use of IGBTs has become the first 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 brought into play. This article introduces the application of IGBTs in UPS and the precautions in use.

　　2. Application of IGBT in UPS

　　绝缘栅双极型晶体管（IGBT）是一种MOSFET 与双极晶体管复合的器件。据东芝公司资料，1200V/100A 的IGBT 的导通电阻是同一耐压规格的功率MOSFET 的1/10，而开关时间是同规格GTR 的1/10。由于这些优点，IGBT广泛应用于不间断电源系统（UPS）的设计中。这种使用IGBT 的在线式UPS 具有效率高，抗冲击能力强、可靠性高的显著优点。

　　UPS 主要有后备式、在线互动式和在线式三种结构。在线式UPS 以其可靠性高，输出电压稳定，无中断时间等显著优点，广泛用于通信系统、税务、金融、证券、电力、铁路、民航、政府机关的机房中。本文以在线式为介绍对象，介绍UPS 中的IGBT 的应用。

　　Figure 1 shows the main circuit of an online UPS. An online UPS power supply has independent bypass switches, AC/DC rectifiers, chargers, DC/AC inverters and other systems. The working principle is: when the mains power is normal, the AC/DC rectifier rectifies the AC power into DC power, charges the battery at the same time, and then converts the DC power into standard sinusoidal AC power through the DC/AC inverter. When the mains power is abnormal, the battery supplies power to the inverter, and when the UPS fails, the output is converted to bypass power supply. The voltage and frequency output of the online UPS are the most stable, and can provide users with truly high-quality sinusoidal power.

　　

　　Figure 1 Main circuit diagram of online uninterruptible power supply

　　①Bypass switch (AC BYPASS SWITCH)

　　Relays and thyristors are often used for bypass switches. Relays are widely used in small and medium-power UPS. The advantages are simple control and low cost, while the disadvantages are that relays have switching time and the life of electromechanical devices. Thyristors are commonly found in medium and large-power UPS. The advantages are large control current and no switching time. However, the disadvantage is that the control is complex, and due to the triggering working characteristics of thyristors, they must be reverse biased before they can be turned off after being triggered to conduct, which will generate a maximum circulating current of 10ms, as shown in Figure 2. If IGBT is used, as shown in Figure 3, this problem can be avoided. Using IGBT has the advantage of simple control, but the cost is higher. Its working principle is: when the input is in the positive half cycle, the current flows through Q1 and D2, and when the input is in the negative half cycle, the current flows through D1 and Q2.

　　

　　Figure 2: SCR delay shutdown phenomenon diagram

　　

　　Figure 3: Bypass switch using IGBT

　　② Rectifier AC/DC

　　UPS rectifier circuits are divided into ordinary bridge rectifiers, SCR phase-controlled rectifiers and PFC high-frequency power factor correction rectifiers. Traditional rectifiers have a base frequency of 50HZ and the filter is heavy. With the development of UPS technology and the requirements of various countries for power input power factor, UPS using PFC power factor correction is becoming increasingly popular. The base frequency of the PFC circuit is at least 20KHZ, and the volume and weight of the filter inductor and filter capacitor used are greatly reduced. The input power factor can reach 0.99 without adding a harmonic filter. IGBT is often used as a power device in PFC circuits. PFC rectifiers using IGBT have the advantages of high efficiency, large power capacity and green environmental protection.

　　③ Charger

　　Common UPS chargers include flyback, BOOST and half-bridge. Single-tube IGBT can be used in high-current chargers for power control, which can achieve high efficiency and large charging current.

　　④DC/AC Inverter

　　Online UPS with power above 3KVA almost all use IGBT as the power device of the inverter part, and full-bridge circuit and half-bridge circuit are commonly used, as shown in Figure 4.

　　

3. Causes of IGBT damage

　　During the use of UPS, it is often subjected to capacitive or inductive load impact, overload or even load short circuit, as well as UPS misoperation, which may cause IGBT damage. The main reasons for IGBT damage during use are as follows:

　　Overcurrent damage;

　　IGBT has a certain ability to resist overcurrent, but care must be taken to prevent damage from overcurrent. There is a parasitic thyristor in the IGBT composite device, so there is a holding effect. Figure 5 is an equivalent circuit of an IGBT. Within the specified drain current range, the positive bias voltage of the NPN is not enough to turn on the NPN transistor. When the drain current reaches a certain level, this positive bias voltage is enough to turn on the NPN transistor, and then the NPN and PNP transistors are in saturation. The parasitic thyristor is turned on, the gate loses its control function, and the holding effect occurs. After the holding effect occurs in the IGBT, the excessive drain current causes excessive power consumption, and finally leads to device damage.

　　Overvoltage damage;

　　When the IGBT is turned off, a spike voltage is generated at the moment of shutdown due to the inductance component in the inverter circuit. If the spike voltage is overvoltage, it may cause the IGBT to break down and be damaged.

　　The bridge arm common conductor is damaged;

　　Overheat damage and static electricity damage.

　　4. Solutions to IGBT damage

　　Overcurrent damage

　　In order to avoid the IGBT from being damaged by the holding effect, the circuit design should ensure that the maximum operating current of the IGBT should not exceed the IDM value of the IGBT. At the same time, it should be noted that the drive resistance RG can be appropriately increased to extend the turn-off time and reduce the di/dt of the IGBT. The size of the drive voltage will also affect the holding effect of the IGBT. If the drive voltage is low, the overcurrent time is long. The IGBT must be negatively biased. IGBT manufacturers generally recommend adding a reverse bias voltage of about -5V. In the case of negative bias, the drive positive voltage is between 10-15V, and the drain current can exceed 4 to 10 times the rated current within 5 to 10μs, so the drive IGBT must be designed with a negative bias. Since the UPS load impact characteristics are different, and the power supply equipment may have a power failure short circuit, it is also necessary to take current limiting measures to limit the current of the IGBT in the UPS design. The drive thick film circuit provided by the IGBT manufacturer can be considered. Such as FUJI's EXB841, EXB840, Mitsubishi's M57959AL, 57962CL, they detect the collector voltage of the IGBT, if the IGBT has overcurrent, the internal circuit will shut down the drive.

　　This method sometimes still cannot protect the IGBT. According to the information of IR Company, the short-circuit protection method recommended by IR Company is: first detect the on-state voltage drop Vce. If Vce exceeds the set value, the protection circuit will immediately reduce the driving voltage to 8V, so that the IGBT will enter the amplification area from the saturation state, the on-state resistance will increase, and the short-circuit circuit will be reduced. After 4us continuous detection of the on-state voltage drop Vce, if it is normal, the driving voltage will be restored to normal. If it is not restored, the drive will be turned off to reduce the collector current to zero. In this way, the short-circuit current is softly turned off, which can avoid the damage of the IGBT caused by excessive di/dt caused by rapid shutdown. In addition, according to the latest IGBT information of Mitsubishi Company, the F series IGBT launched by Mitsubishi all contain overcurrent limiting circuits (RTC circuit), as shown in Figure 6. When overcurrent occurs, the starting voltage of the IGBT will be reduced to 9V within 10us. In conjunction with the M57160AL driving thick film circuit, the IGBT can be quickly softly turned off to protect it.

　　

　　Figure 5: IGBT equivalent circuit diagram

　　

　　Figure 6 RCT circuit of Mitsubishi F series IGBT

　　Overvoltage damage

　　Methods to prevent overvoltage damage include: optimizing the process structure of the main circuit, reducing the parasitic inductance of the line by narrowing the path of the large current loop; appropriately increasing the IGBT drive resistance Rg to slow down the switching speed (but the switching loss also increases); designing a buffer circuit to suppress the peak voltage. The diode used in the buffer circuit must be a fast recovery diode, and the capacitor must be a high-frequency, low-loss, and film capacitor with good frequency characteristics. Only in this way can a good absorption effect be achieved. Common circuits include energy-consuming and feedback buffer circuits. There are two types of feedback types: passive and active. For detailed circuit design, please refer to the technical manual of the selected device.

　　Bridge arm common conduction damage

　　In UPS, the two drives of the same arm branch of the inverter bridge must be interlocked, and the dead time (i.e. the time when they are not conducting together) should be set. If common conduction occurs, the IGBT will be damaged quickly. The control circuit should take into account the drive problems and control timing problems under various operating conditions.

　　Overheating damage

　　The problem of overheating damage can be solved by reducing the rating, increasing the size of the heat sink, applying thermal conductive adhesive, forcing fans to cool, setting over-temperature protection, etc.

　　In addition, attention should be paid to the problem of electrostatic damage during the installation process, and operators and tools must be protected against static electricity.

　　5. Conclusion

　　IGBT has the advantages of both power MOSFET and GTR, and is an ideal device for power conversion such as charging, bypass switch, inverter, rectifier, etc. in UPS.

　　Only by using IGBT reasonably and taking effective protection schemes can the reliability of IGBT in UPS be improved.