IGBT (insulated gate bipolar transistor) is a composite fully controlled voltage driven power semiconductor device composed of BJT (bipolar transistor) and MOS (insulated gate field effect transistor). It has the advantages of high input impedance of MOSFET and low on-off voltage drop of GTR. When the saturation voltage of GTR is reduced, the current carrying density is high, but the driving current is large; the driving power of MOSFET is very small, the switching speed is fast, but the on-off voltage drop is large, and the current carrying density is small. IGBT combines the advantages of the above two devices, with low driving power and low saturation voltage. It is very suitable for the converter system with DC voltage of 600V and above, such as AC motor, inverter, switching power supply, lighting circuit, traction drive and other fields.

1、 Introduction

IGBT (insulated gate bipolar transistor) is a composite fully controlled voltage driven power semiconductor device composed of BJT (bipolar transistor) and MOS (insulated gate field effect transistor). It has the advantages of high input impedance of MOSFET and low on-off voltage drop of GTR. When the saturation voltage of GTR is reduced, the current carrying density is high, but the driving current is large; the driving power of MOSFET is very small, the switching speed is fast, but the on-off voltage drop is large, and the current carrying density is small. IGBT combines the advantages of the above two devices, with low driving power and low saturation voltage. It is very suitable for the converter system with DC voltage of 600V and above, such as AC motor, inverter, switching power supply, lighting circuit, traction drive and other fields. Figure 1 shows an n-channel enhanced IGBT structure. The N + region is called the source region, and the electrodes attached to it are called the source. The N + zone is called leakage zone. The control area of the device is the gate area, and the electrode attached to it is called the gate electrode. The channel is formed close to the grid boundary. The p-type region (including P + and p-1) between the source and the drain (where the channel is formed) is called the subchannel region. The P + region on the other side of the drain region is called drain injector, which is the unique functional area of IGBT. Together with the drain region and sub channel region, PNP bipolar transistor is formed, which acts as emitter, injects holes into the drain to conduct conductive modulation, so as to reduce the on state voltage of the device. The electrode attached to the leakage injection area is called drain electrode. The switching function of IGBT is to form a channel by adding forward gate voltage to provide base current for PNP transistor and make IGBT turn on. On the contrary, reverse gate voltage is added to eliminate the channel and cut off the base current to turn off IGBT. The driving method of IGBT is basically the same as that of MOSFET. It only needs to control the n-channel MOSFET, so it has high input impedance characteristics. When the channel of MOSFET is formed, a hole (minority carrier) is injected into the n-layer from the P + base to modulate the conductance of the n-layer to reduce the resistance of the n-layer, so that IGBT also has a low on-state voltage at high voltage.

Selection of IGBT module

2、 Precautions in use

Because the IGBT module is MOSFET structure, the gate of IGBT is electrically isolated from the emitter by a layer of oxide film. Because the oxide film is very thin, the breakdown voltage generally reaches 20 ~ 30V. Therefore, grid breakdown caused by static electricity is one of the common reasons for IGBT failure. Therefore, attention should be paid to the following points in use:

When using the module, try not to touch the driving terminal by hand. When it is necessary to touch the module terminal, the static electricity on human body or clothes should be grounded with high resistance before touching; when connecting the drive terminal with conductive materials, do not connect the module before wiring is well connected; try to operate under the condition that the bottom plate is well grounded. In some cases, although the gate driving voltage is guaranteed not to exceed the maximum rated voltage of the gate, the parasitic inductance of the grid connection and the capacitive coupling between the grid and the collector can also produce oscillation voltage that damages the oxide layer. In order to reduce the parasitic inductance, twisted pair is usually used to transmit the driving signal. The oscillation voltage can also be suppressed by connecting a small resistor in series in the grid connection.

In addition, if a voltage is applied between the collector and the emitter in the open circuit between the grid and the emitter, the current will flow through the collector due to the leakage current flowing through the collector and the grid potential increasing with the change of the collector potential. At this time, if there is a high voltage between the collector and the emitter, the IGBT may be heated and damaged.

In the case of using IGBT, when the grid circuit is abnormal or the grid circuit is damaged (the grid is in open circuit state), if the voltage is applied to the main circuit, the IGBT will be damaged. In order to prevent this kind of fault, a resistance of about 10K Ω should be connected between the grid and the emitter in series.

When installing or replacing the IGBT module, great attention should be paid to the contact surface status and tightening degree between the IGBT module and the heat sink. In order to reduce the contact thermal resistance, it is better to apply thermal conductive silicone grease between the radiator and the IGBT module. Generally, there is a cooling fan installed at the bottom of the heat sink. When the cooling fan is damaged, the poor heat dissipation of the radiator will cause the IGBT module to heat up and cause failure. Therefore, the cooling fan should be checked regularly. Generally, there is a temperature sensor installed on the radiator near the IGBT module. When the temperature is too high, it will alarm or stop the IGBT module.

The voltage specification of IGBT module is closely related to the input power supply voltage of the device used. The relationship between them is shown in the table below. In use, when the collector current of IGBT module increases, the rated loss also increases. At the same time, the switch loss increases, which makes the original heating more serious. Therefore, when selecting IGBT module, the rated current should be greater than the load current. Especially when it is used as high-frequency switch, it should be used in a lower grade due to the increase of switch loss and heating.

3、 Precautions during storage

Generally, the place where IGBT module is stored should be kept in normal temperature and humidity, and should not deviate too much. The normal temperature is 5-35 ℃, and the normal humidity is about 45-75%. In the dry area in winter, humidifier is needed; Keep away from places with corrosive gas or dust as far as possible; in the place where the temperature changes rapidly, the surface of IGBT module may have condensation water, so the IGBT module should be placed at the place with small temperature change; when keeping, pay attention not to stack heavy objects on the IGBT module; the container containing IGBT module should choose the container without static electricity. IGBT module has a variety of excellent characteristics, so it has been rapid development and popularization, has been applied to all aspects of power electronics. Therefore, it is necessary to be familiar with the performance of IGBT module, and to understand the selection and use of precautions.