Research on IGBT module driving and protection technology
The IGBT is a composite device of a MOSFET and a bipolar transistor. It has the characteristics of easy driving of MOSFET, and has the advantages of power transistor voltage and current capacity. Its frequency characteristics are between the MOSFET and the power transistor, and it can work normally in the frequency range of several tens of kHz, so it dominates the large- and medium-power applications at higher frequencies. The IGBT is a voltage-controlled device in which a DC voltage of a few ten volts is applied between its gate and emitter, and only a leakage current of the μA level flows, and substantially no power is consumed. However, there is a large parasitic capacitance (several thousand to tens of thousands of pF) between the gate and the emitter of the IGBT. It is necessary to provide a charge and discharge current of several A at the rising and falling edges of the driving pulse voltage to satisfy the dynamics of turn-on and turn-off. This requires that its drive circuit must also output a certain peak current. As a high-power composite device, IGBT has the problem of damage caused by locking phenomenon during overcurrent. In the overcurrent, if the gate voltage is blocked by the general speed, the excessive current change rate will cause overvoltage. Therefore, soft turn-off technology is needed, so it is necessary to master the driving and protection characteristics of the IGBT. Gate characteristics The gate of the IGBT is electrically isolated from the emitter by an oxide film. Since this oxide film is very thin, its breakdown voltage can only reach 20 to 30V, so gate breakdown is one of the common causes of IGBT failure. In the application, although the gate driving voltage is not ensured to exceed the maximum rated voltage of the gate, the parasitic inductance of the gate wiring and the capacitive coupling between the gate and the collector also generate an oscillating voltage that damages the oxide layer. to this end. Stranded wires are typically used to transmit drive signals to reduce parasitic inductance. The oscillating voltage can also be suppressed by connecting a small resistor in series with the gate wiring. Due to the distributed capacitance Cge and Cgc between the gate-emitter and the gate-collector of the IGBT, and the distributed inductance Le in the emitter driving circuit, the influence of these distributed parameters makes the actual driving waveform and ideal driving of the IGBT The waveforms are not exactly the same and create factors that are not conducive to IGBT turn-on and turn-off. This can be verified with an inductive load circuit with a freewheeling diode (see Figure 1). Figure 1 IGBT switch equivalent circuit and turn-on waveform At time t0, the gate drive voltage begins to rise. At this time, the main factors affecting the rising slope of the gate voltage uge are only Rg and Cge, and the gate voltage rises faster. The gate threshold of the IGBT is reached at time t1, and the collector current begins to rise. From this point on, there are two reasons why the uge waveform deviates from the original trajectory. First, the induced voltage on the distributed inductance Le in the emitter circuit increases as the collector current ic increases, thereby weakening the gate drive voltage and reducing the rise rate of the gate-emitter uge, slowing down The collector current is increasing. Second, another factor that affects the voltage of the gate drive circuit is the Miller effect of the gate-collector capacitance Cgc. At time t2, the collector current reaches a maximum value, and the gate-collector capacitance Cgc starts to discharge, and a capacitive current of Cgc is added in the driving circuit, so that the voltage drop in the impedance of the driving circuit is increased, and the gate is also weakened. Polar drive voltage. Obviously, the lower the impedance of the gate drive circuit, the weaker this effect is. This effect is maintained until t3, and uce drops to zero. Its effect also slows down the turn-on process of the IGBT. After the time t3, ic reaches the steady state value, and after the factor affecting the gate voltage uge disappears, the uge reaches the maximum value with a faster rising rate. It can be seen from the waveform of Fig. 1 that due to the existence of Le and Cgc, the rate of rise of uge in the actual operation of the IGBT is slowed down a lot, and the effect of hindering the rise of the driving voltage is manifested as an obstacle to the rise and the opening process of the collector current. . In order to alleviate this effect, the internal resistance of the Le and Cgc of the IGBT module and the gate drive circuit should be as small as possible to obtain a faster turn-on speed. The waveform when the IGBT is turned off is shown in Figure 2. At t0, the gate drive voltage begins to decrease. At the time t1, the level at which the collector's normal operating current can be maintained is reached. The IGBT enters the linear working region, and the uce starts to rise. At this time, the Miller effect of the gate-collector capacitance Cgc dominates. The rise of uce, due to Cgc coupling charging, uge ​​is basically unchanged during t1-t2, at time t2, uge ​​and ic start to decrease at the speed determined by the inherent impedance between the gate and emitter. At t3, uge ​​and ic Both fell to zero and the shutdown ended. Figure 2 Waveform when the IGBT is turned off Lamp Holder,Bulb Holder,Light Holder,Lamp Socket WENZHOU TENGCAI ELECTRIC CO.,LTD , https://www.tengcaielectric.com