1. IGBT and its characteristics
IGBT (Insulated Gate Bipolar Transistor), or Insulated Gate Bipolar Transistor, is a composite semiconductor device composed of MOSFET (Insulated Gate Field Effect Transistor) and BJT (Bipolar Transistor Transistor). Although IGBT has the disadvantage of current tailing, it has the advantages of low driving power of MOSFET and high current of BJT at the same time, and is widely used in high-voltage, high-current and high-speed switching environments. IGBTs have static and dynamic characteristics. Static characteristics mainly include transfer characteristics and output characteristics. The IGBT transfer characteristic describes the relationship between the collector current IC and the gate-emitter voltage VGE. The output characteristics of the IGBT describe the relationship between the collector current IC and the collector-emitter voltage VCE when the gate voltage VGE is used as a variable, and the dynamic (switching) characteristics of the IGBT mainly include turn-on characteristics and turn-off characteristics. Generally, the switching characteristics are tested by the double-pulse test method.
2. Technology to improve IGBT switching characteristics (turn-on and turn-off)
2.1 Single-level gate drive technology
Single-level gate drive technology controls the process of IGBT turn-on and turn-off. Generally, single-level turn-on resistance Rgon and turn-off resistance Rgoff are used to control switching characteristics, as shown in the schematic diagram of single-level gate drive in Figure 1. The disadvantages of this control method mainly include:
Fig 1: Schematic diagram of a single-stage gate drive.png
1) During the switching process, the switching speed of each stage cannot be changed flexibly, which to a certain extent causes large switching losses and large turn-on/turn-off delays;
2) Diode recovery characteristics are poor, because the parameters are not adjustable, it may cause gate waveform oscillation and body diode stress and power consumption exceed the standard;
3) Due to the poor turn-off speed regulation, the poor response of active clamping leads to excessive turn-off peaks and other problems. Due to the poor flexibility of the single-stage gate drive structure, it is usually impossible to achieve the best switching performance, and the ability to adapt to diverse systems is poor, as well as its EMC (current change rate di/dt and voltage change rate dv/dt) characteristics Poor, and there is an increased risk of latching effects, large interference to the system, reduced reliability, and poor ability to adapt to different systems (especially modules).
2.2 Multi-level gate drive technology
With the miniaturization of the system structure, the change of the module structure, and the improvement of the switching speed brought about by the replacement of the module, the single-level gate control can no longer meet the driving requirements. In order to ensure the reliable operation of the system and improve the switching characteristics, from the aspect of drive technology, it is necessary to control the dynamic process of the switch more accurately, and each dynamic stage provides the basis for the drive requirements, which is the multi-level drive technology. Figure 2 is a schematic diagram of multi-level gate drive, its advantages:
Fig 2: Schematic diagram of multi-level gate drive
1) Reduce the on-off delay, improve the system response speed, reduce the PWM dead time, and improve the pulse width utilization;
2) Improve the EMC characteristics, especially the di/dt of the reverse recovery of the diode when it is turned on, prevent the latching effect from occurring, and reduce the recovery current oscillation;
3) Reduce the turn-on loss caused by the Miller effect. The above advantages are all reflected in the improvement of the switching characteristics.
2.2.1 Application of multi-level gate drive technology in the turn-on process
As shown in Figure 3, according to the requirements of the turn-on process, we adopt multi-level drive turn-on technology, and we invest in different drive turn-on resistors to achieve better drive turn-on characteristics.
Fig 3: Open process analysis diagram
t0~t1: In the turn-on transition stage, the drive rises from negative voltage to Vth, and the current IC and voltage VCE do not change. In order to reduce the turn-on delay, a small gate resistance is used to charge the gate quickly;
t1~t2: From the time t1, the IGBT current starts to increase rapidly until the diode reverse recovery is completed, and VCE begins to drop. At this stage, the current change rate di/dt needs to be controlled, so a larger gate resistance is used;
t2~t3: Miller effect stage, to accelerate the VCE drop, input a small gate resistor, reduce the Miller effect, and reduce the turn-on loss;
t3~t4: Still put in a small gate resistance, speed up the gate to enter the steady state, and speed up the speed of the transistor completely entering saturation.
Note: During the period t2~t3, the charging process of the gate is determined by the Miller capacitor CGC. At this time, the collector-emitter voltage VCE keeps decreasing, and the current IGC discharges the gate through the CGC. This part of the current needs to be compensated by the driving current Idriver. The IGC current is shown in formula (1):
IGC =CGC * dUCE / dt (1)
2.2.2 The application of multi-level gate drive technology in the turn-off process
As shown in Figure 4,.According to the previous analysis of the turn-off process, the multi-level drive turn-off technology is adopted, and different drive turn-off resistors are used to achieve better turn-off. break feature.
Fig 4: Comparative analysis chart of shutdown process
t5~t6: The driver issues a shutdown command, Ciss discharges, and the gate VGE begins to drop. To reduce the shutdown delay, a small gate resistor is sufficient;
t6~t7~t8: Miller effect occurs, Vds rises, and the switching tube Miller capacitor Crss is charged. According to the system situation, a larger resistor is used to control dv/dt and di/dt, and reducing dv/dt is helpful for clamping response , reducing di/dt helps to turn off the peak;
After t8: The MOSFET in the IGBT is completely turned off, and then there is a current tail, which is mainly caused by the process of turning off the internal PNP transistor. At this time, a small resistance can be used.
3. Conclusion
The switching characteristic is an important characteristic of any switching device. Optimizing and improving the switching characteristic is the key task of the corresponding driver. This paper describes the detailed process of IGBT turn-on and turn-off. Based on the different requirements in the switching process, it is a better choice to improve the switching characteristics through multi-level driving.
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