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Definition of IGBT switching time


The switching process of the IGBT is mainly controlled by the gate voltage VGE. Since there is a parasitic capacitance between the gate and the emitter, the turn-on and turn-off of the IGBT is equivalent to charging and discharging the CGE. It is assumed that the initial state of the IGBT is an off state, that is, VGE is a negative pressure VGC-, and the output of the latter stage is a resistive inductive load with a freewheeling diode. So how much do you know about the definition of IGBT switching time? details as follows.


1. Opening time ton


The turn-on time can also be divided into two parts: turn-on delay time td(on) and rise time tr, during which the IGBT mainly works in the active region.


When a step forward driving voltage is applied between the gate and the emitter, the CGE starts to charge, and the VGE starts to rise. The time constant of the rising process is determined by the resistance of the CGE and the gate drive network. Once 'VGE reaches the turn-on voltage VGE(th), the collector current Ic starts to rise. From the rise of VGE to VGE(th), until the IC rises to 10% of the load current IL, this time is defined as the turn-on delay time td(on).


Thereafter, the collector current Ic continues to rise, and when Ic rises to 90% of the load current IL, this period of time is called the rise time tr. The sum of the turn-on delay time td(on) and the rise time tr is the turn-on time ton. During the entire turn-on time, it can be seen that the current gradually rises and the collector-emitter voltage drop is still considerable, so the main turn-on losses occur during this time.


2. IGBT turns on


When the IGBT is turned on, it mainly works in the saturation region.


After the IGBT is turned on, the collector current Ic will continue to rise, and a peak value of the turn-on current will be generated. This peak value is generated by the resistive inductive load and the freewheeling diode. If the peak current is too large, the IGBT may be lost. After Ic reaches the peak value, it will gradually decrease to the level of the load current Ic. At the same time, VCE also decreases to the saturation voltage drop level, and ICBT enters a relatively stable conduction stage. The main parameters in this stage are the on-state current IL determined by the load and a lower saturation voltage drop VCEsat. It can be seen that the loss of the IGBT working in the saturation region is not particularly large.


3. Off time toff


Like the turn-on time ton, the turn-off time toff can also be divided into two sections: the turn-off delay time td (off), and the fall time tf.


When the forward voltage between gate and emitter is abruptly removed and a negative voltage is applied at the same time, VCE begins to drop. The time constant of the drooping process is still determined by the input capacitor CGE and the resistance of the gate drive loop. At the same time, VCE started to rise. But as long as VCE is less than VCC, the freewheeling diode is in an off state and cannot continue current. Therefore, the collector current Ic of the IGBT does not drop significantly during this period. Therefore, the gate-emitter voltage VCE falls to 90% of its turn-on value until the collector current falls to 90% of the load current; this period is defined as the turn-off delay time td(off).


Once the collector-emitter voltage of the rising IGBT exceeds the operating voltage VCC, the freewheeling diode is in a forward biased state, the load current can be commutated to the freewheeling diode, and the collector current is therefore reduced from the collector. The time between the electrode current IC falling from 90% to 10% of the load current k is called the falling time tf. As can be seen from Figure 1, while the IC is falling, VCE will generate a peak value that greatly exceeds the operating voltage Vcc, which is mainly caused by the load inductance, and its magnitude is linearly related to the turn-off speed of the IGBT. Excessive peak current may cause damage to the IGBT.


The sum of the turn-off delay time and the fall time tf is called the turn-off time toff.


4. Trailing time, trailing current


Compared with MOSFET, IGBT adopts a new way to reduce the on-state loss, but this design also causes the tail current It, the time for the tail current to decay to the off-state leakage current is called the tail time tt, The tail current seriously affects the turn-off losses, because during this time, VCE has risen above the operating voltage VCC. The generation of trailing current also tells us that even if a shutdown signal is given at the gate, the IGBT cannot be completely turned off in time. dead zone.


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