Switching characteristics of IGBT
Switching characteristics of IGBT:
- The switching waveforms of an IGBT is, in many respects, similar to that of a Power MOSFET. This is expected, since the input stage of an IGBT is a MOSFET. In a modern IGBT a major portion of the total device current flows through the MOSFET.
- Therefore, the switching voltage and current waveforms exhibit a strong similarity with those of a MOSFET. However, the output p-n-p transistor does have a significant effect on the switching characteristics of the device, particularly during turn off. Another important difference is in the gate drive requirement.
- To avoid dynamic latch up, (to be discussed later) the gate emitter voltage of an IGBT is maintained at a negative value when the device is off. The switching waveforms of an IGBT. Similarity of these waveforms with those of a MOSFET is obvious.
- To turn on the IGBT the gate drive voltage changes from –Vgg to Vgg. The gate emitter voltage vgE follows Vgg with a time constant τ1. Since the drain source voltage of the drive MOSFET is large the gate drain capacitor assumes the lower value CGD1.
- The collector current ic does not start increasing till vgE reaches the threshold voltage vgE(th). Thereafter, ic increases following the transfer characteristics of the device till vgE reaches a value vgEIL corresponding to ic = iL. This period is called the current rise time tri
- The free wheeling diode current falls from IL to zero during this period. After ic reaches IL, vgE becomes clamped at vgE IL similar to a MOSFET. vCE also starts falling during this period. First vCE falls rapidly (tfv1) and afterwards the fall of vCE slows down considerably.
- Two factors contribute to the slowing down of voltage fall. First the gate-drain capacitance Cgd will increase in the MOSFET portion of the IGBT at low drain-source voltages. Second, the pnp transistor portion of the IGBT traverses the active region to its on state more slowly than the MOSFET portion of the IGBT. Once the pnp transistor is fully on after tfv2, the on state voltage of the device settles down to vCE(sat).
- The turn ON process ends here. The turn off process of an IGBT follows the inverse sequence of turn ON with one major difference. Once vgE goes below vgE(th) the drive MOSFET of the IGBT equivalent circuit turns off. During this period (tfi1) the device current falls rapidly.
- However, when the drive MOSFET turns off, some amount of current continues of flow through the output p-n-p transistor due to stored charge in its base. Since there is no reverse voltage applied to the IGBT terminals that could generate a negative drain current, there is no possibility for removing the stored charge by carrier sweep-out.
- The only way these excess carriers can be removed is by recombination within the IGBT. During this recombination period (tfi2) the remaining current in the IGBT decays relatively slowly forming a current fail. A long tfi2 is undesirable, because the power dissipation in this interval will be large due to full collector-emitter voltage. tfi2 can be reduced by decreasing the excess carrier life time in the p-n-p transistor base. However, in the process, on state losses will increase. Therefore, judicious design tradeoffs are made in a practical IGBT to give minimum total loss.