A transient model for insulated gate bipolar transistors (IGBTs)

The Insulated Gate Bipolar Transistor is widely accepted as the preferred switching device in a variety of power converters and motor drive applications. The device combines the advantages of high current density bipolar operation that results in low conduction losses with the advantages of the fast switching and low drive power of MOSFET gated devices. The basic idea behind IGBT is to increase the conductivity of a thick lightly doped epitaxial layer, thus reducing the on-resistance and losses associated with power MOSFET. This reduction in resistivity resulting from high level of carrier injection is referred to as conductivity modulation. When the IGBT is turned OFF, however, injected carriers must be extracted first before the device can sustain the reverse blocking voltage. Several models have been proposed in the literature to describe both DC and transient behaviors of IGBTs. These models can be broadly classified into two main categories: physics based models and behavioral or compact models. The dissertation compares the various approaches made in the literature to model the transient behavior of IGBTs. A new physics-based model for a Non Punch Through (NPT) IGBT during transient turn OFF period is presented. The steady state part of the model is derived from the solution of the ambipolar diffusion equation in the drift region of the IGBT. The transient component of the model is based on the availability of a newly developed expression for the excess carrier distribution in the base. The transient voltage is obtained numerically from this model. Alternatively, an analytical solution for the transient voltage is presented. The theoretical predictions of both approaches are found to be in good agreement with the experimental data. The model is used to calculate the instantaneous power dissipation P( t) and the switching losses E_off in an IGBT for different device carrier lifetimes.