Circuit analysis of active mode parasitic oscillations in IGBT modules

The susceptibility of multi-chip IGBTs to differential oscillations between chips has been noted experimentally. The authors present a theoretical analysis of such oscillations during the transient active operation at turn off and turn on. They take into account the stored charge within the IGBT, the nonlinear transconductance and capacitances and the basic stray inductance elements found within an IGBT module or capsule. Two small-signal analyses are given. The first of these is based on the characteristic equation and exploits symmetry within the module, if appropriate. The second extends the analysis to asymmetrical designs. It is concluded that the methods are complementary and aid understanding of the operation of such devices. Both will also be of use in the design of future devices, especially in making the choice of the gate resistance.