Current Sharing and Redistribution in High Power IGBT Modules

The Insulated Gate Bipolar Transistor (IGBT) is now the favoured power semiconductor device for most medium power applications. One major difficulty with current IGBT manufacturing technologies is the inability to produce chips rated at more than 100 amps both reliably and cost-effectively. To achieve higher current ratings, a number of chips are connected in parallel to form a module; up to 32 chips operating in parallel have been reported. The aim of this investigation is to examine what measures can be taken to ensure that current is evenly distributed amongst the chips in such a module, during both conduction and switching. An even inter-chip current distribution will make optimum use of the chips in a module, enhancing reliability. The current distribution within existing modules is investigated, and tests indicate that the turn-off transient can produce uneven current distributions and switching losses. Attention then turns to investigation of the matching of chips for selective assembly into power modules. It is found that matching on the characteristics currently measured can improve transient performance. It is also seen that gate resistors, and their tolerances and materials, are important aspects of module construction. The focus then moves to susceptibility to differential oscillations, a problem encountered when paralleling MOSFETs. Analysis of a simple circuit model of the IGBT in its saturated region leads to a method for predicting the minimum gate resistor required to avoid sustained differential oscillations during turnoff. Finally, the device simulator ATLAS enables the investigation of the parallel operation of IGBTs under transient operation, and in particular the effects of structural differences between chips caused by processing variability. Certain aspects of device processing are found to be more critical than others.