Recent technology advances in power electronics have arisen primarily from improvements in semiconductor power devices, with insulated gate bipolar transistors (IGBT) leading the market today for medium power applications. IGBT's feature many desirable properties including a MOS input gate, high switching speed, low conduction voltage drop, high current carrying capability, and a high degree of robustness. Devices have drawn closer to the 'ideal switch', with typical voltage ratings of 600 - 1700 volts, on-state voltage of 1.7 - 2.0 volts at currents of up to 1000 amperes, and switching speeds of 200 - 500 ns. The availability of IGBT's has lowered the cost of systems and enhanced the number of economically viable applications.

The safe operating area (SOA) of a power semiconductor device is a graphical representation of the maximum operational voltage and current limits of the device subjected to various constraints. The forward bias safe operating area (FBSOA) and the reverse bias safe operating area (RBSOA) represent the device SOA with the gate emitter junction forward biased or reverse biased, respectively.

The IGBT has robust SOA during both turn-on and turn off. The FBSOA, is square for short switching times, similar to that of power MOSFET's. The IGBT is thermally limited for longer switching times.

The RBSOA of IGBT's, is different than the FBSOA. The upper half corner of the RBSOA is progressively cut out which reduces the RBSOA as the rate of change of the collector to emitter voltage across the device, dVce/dt, is increased. The RBSOA is reduced as the dVce/dt is increased so as to avoid latch up within the device. This condition exists when higher values of dVce/dt are applied may give to the rise to a pulse of forward decaying current in the body region of the device which acts as a pulse of gate current that can turn on the device. Fortunately, the dVce/dt values that would cause latch up in IGBT's are much higher compared to other devices.

The maximum value of ICM is set so as to avoid latch up which is determined based on the dynamic latch up condition. In addition, a maximum VGE voltage is specified in order to limit the current during a fault condition to ICM by forcing the device out of the on-state into the active region where the current becomes constant regardless of the drain to source voltage. The IGBT must be turned off under these conditions as quickly as possible to avoid excessive dissipation. The avoidance of latch up and the continuous gate control over the collector current are very desirable features.