What is a safe operating area?

The safe operating area (SOA) is defined as the current and voltage conditions over which an IGBT can be expected to operate without self-damage or degradation. In practice, it is necessary not only to use an IGBT within the safe operating area but also to derate its area for temperature. There are forward-bias and reverse-bias safe operating areas (FBSOA and RBSOA). The forward-bias safe operating area defines the usable current and voltage conditions for the period of time while the IGBT is on. The reverse-bias safe operating area defines the usable current and voltage conditions during the turn-off period of the IGBT.

Figure (a) shows an example of a forward-bias safe operating area, which consists of four regions: (1) a region limited by the maximum collector current rating, (2) a region limited by collector power dissipation (thermal breakdown), (3) a region limited by secondary breakdown, and (4) a region limited by the maximum collector-emitter voltage rating. Care should be exercised as to the region limited by secondary breakdown because it differs, depending on the device design.

Figure (b) shows an example of a reverse-bias safe operating area, which consists of three regions: (1) a region limited by the maximum collector current rating (I_CP), (2) a region limited by the inherent characteristics of a device, and (3) a region limited by the maximum collector-emitter voltage rating (V_CES). In the second region, the maximum current is in inverse proportion to V_CE. Hard-switching applications, in particular, should be designed so as to satisfy the limit of this SOA region.