Safe Operating Area for BJT (Power Transistors)

Safe Operating Area (SOA) for BJT

The safe operating area (SOA) defines the current and voltage limitations of power devices.

The above figure shows the typical SOA of a power bipolar transistor. It can be partitioned into four regions. The maximum current limit (section a-b) and the maximum voltage limit (d-e) are determined by the technological features and construction of the particular device. The maximum power dissipation limits the product of the transistor's currents and voltages (section b-c). A secondary breakdown (c-d) occurs when high voltages and high currents appear simultaneously when the device is turned off. When this happens, a hot spot is formed and the device fails due to thermal runaway.

SOA for MOSFET & IGBT

The above figure shows the safe operating area (SOA) of MOSFET transistors. This area is bounded by three limits: current limit (section a-b), maximum power dissipation limit (b-c), and the voltage limits (c-d). The SOA of an IGBT is identical to that of the MOSFET SOA.
Since the drain current decreases when the temperature increases in MOSFET transistors, the possibility of secondary breakdown is almost nonexistent. If local heating occurs, the drain current - and consequently the power dissipation - both diminish. This avoids the creation of local hot spots that can cause thermal runaway.

The above figure demonstrates how the SOA of a device increases when the device is operating in pulse mode.
When the device is operating in DC mode the safe operating area is at its smallest. The SOA grows when pulse mode is used. The shorter the pulse signal, the higher the SOA.