Safe operating area

The SOA specification combines the various limitations of the device — maximum voltage, current, power, junction temperature, secondary breakdown — into one curve, allowing simplified design of protection circuitry.

The safe operating area curve is a graphical representation of the power handling capability of the device under various conditions.

The SOA curve takes into account the wire bond current carrying capability, transistor junction temperature, internal power dissipation and secondary breakdown limitations.

Where both current and voltage are plotted on logarithmic scales, the borders of the SOA are straight lines: SOA specifications are useful to the design engineer working on power circuits such as amplifiers and power supplies as they allow quick assessment of the limits of device performance, the design of appropriate protection circuitry, or selection of a more capable device.

[2] Except at low collector-emitter voltages, the secondary breakdown limit restricts the collector current more than the steady-state power dissipation of the device.

[4] However, power MOSFETs have parasitic PN and BJT elements within the structure, which can cause more complex localized failure modes resembling secondary breakdown.

This can lead to thermal runaway and the destruction of the MOSFET even when it is operating within its Vds, Id and Pd ratings.

The most common form of SOA protection used with bipolar junction transistors senses the collector-emitter current with a low-value series resistor.