About SOA （Safe Operating Area） Destruction

About SOA （Safe Operating Area）

SOA is an abbreviation of Safe Operating Area, meaning the range within which operation is safe. In order to use a MOSFET safely, it must be used within the SOA range; if this range is exceeded, the occurrence of destruction is a possibility. Destruction when a device is operated outside the SOA range is called SOA destruction. As an example, the SOA of the R6024KNX SJ (Super Junction) MOSFET is shown below.

SOA of the R6024KNX SJ MOSFET

The SOA is represented by the drain current I_D along the vertical axis and the drain-source voltage V_DS along the horizontal axis. That is, the range within which the MOSFET can operate safely is determined by V_DS, I_D, the power dissipation P_D that is the product of these two quantities, and the secondary breakdown area. The applied power pulse width P_W is another important factor determining the SOA. The SOA has regions (1) to (5), indicated in the graph.

Relationship of Limits and Destruction to Areas of SOA

Areas (1) to (5) in the graph are explained below.

Area （1）: Area in which the drain current I_D is limited by the MOSFET on-resistance R_DS(ON)

This is an area in which I_D is limited by R_DS(ON), even when the applied V_DS is under the absolute maximum rating. From Ohm’s law I=V/R, only I_D values up to the red line can be passed.　　※The area shown is for V_GS=10 V

■Area （2）: Area determined by the absolute maximum rating I_DP of the drain current under pulse application

The green line (2) is the absolute maximum rating I_DP, stipulated in the specifications. The absolute maximum rating is of course a value that must not be exceeded, and therefore the device cannot be used at I_DP values above this value. If the device is used in the area (at current values) exceeding this, operation is outside the range of guaranteed operation, and there is a risk of destruction.

■Area （3）: Thermal limitation area

This area is determined by the allowed power dissipation P_D for the MOSFET, and is limited by the applied power pulse width P_W and the transient thermal resistance. Within this range, in general Tj does not exceed the absolute maximum rating Tj_MAX, and therefore the device can be used safely. However, this line varies depending on the ambient temperature, the MOSFET mounting conditions, heat dissipation conditions, and other factors, and so caution is required. Moreover, when a MOSFET is switched during use, there may be instantaneous application of high voltages or large currents, and so care must also be taken to ensure that the limits of this area (3) are not exceeded in transient states during switching as well.

■Area （4）: Secondary breakdown area

When a current is passed in a state of high voltage application, large currents may flow locally within a device to cause destruction; this is called secondary breakdown. This line in the graph is a limit line to ensure that a secondary breakdown state does not occur. As with the thermal limitation area (3), the secondary breakdown area is affected by the ambient temperature and other factors.

■Area （5）: Area determined by the absolute maximum rating V_DSS of the MOSFET drain-source voltage

This is an area limited by the V_DSS stipulated by the specifications; exceeding this value results in the occurrence of breakdown, which can cause destruction. Due to an emf induced by a flyback voltage and parasitic inductance, this limit may be exceeded instantaneously, so caution is required.