Transistors｜Evaluation

About SOA （Safe Operating Area） Destruction

2023.03.29

Points of this article

・SOA is an abbreviation of Safe Operating Area, meaning the range within which operation is safe.

・MOSFETs and other devices are used within the SOA range.

・There are five conditions that set limits on the SOA; if any of these are exceeded, destruction is possible.

table of contents

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 ID along the vertical axis and the drain-source voltage VDS along the horizontal axis. That is, the range within which the MOSFET can operate safely is determined by VDS, ID, the power dissipation PD that is the product of these two quantities, and the secondary breakdown area. The applied power pulse width PW 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 ID is limited by the MOSFET on-resistance RDS(ON)

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

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

The green line (2) is the absolute maximum rating IDP, 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 IDP 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 PD for the MOSFET, and is limited by the applied power pulse width PW and the transient thermal resistance. Within this range, in general Tj does not exceed the absolute maximum rating TjMAX, 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 VDSS of the MOSFET drain-source voltage

This is an area limited by the VDSS 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.

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