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  1.   Abstract
  2.   Trademarks
  3. 1Using MOSFET Safe Operating Area Curves in Your Design
  4. 2Review the SOA Graph
  5. 3Applications Where SOA is Important
  6. 4Using the Data Sheet SOA Graph
  7. 5Temperature Derating of SOA
  8. 6Estimating SOA for Different Pulse Widths
  9. 7Estimating SOA for Non-Square Waveforms
  10. 8Summary
  11. 9References

3 Applications Where SOA is Important

Power MOSFETs are found in a wide variety of applications but typically fall into two categories: switch-mode and linear-mode. Some examples of switch-mode applications are DC-DC converters, class-D audio amplifiers and motor drives. Inrush control for hot-swap, load switching and as a pass element in a linear regulator are common linear-mode applications.

First, a quick review of MOSFET output characteristics as shown in Figure 3-1. The family of IDS vs. VDS curves at different values of VGS displayed in this chart can be divided into two regions: linear, where VDS << VGS – VGS(th), and saturation, where VDS > VGS – VGS(th). In the linear region the output is ohmic and increasing VDS results in proportionally higher IDS. In the saturation region the output is flat or saturated and IDS only increases slightly with increasing VDS.

Figure 3-1 MOSFET Output Characteristics

Figure 3-2 shows switch-mode (blue circles) and linear-mode (orange triangle) operating points. In a switch-mode application, the FET transitions between the off state (VGS << VGS(th), and IDS = 0A) and the linear region. During the switching transitions, the FET rapidly passes through the saturation region as depicted by the blue dashed line. Because of the short duration in the saturation region, the FET does not incur excessive power loss and is of little concern for SOA. In a linear-mode application, the FET operates for long periods of time in the saturation region where there is simultaneously voltage across and current through the device leading to high power dissipation and elevated junction temperature. The focus of this article is on linear-mode operation and how to use the data sheet SOA curves to make sure the FET operates within safe limits with no damage.

Figure 3-2 Switch-Mode and Linear-Mode Operating Points