design ideas MOSFET Safe Operating Area and Hot Swap Circuits
Dan Eddleman “Is this MOSFET’s SOA (safe operating area) adequate for my application?” This is the
most frequently asked question by designers of hot swap circuits. When evaluating a
MOSFET’s suitability for a specific application, it helps to have an intuitive understanding
of SOA. After a short review of the SOA specifications found in MOSFET data sheets,
this article presents a simple explanation of SOA, with a focus on MOSFET thermal
behavior. With this understanding, designers can confidently use SOAtherm, a tool
included with LTspice, to accurately evaluate MOSFET SOA in their circuit simulations.
WHAT IS SOA? Every MOSFET data sheet includes an SOA
plot, which describes the maximum time
a MOSFET can be exposed to a specific
voltage and current. Figure 1 shows the
SOA plot from NXP Semiconductor’s data
sheet for the PSMN1R5-30BLE 30V 1.5mО©
N-channel MOSFET. Consider the condition where 10V at 100A is applied to the
MOSFET. Looking at the corresponding
point on the SOA plot, we see that it falls
between the 1ms and 10ms lines. The plot
indicates that you can apply 10V and 100A
for at least 1ms without damaging the
MOSFET if the case (tab) is held at 25В°C. Of course, holding the case at 25В°C is
impossible, requiring an unattainably
perfect heat sink, but fortunately, for short …