Understanding MOSFET Safe Operating Area (SOA) and How to Derate It

When designing power electronics, the Safe Operating Area (SOA) graph is your boundary between a reliable design and a catastrophic failure. This chart indicates the maximum power a device can safely handle before failing.

What is Linear Mode?

While MOSFETs are often used as simple switches, many applications—like soft starts, hotswap and surge voltage operation—require them to operate in linear mode.

In linear mode, the MOSFET behaves as a voltage-controlled current source. Because both the drain current (ID) and the drain-source voltage (VDS) are high at the same time, the power dissipation is significant (Power = ID x VDS).

The "Spirito Effect" and Thermal Instability

Standard thermal models sometimes fail to predict the real limits of a MOSFET at high voltages. This is known as the Spirito effect.

At certain operating points, the MOSFET enters a region of thermal instability. If a small spot on the silicon die gets slightly hotter than the rest, it begins to draw more current. This creates a positive feedback loop: more current leads to more heat, which leads to even more current. This eventually results in thermal runaway and the destruction of the device.

How to Derate the SOA for High Temperatures

Datasheet SOA graphs are only valid for a mounting base temperature (Tmb) of 25°C. If your system runs hotter, you must derate the curve to find the new safe limits.

Step 1: Calculate the Power Scale Factor (kPSF)

First, determine how much the power capability is reduced using the following formula:

• kPSF = (175°C - Tmb) / (175°C - 25°C)

For example, if your mounting base is at 100°C, your kPSF is 0.5 (or 50%).

Step 2: Apply the Scaling Method

There are three main ways to apply this factor to your SOA curve:

1. Current Scaling: Multiply the original current limit by your kPSF while keeping the voltage constant. This is usually the best approximation for the Spirito region.
2. Voltage Scaling: Multiply the voltage limit by kPSF while keeping the current constant.
3. Power Scaling: Scale both voltage and current by the square root of kPSF to keep the total power constant.

Step 3: Redraw the Curve

When redrawing your safe area for a higher temperature, remember these rules:

• Do NOT derate the RDSon limit: This line is already calculated using the worst-case resistance at 175°C.
• Shift the Linear/Spirito region: Move these lines downward based on your calculated kPSF.

Design Tips

• Package Choice: Using advanced packages like LFPAK (which uses a copper clip) can help prevent local hot spots better than older wire-bonded packages like D2PAK.
• Pulse Duration: SOA limits change drastically based on the pulse width (e.g., 10 microseconds vs. 100 milliseconds).
• Enhanced SOA MOSFETs: For very demanding applications, consider using ASFETs specifically designed for linear mode robustness.