Thermal Runaway Stability Check

Formula

Description

Thermal runaway occurs when a component power dissipation increases with temperature faster than the cooling system can remove heat, creating a positive feedback loop that leads to destruction. The stability condition requires that the rate of power increase with temperature (dP/dT) must be less than the cooling capability (1/Rth_ja). The forward result shows the stability margin: positive means stable, negative means thermal runaway risk. This is critical for BJTs (which have positive temperature coefficient of current), MOSFETs in linear mode, and voltage regulators at high power.

Variables

• dP/dT — Rate of power increase with temperature (W/°C)
• Rth_ja — Junction-to-ambient thermal resistance (°C/W)

Practical Notes

A positive result indicates stability margin. A negative result means the thermal design is unstable — either improve cooling (lower Rth) or reduce the power-temperature sensitivity. BJTs are particularly susceptible because collector current increases with temperature (about 7%/°C), which increases power dissipation. MOSFETs have a negative temperature coefficient in saturation mode, providing inherent stability, but a positive coefficient in linear (ohmic) mode.