Thermal Runaway
Understanding Thermal Runaway
Thermal runaway is one of the most catastrophic failure modes in power amplifier design. It can destroy a transistor in milliseconds once the cycle begins. Understanding and preventing thermal runaway is essential for reliable PA design.
Thermal Runaway Mechanism
- Temperature increases (from self-heating or ambient).
- Device current increases with temperature (negative temperature coefficient of threshold voltage in BJTs, positive temperature coefficient of IDSS in some FETs).
- Increased current generates more heat (P = I x V).
- More heat raises temperature further. Positive feedback loop.
- If heat generation exceeds heat removal, temperature increases without limit until failure.
Prevention
- Emitter/source resistors: Negative feedback limits current increase.
- Temperature compensation: Bias circuit adjusts with temperature.
- Adequate heatsinking: Ensure thermal resistance is low enough.
- GaN: Wider bandgap means less sensitivity to temperature. Naturally more stable.
Frequently Asked Questions
What is thermal runaway?
Thermal runaway is destructive positive feedback: higher temperature increases current, which increases heat, which increases temperature, until the device is destroyed. Common in BJTs and GaAs PAs. Prevented by proper bias design and thermal management.
Why are BJTs prone to thermal runaway?
BJT collector current increases exponentially with temperature (approximately doubles every 10C for silicon). Without current-limiting feedback, a small temperature increase triggers a runaway spiral. Emitter resistors and temperature-compensated bias prevent this.
Is GaN immune to thermal runaway?
GaN is much more resistant but not immune. GaN has a wider bandgap (less intrinsic carrier increase with temperature) and negative temperature coefficient of current above a certain bias point. However, at extreme conditions, thermal runaway can still occur.