Thermal Derating Curves
How component ratings reduce with ambient temperature — capacitors, resistors, semiconductors.
Reference
General pattern
- Components are rated at a reference ambient (typically 25 °C or 70 °C).
- Above reference, maximum power / current / voltage must be reduced linearly to zero at T_max.
- Derate further for reliability — 50% of rated is conservative engineering practice.
Typical derating references
| Part | T_max rating | Reference temp | Derate from |
|---|---|---|---|
| Power resistor | 155 °C case | 25 °C | Linear to 0 W at T_max |
| Aluminum electrolytic cap | 85 or 105 °C | 40–85 °C | Life halves per 10 °C rise |
| MLCC (ceramic cap) | 85–125 °C | — | Also DC-bias derate (capacitance loss) |
| TO-220 transistor | 150 °C junction | 25 °C | Linear to 0 W at T_J,max |
| LED | 85 °C junction | 25 °C | Linear to 0 mA at T_J,max |
| Crystal oscillator | Varies | 25 °C | Frequency drifts with T |
Capacitor lifetime (rule of thumb)
- Arrhenius
- Life doubles per 10 °C below rated
- Example
- Cap rated 2 000 h @ 85 °C operated at 55 °C → 2³ × 2 000 h = 16 000 h
Voltage derating
- X7R ceramic at DC
- Capacitance drops to ~50% at rated DC voltage — derate to 50% for bulk
- High-voltage film
- Derate 25–50% below V_rated for reliability
- MLCC in automotive
- Typical guidelines derate 50% of rated DC voltage
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