Derating
Understanding Derating
A waveguide termination rated at 500 Watts CW at 25°C ambient does not survive 500 Watts at 85°C. The absorbing element inside the termination has a maximum operating temperature (say 200°C). At 25°C ambient, there is 175°C of thermal headroom. At 85°C ambient, only 115°C of headroom remains, which means the component can only dissipate 115/175 = 66% of its rated power, or approximately 329 Watts. Operating above this derated limit causes the absorber temperature to exceed its design maximum, accelerating material degradation and eventually destroying the component.
The same principle applies to semiconductors. A GaN power amplifier with a 200°C maximum junction temperature and a thermal resistance of 5°C/W can dissipate 35 Watts at 25°C ambient (175°C headroom / 5°C/W). At 85°C ambient, the headroom drops to 115°C and maximum dissipation falls to 23 Watts. This is why all RF component datasheets specify power ratings at a reference temperature and provide a derating factor or curve for elevated temperatures.
Derating Calculation
Pmax(T) = Prated × (Tmax − Tambient) / (Tmax − Tbaseline)
Worked Example (500 W termination):
Prated = 500 W at Tbaseline = 25°C
Tmax = 200°C (absorber limit)
At Tambient = 50°C: Pmax = 500 × (200−50)/(200−25) = 500 × 0.857 = 429 W
At Tambient = 85°C: Pmax = 500 × (200−85)/(200−25) = 500 × 0.657 = 329 W
Derating Factor:
DF = Prated / (Tmax − Tbaseline) = 500 / 175 = 2.86 W/°C
Typical Derating by Component Type
| Component | Rated Power (25°C) | At 50°C | At 85°C | Tmax |
|---|---|---|---|---|
| Waveguide Termination (OFHC Cu) | 500 W CW | 429 W | 329 W | 200°C |
| Waveguide Termination (Al body) | 200 W CW | 167 W | 120 W | 175°C |
| Coaxial Connector (SMA) | 500 W peak | 450 W | 375 W | 165°C |
| GaN PA (junction-limited) | 100 W | 77 W | 43 W | 200°C |
Frequently Asked Questions
Why do RF components need to be derated at elevated temperatures?
Every RF component has a maximum internal temperature beyond which permanent damage occurs. In a waveguide termination, the absorbing element degrades above its limit, causing increased VSWR and eventual failure. As ambient temperature rises, less thermal headroom exists between operating and failure temperatures, so maximum dissipated power must decrease proportionally. A 500 W termination at 25°C has 175°C of headroom; at 85°C, only 115°C remains, limiting safe operation to 329 W.
How do you read a derating curve?
A derating curve plots maximum allowable power on the vertical axis against ambient or baseplate temperature on the horizontal axis. The curve stays flat from minimum operating temperature up to the baseline (typically 25°C), then slopes linearly downward to zero at the maximum rated temperature. Find your ambient temperature, read up to the curve, then across to find the maximum safe power. Apply an additional 20-30% safety margin below the derated value for mission-critical systems.
What is the difference between CW and peak power derating?
CW derating addresses average thermal dissipation, which sets the steady-state temperature of the component. Peak power derating addresses instantaneous voltage stress, which determines the risk of dielectric breakdown or arcing. A termination rated at 500 W CW and 5 kW peak handles 5 kW pulses as long as average power (peak × duty cycle) stays within the CW derating limit. Both constraints must be met simultaneously; violating either one risks damage.