What is the duty cycle derating factor for power handling in pulsed RF applications?
Pulsed Power Derating
The thermal and voltage breakdown limits of RF components respond differently to pulsed signals. The thermal limit depends on average power and thermal time constants. If the pulse period is much shorter than the thermal time constant of the component, the component responds to the average power, not the peak. Since average power = peak power × duty cycle, the allowable peak power increases inversely with duty cycle.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
Frequently Asked Questions
Do I need to consider pulse width?
Yes, if the pulse width approaches or exceeds the thermal time constant. For very short pulses (< 1 μs), localized heating at the junction may cause failure even though the average power is within rating. For very long pulses (> 1 s), the component may approach thermal equilibrium during the pulse.
What about the transition to CW?
As duty cycle approaches 100%, the peak power limit approaches the CW rating. There is no discontinuity; the thermal limit smoothly transitions from the derating curve to the CW rating. Most datasheets provide derating curves showing peak power versus duty cycle.
How does this apply to radar?
Radar systems typically operate at 0.1-10% duty cycle with peak powers of kilowatts to megawatts. The waveguides, connectors, and antenna feed handle the peak power; the average power determines the cooling requirements. Both limits must be verified independently for each component in the transmit chain.