How does pulsed operation affect the thermal management requirements of a radar transmitter?

Pulsed operation significantly reduces the average thermal load compared to CW operation, but the thermal design must account for both the average and transient (peak) temperatures: (1) Average power dissipation: for a pulsed radar PA: P_diss_avg = P_diss_peak × duty_cycle. Where duty_cycle = pulse_width × PRF (pulse repetition frequency). Example: P_diss_peak = 500W, pulse_width = 10 μs, PRF = 1000 Hz. Duty_cycle = 10 × 10^-6 × 1000 = 0.01 (1%). P_diss_avg = 500 × 0.01 = 5W. The heat sink only needs to dissipate 5W average (not 500W). This dramatically reduces the heat sink size. (2) Transient temperature rise: during each pulse, the junction temperature rises above the steady-state average. The peak temperature depends on the thermal time constant (τ_th) of the device: if pulse_width << τ_th: the junction temperature barely rises during the pulse (the thermal mass of the die absorbs the heat). The peak ΔT ≈ P_peak × R_θJC × (t_pulse / τ_th). If pulse_width >> τ_th: the junction temperature approaches the CW value during the pulse. The heat sink must handle the peak power. (3) Thermal time constant: τ_th depends on the die size, package, and mounting. Small GaN die (1 × 1 mm): τ_th ≈ 0.1-0.5 ms. Medium GaN die (3 × 3 mm): τ_th ≈ 1-5 ms. Large LDMOS die (10 × 10 mm): τ_th ≈ 5-20 ms. For the example above (10 μs pulse, τ_th = 1 ms): t_pulse / τ_th = 0.01. Peak ΔT ≈ P_peak × R_θJC × 0.01 = 500 × 0.8 × 0.01 = 4°C above the average temperature. This is negligible. (4) Duty cycle limits: the device datasheet specifies the maximum duty cycle for each peak power level. At maximum peak power: the duty cycle may be limited to 1-10% to keep the average junction temperature within limits. At reduced peak power: higher duty cycles are allowed. The designer must verify that both the average and peak junction temperatures are within specification.