Thermal simulation of transients in microwave devices

The optimal design of solid state microwave power devices, taking thermal and electrical parameters into consideration is a complex subject requiring extensive use of numerical simulation tools. The authors address the problem of the thermal simulation of pulsed microwave solid state power sources designed to operate at a particular duty cycle. The electrical design dictates that the device be as small as possible, in conflict with the thermal criteria. The size of the device will increase with increasing duty cycle and the length of each power pulse it is required to handle. A significant factor is the amount of computing resource necessary to simulate the thermal transient for a sufficiently large number of duty cycles so that the device will reach its maximum working temperature. A solution to this problem is presented that requires the combination of one steady state, and one transient simulation, to predict the maximum working temperature for a given duty cycle and any pulse length. A particular hetero-junction bipolar transistor (HBT) operated under varying duty cycle is analysed as an example