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Coolit hits the mark with power amplifier predictions.

Courtesy of Miteq Corp.

Developer/manufacturer Miteq Corp had been using a healthy safety factor when calculating thermal loads in its satellite communications subsystems. But this approach was adding unnecessary cost and weight to the equipment, so the company enlisted Coolit to optimize its designs.

To assess the accuracy of the Coolit predictions, Miteq benchmarked a 3 RU rack-mounted SSPA (Solid State Power Amplifier) whose main components are a 3-stage power amplifier and power supply. A bonded-fin heatsink is mounted beneath each main component, and the assembly is cooled by a side-by-side blower and axial fan arrangement that pulls cooling air into the enclosure and forces it through the SSPA and power supply heat sink fins before exhausting it out the rear vents.

Since the subsystem was still in the design stage, a mockup was required. An aluminum housing was machined to match the 'form factor' and thermal mass of the final SSPA housing, and the actual 100 Watt power supply was mounted inside the 3RU enclosure. To simulate the FET amplifiers, engineering used resistors, bolted inside the machined housing and driven by the power supply, that delivered equivalent power dissipation (200 Watts).

Thermocouples were mounted in 3 locations: at the lowest powered FET amplifier (SSPA rear), at the highest powered FET (SSPA front), and in the exhaust air stream exiting the power supply heat sink. After power-up, the subsystem reached steady-state temperature in 30 minutes. Temperature measurements were taken once every 60 seconds for 4 hours. The results are shown in the table below; the Coolit predictions matched test temperatures within 3%.

 

SSPA Rear

SSPA Front

Power Supply

Coolit prediction (deg. C)

38.5

49.4

26.9

Actual (deg. C)

37.9

48.8

26.3

 

With access to highly accurate predictions, Miteq expects to eliminate a minimum of three design/test iterations per subsystem design and reap significant savings in development time and expense.

Figure 1. Schematic of the experiment.
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Figure 2. Experiment results showing temperature and airflow.
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