Power Electronics

Power Supply Elevates Aviation-Lighting Capabilities

Aiming to provide avionics-system designers with a low-cost, military-grade power source for universal aircraft lighting applications, Executive Engineering is redesigning its universal aviation-lighting power supply. To achieve this military rating, the supply will fully adhere to RTCA/DO-160E, the most current international standard for testing airborne electronic equipment. Slated for release in the third quarter of 2008, the redesigned supply will provide power designers with flexible options to meet modern aviation lighting challenges, especially the difficult requirements facing military avionics power designers.

The redesigned supply, the TPS28D, will be based on the previous version C of the unit, which is nearly compliant with RTCA/DO-160E (except for category Z of section 16, which requires a military supply to withstand excessive input-power noise). According to David Weber, founder and chief design engineer at Executive, version D will incorporate design changes to achieve full compliance, as well as significant improvements in the performance and efficiency of the power supply. These improvements will reduce the assembly time by 25%, while increasing the output power by approximately 100% (280 W for the D version), as compared to the C version of the supply.

One modification involves replacing an n-channel MOSFET driven by a floating gate-drive transformerwith a p-channel power MOSFET that did not exist during the design of version C (which has been in service some 20 years and is deployed on several types of fixed-wing and rotary-wing aircraft). Another modification that will enable full compliance relates to the design of the main switching transformer, which is a critical component in the buck-converter topology used in the power supply. These transformers are custom-built at Executive, and the improved versions will leverage improved magnet wire and core materials that were also not available for version C.

Once these modifications have been implemented, compliance with RTCA/DO-160E can be easily achieved, because the power supply was specially designed for universal aviation lighting applications from the outset. According to Weber, this focus—even apart from its future regulatory compliance—makes the TPS28 unique. For example, the soft-starting output extends the life of all incandescent lamp filaments, from grain-of-wheat to high-power-quartz types, a feature that enables the unit to quickly pay for itself when driving expensive legacy lamps.

The power supply is also well suited for driving LEDs (requiring the use of a series resistor) in aviation applications. In fact, one interesting application for the TPS28 has been the driving of high-brightness white LEDs used to illuminate patients in medical evacuation helicopters. According to Weber, white LEDs provide the best light for monitoring patient color during transport, and are superior to both incandescent and fluorescent lamps in that application. However, Weber states the TPS28 was developed for universal aviation lighting applications long before RTCA/DO-160 was formally issued, making it impossible to achieve full compliance using the C version.

Therefore, to both meet and exceed this challenge, version D of the TPS28 will have the previously mentioned design changes, while retaining the unique features of previous versions. For example, the D version will retain the half-bright dimming feature that allows the voltage of the power supply to be reduced by 50% with a single switch. This capability improves aircraft safety by allowing pilots to quickly adjust cockpit illumination when visibility changes rapidly, as it can during night flying under a full moon in cloudy conditions. Furthermore, unlike version C, the D version’s output can drop to 50 mV (due to its rail-to-rail switching stage), which is critical for darkening instrument panels when pilots wear night vision goggles.

Another feature version D will have is the slow-changing output that maintains good output-voltage stability, despite the potentially harsh RF environment of aircraft equipment bays. Output voltage stability is maintained for both fixed-output (5 Vdc, 14 Vdc, and 28 Vdc) and voltage-tracking-output (0 Vdc to 5 Vdc, 0 Vdc to 14 Vdc, and 0 Vdc to 28 Vdc) configurations of the supply. This feature can improve cockpit safety by eliminating distracting flicker in cockpit indicators.

The dc output of the TPS28 can track dc input voltages across ranges spanning 5 V, 14 V and 28 V. The output can also track the rms voltage of an ac power signal from 0 Vac to 90 Vac at frequencies from 60 Hz to 1000 Hz. In some cases, an ac voltage may be the only control signal available where the supply is installed.

Output-voltage configurations of the TPS28 are programmed through jumper connections in the mating 25-pin D-connector jack (another feature unique to the industry, according to Weber), and the supply operates from any input voltage between 28 Vdc to 33 Vdc. It also withstands input surges of 50 V, a requirement for any power supply that is fully compliant with RTCA/DO-160E.

The power supply’s chassis serves as both electrical ground and heatsink, relying on good thermal and electrical connections with a metal structural component within the airframe for both functions. However, the wide ranges of input, output and tracking voltages supported by the supply provide designers with great flexibility in placing the unit, especially since full access to airframe wiring is often limited in aerospace applications.

The benefits of all these capabilities to avionics engineers will vary depending on the application. However, weight reduction is of universal importance in aircraft design, in contrast to advanced features or even regulatory compliance. Fortunately, the TPS28D will address this challenge, too, weighing in at less than one pound.

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