Designing-out risk in an air-borne power supply

Designing-out risk in an airborne power supplyA major manufacturer of civil and military aircraft presented Acal BFi’s design team the challenge of creating a power supply for an airborne communication system, which need to meet tight mechanical constraints, achieve very low electrical noise, keep heat dissipation to a minimum, and provide a constant output voltage despite an intermittent input supply.

The design process not only covered the usual requirements of concept review for the power supply, component selection, PCB layout and prototyping, but also featured a degree of risk management. There was also a need to develop some innovative circuitry to meet the specification.

Setting tight design targets

The manufacturer had already selected the dual computer boards which were to be used in the platform-based computer for an air-based communication system and had also developed the communications links. All that remained was to design a power supply which would fulfil all of the critical design objectives.

The electrical parameters were challenging – with a target of 20dB below the level specified by Mil-Std 461 for conducted noise, the efficiency of the power supply had to exceed 80% to minimise internal temperature rise, and the supply had to provide continuous power whilst the input from the aircraft was interrupted for periods of up to 50mS.

Managing risk at every stage

Acal BFi started the design and risk management process by helping the manufacturer to specify the electrical and mechanical requirements which would enable the power supply to operate at or better than the defined levels for EMC, reliability and thermal performance.

The design which Acal BFi suggested was a closed-box power supply, based on standard DC-DC converters and filters, with additional circuitry to limit the current to the internal circuitry and provide support during the breaks in the input power. The sealed enclosure featured one compartment for the power supply and a second compartment for the filtering circuitry, to allow the design to meet the targets for EMC integrity. Careful consideration was given to the layout and interconnection of the components on the PCB to minimise the footprint of the power supply within the enclosure.

The power-supply enclosure had to achieve a height profile of just 42.00 mm (including casing) and a tight tolerance of +/- 0.01mm for aligning the input connector to the customers own structure. Heat would be dissipated via the base of the power supply, which was fitted onto the internal wall of the lid of the equipment casing.

To provide a continuous output during the breaks in input power, Acal BFi developed a custom dual-circuit system in which critical loads were supported during the loss of power, whilst non-critical loads were allowed to fail. Internal capacitors at the front end, and additional filtering, were also required to ensure a constant output power during disturbances in the input supply. To minimise the risk of fire, the modules also had accurately set independent current-limiting circuitry feeding the internal cabling.

The risk management process continued with pre-compliance EMC measurement using Acal BFi’s in-house test chamber. Conducted EMC was measured at the beginning of the design process as well during the development to ensure compliance was maintained as changes in the design inevitably occurred. In addition to in-house testing, Acal BFi also provided expertise and support during the formal EMC qualification process to ensure that time-to-market was not compromised by the power supply failing to meet the EMC regulatory requirements.

Early in the design process, Acal BFi also prepared component-level reliability predictions using the MIL Handbook 271F predictive techniques. This helped to identify the components which would require critical selection and derating to meet the required reliability target. When the prototypes had been tested by the manufacturer, Acal BFi also carried out a post shock and vibration testing evaluation by analysing the components for stress-related wear.

Conclusion – fast, safe and constant

The combination of specialist engineering skills, an innovative circuit design and in-house test facilities allowed Acal BFi to develop a custom power-supply which addressed every one of the manufacturer’s challenging design criteria. Acal BFi’s ability to risk-manage the design at every stage of the process was critical. It allowed the design time-scale to be minimised, and ensured that the finished power supply would be a safe, reliable and compliant product.

The whole process, from the initial approach by the manufacturer, to the finished and approved design, took nine months, with the first prototypes being supplied just ten weeks after the initial meeting.

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