Historically, military equipment of any type has been characterised by long development times including extensive procedures to define specifications, as well as exacting performance and environmental qualifications. But modern armed forces need new and better equipment on a more regular basis than traditional design methodologies have allowed, also benefiting from the latest and best technologies to outperform enemy capabilities.

In response to these demands UR Group saw an opportunity to improve performance, reduce weight and form factor and cut development time with a configurable power supply platform using pre-approved DC-DC modules. These improvements can benefit land-based vehicles as well as aircraft, subject to meeting applicable environmental and electrical standards.

The design team began by developing a specification for use in land vehicles, with a view to extending its modular concept to fully air-specified units following successful completion. The resulting specification has a wide input voltage range, from 10V to 36V, to allow use in vehicles with either 12V or 24V electrical infrastructure.

There is little demand for multi-output PSUs among military equipment designers; power architectures tend to rely on a common distribution voltage with conversion to individual voltages close to the point of load. Mindful of this trend the power supply platform was designed to support single-output or triple-output configurations, delivering greater flexibility for designers to meet a much wider variety of system requirements. The Vicor power modules at the heart of this design are available with industry-standard output voltages from 2.5V to 48V including 28V, which is commonly used for distribution within military vehicles. The larger modules for single-output power supplies enable total power up to 400W. An auxiliary 5V/1A output is also available as an option.

The Vicor modules are pre-approved to the relevant standards, and are pre-tested and carry documentation of 24-hour stress-cycle approval. Basing the design around these modules has enabled the PSU to be certified quickly according to the military environmental specification MIL-STD810. The design also meets MIL-STD-1275 covering immunity to input transients. In addition, the units are able to meet MIL-STD704, and are thus suitable for use within a 28V aircraft power infrastructure. The next design, which will be optimised for aircraft applications, will also support operation from a 400V aircraft supply.

The design and integration carried out at Swindon has included design of the enclosure enabling the unit to pass environmental testing. Whereas a significant proportion of power supplies positioned for military applications are supplied as open-frame units or with a basic enclosure, expecting the system designer to take responsibility for final packaging within the subsystem, these units are built within a milled aluminium enclosure. The enclosure achieves high strength and sealing integrity, as well as high EMC performance by preventing emission or ingress of EMI. The modular architecture of this design has enabled an extremely low-profile enclosure measuring just 25mm thick. The case footprint of 135mm x 171mm allows unobtrusive installation inside almost any vehicle. Mounting holes are provided, enabling easy fixing to a bulkhead. An optional mounting plate is also available.

All connections to the PSU, including an input to enable an external hold-up capacitor to be connected, are made through the unit’s three sealed D-type connectors. This makes the power supply easier to install and commission, while also maintaining effective sealing of the enclosure.

Although military design is less focused on survival of extremely high shocks in direct battle situations, certain minimum toughness is nevertheless expected. Features to protect against Nuclear and Electromagnetic Pulse (NEMP) damage, for example, are built in. The unit is able to detect an impending NEMP and power down sensitive PSU circuits for a limited duration to prevent pulse damage from occurring internally. The NEMP protection circuit automatically recovers normal power supply operation after the pulse has passed.

In addition, a safety-discharge circuit is provided, which safely discharges the PSU’s large output capacitor when the case is opened – for example in the event that maintenance is required. Other built-in protection features include current limiting, short-circuit immunity up to 115% and overvoltage protection to 125% of the maximum input voltage.

This modular architecture, using pre-approved, pre-tested power modules, provides access to miniature, lightweight, high-output power supplies up to 400W, in sealed enclosures suitable for direct mounting in the end application, on a fast-turnaround basis. Units can be delivered within as little as six weeks, to help equipment designers and armed forces to respond quickly to rapidly evolving battlefield demands.

John Quinlan is engineering manager at UR Group

Printer friendly version

Email this article to a friend