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Designers of military electronic systems face challenges of ever more rapid product development, increased time-to-market pressures and shrinking budgets. In the light of these factors, the adaptation of COTS parts is attractive, providing overall system performance and that reliability is not compromised and that the resulting power supplies meet the requirements of the appropriate MIL-STD, DEF-STAN or relevant industry standard for the application. This article discusses the special demands made upon military power supplies.

If power fails, the system fails, so power supplies are critical system components. Furthermore, the power supply is often an essential element in the protection of sensitive electronics from excessive input voltage variation. It also provides input filtering for the system. With the exception of interconnect components and fans, power supplies are potentially the most unreliable parts of the system due to the heat generated by even the most efficient units. Good power supply design is, therefore, essential for reliable system design.

Some military systems designers opt to build their own power supplies using discrete components. It can appear to be a relatively cheap solution at the outset, but issues of unpredictable performance, time for experimentation and potential delays in product qualification can add to the real costs of the product. Furthermore, companies often employ many digital design engineers but few, if any, analog designers, so this approach is not always available due to limited in-house skills.

Buying in a custom designed power supply is another option. This can deliver an optimum technical solution, but with the risks of high up-front engineering charges and a lengthy design cycle. The latter often runs to between four and six months and the small volumes required by many military applications can make unit costs unacceptably high.

In many applications, the ideal solution is found in a combination of standard DC-DC power modules, or ‘bricks,’ to which customized input and output circuits and customized packaging are applied. Key to the successful implementation of such a solution is a detailed understanding of military specifications and the special conditions under which the power supplies will be required to operate.

The special technical requirements for military power supplies fall into three main categories: environmental, input and output voltage and EMC.

1. Environmental considerations: Extremes of temperature are encountered in military applications and it's not just high temperatures that cause concern. Some applications require equipment that can operate at -40 °C or even -55 °C and while a first glance at product datasheets might indicate that this is possible, it's an entirely different matter when the detailed performance and EMC specifications at these temperatures are considered. Compensation for degraded performance at low temperatures needs to be included in the overall power solution.

High temperatures cause similar problems but with implications for reliability too. As a rule of thumb, the mean time between failures (MTBF) halves for every 10 °C increase in operating temperature. In some applications designers must avoid the use of forced-air cooling. Fans are electromechanical components that can compromise system reliability. They require monitoring circuits that add to system complexity and cost, and their associated filters are prone to clogging in many environments, reducing cooling effectiveness and increasing maintenance work. Power modules with base plates are most effective in dissipating excess heat and they need to be carefully designed into packaging that effectively conducts and convects heat away from sensitive electronics. Using appropriate power modules, base plate operating temperatures of between +85 °C and +125 °C can be achieved.

Shock and vibration resistance are other important environmental considerations. Normal potting compounds used to encapsulate DC-DC power modules are prone to crystallisation at very low temperatures leading to damage to internal components. Specially developed soft-potting compounds and spin-fill techniques can be used to overcome these problems. Finally, dust and moisture ingress needs to be avoided.

2. Input and output voltages: The input voltages needed for military applications rarely coincide with those used in commercial ones. Military specifications, as shown in Figure 1, are stringent for other input characteristics too, including low and high line conditions and the capability of power supplies to handle voltage spikes, surges and excessive input ripple. These special input requirements demand flexibility from the DC-DC power module manufacturer and mean that input conditioning circuits are invariably needed.

Output voltages are often non-standard when compared with commercial products. A power module manufacturer that offers a wide product choice may be able to provide a module with an output that is trimmable to the required voltage but it's important to remember that overvoltage and undervoltage trips will probably not move to accommodate the change in nominal output voltage.

A few manufacturers will adapt DC-DC power modules for user-defined output voltages, with appropriate overvoltage and undervoltage trip points. Trip points can be set or deleted depending upon design requirements.

3. EMC compliance: Military EMC specifications such as MIL-STD 461C/D and DEF-STAN 59-41 have important differences over commercial specifications such as Europe's EN55022. The main one is that EN55022 specifies limits over a 150 kHz to 30 MHz frequency range while the military standards typically specify limits from 10 Hz to 1 GHz, adding a range of conducted and radiated susceptibility and emissions requirements.

There are also significant differences in measurement methods. For example, different line impedance stabilization network (LISN) measurements are used in EN55022, MIL-STD-461E and DEF STAN 59-41 specifications. There are different test limits and measurements are made in dBuV for EN and MIL-STD and dBuA for the DEF-STAN standard.

Once again input and output conditioning are needed to achieve compliance. EMC filtering can be achieved using discrete components or filter modules. Active filters remove spikes and filter both conducted emissions and conducted susceptibility such as transients or input ripple appearing at the output. Radiated emissions are dealt with by complete screening of the final power supply.

A few companies now produce reliable and cost-effective, customized power supplies for military applications based upon COTS DC-DC power modules. The block diagram of a typical design is shown in Figure 2. Successful designs demand an-depth understanding of the special requirements of military systems and access to the widest possible range of reliable DC-DC power modules.

Customization involves providing the analog design for the input and output circuits and 3-D mechanical modeling of power supplies based on COTS units. Pre-compliance testing then assists users in the final product qualification process.

Some examples of custom DC-DC converter designs using standard DC-DC converters are shown in Figure 3. Although much lower cost than a custom design, they are still more expensive and have a longer lead time than a standard off-the-shelf COTS solution.

Over the development of several customized solutions, power specialists XP Power noted that there were often common electrical requirements but widely varying mechanical demands. Power levels, output types and control requirements were reasonably consistent across several designs but some units needed to be enclosed, some required base plates for conduction cooling via a heat sink, and some needed to be cooled in free air. This led the company to develop a factory-configurable COTS power supply for military customers. The new unit is designated the MCC.

The MCC has two printed circuit boards, separating the input and output parts of the design. This allows flexibility in packaging and form factor. It is a DC-DC converter based on standard DC-DC modules that operates from 18 VDC to 36 VDC input to deliver from one to four factory-configurable filtered outputs with a total power rating of 400 W.

The circuit in Figure 4 provides input transient suppression, input EMI filtering and active surge and spike protection to MIL STD 1275 A/B. It also enables the power supply to meet the EN55022B specification. An inhibit function enables remote on-off switching of the DC-DC converter, there is also reverse voltage protection and output filtering. Figure 5 compares the specification of the complete MCC power supply with that of the DC-DC modules alone. All of the performance characteristics needed to conform to the various military specifications are achieved using the input and output conditioning circuits.

There are four standard output boards. Each is populated with standard DC-DC power modules and each module is isolated and regulated allowing them to be connected in parallel or series to achieve the required voltage or current rating. Modules offer 2 VDC to 200 VDC outputs and come in 100 W, 200 W or 400 W nominal power ratings. A global DC OK signal is available as an option and the power supplies are available with application-specific signals.

Because many military applications require an external feed for peripherals such as cameras, motors, fans or other equipment, the MCC unit has as option for an auxiliary output filter on the input board to take the rating up to 600 W. The 600 W is split into a 400 W feed for the DC-DC PCB and a 200 W external feed that enables customers to power peripherals that do not have an MIL-STD 1275A/B compliant filter. This external feed is clamped at 36 V.

The DC-DC converters used have been proven in numerous applications with harsh environmental specifications such as MIL-STD 810E. They deliver the required output current and voltages from a nominal 28 VDC input. The modules use base plates and advanced power semiconductor packaging for effective heat dissipation and feature a power density of around 60 W/in³, making them compact. The modules use zero-current switching together with control and packaging technologies that deliver a combination of high efficiency and high power density. They are encapsulated with a soft potting compound using a void-free spin fill process.

The complete MCC power supply is built onto a base plate for conduction cooling. All of the main heat-dissipating components are fixed directly to this base plate to maximize thermal conductivity. This removes the requirement for fans that could compromise system MTBF and add ongoing maintenance costs. The 3-D CAD model produced during development is shown in Figure 6. The base plate is fitted with a cover that provides protection to IP20 standards. The MCC power supply and one of its first applications in a military ATR enclosure are shown in Figure 7. The MCC will operate over an ambient temperature range of -40 °C to +70 °C with a permissible maximum base plate temperature of up to +90 °C. The necessary filtering introduces some losses but typical efficiency remains high for a power supply of this type at approximately 75% at full load. Line and load regulation are better than 1%. The MCC is also easily customisable to incorporate application-specific signals and paralleling multiple MCC units.

The MCC is a standard product that is suited to a wide range of applications. After evaluating the power supplies in-house XP Power obtained formal product qualification through an external test house.

Although, originally designed for military applications, the rugged nature of the construction of the MCC DC-DC converter makes it suited to demanding industrial applications. This uses a passive input filter design and less stringent electrical and environmental screening to lower the cost of the product. The MCC was launched this month and has secured several design-ins including a military 19-inch rack system and a specialist piece of medical test equipment that will be flown on the space shuttle.

Martin Brabham is the industry director for Avionics and Defence at XP Power in Berkshire, United Kingdom.