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This article was posted on 03/01/2011
Power supplies evolve to meet military needs
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Power supplies are a critical component impacting size, cooling and mobility for computing density in military systems
BY GORDON SMITH and MIKE SOUTHWORTH
Parvus
www.parvus.com (http://www.parvus.com)
Military electronics are continually pushing the boundaries of electrical design as these specialized components must
prove reliable amidst harsh conditions. To ensure electronics deployed onboard a military platform, albeit airborne,
ground or shipboard meet the military’s rigorous demands, formal military standards are implemented to ensure the
safety and effectiveness of equipment deployed in tactical operations.
One electrical component that must particularly keep pace with these growing demands is the subsystem power
supply. Tasked with ensuring consistent reliable power to military computing devices – including mission computers
and communications gear the power supply must meet MIL standard and application requirements, not to mention
size, weight, and power (SWaP) constraints and cost considerations.
Military Standards 1275 and 704 (MIL-STD-1275 / MIL-STD-704) are of particular interest to vehicle electronics
(vetronics) and aircraft engineers, respectively, when onboard subsystems are powered by a 24-V battery or 28-V
generator. This article highlights aspects from these and related MIL standards with particular emphasis on the 1275
standard.
Meeting military standards
MIL-STD-1275 covers the characteristics of 28-Vdc electrical systems in military vehicles and provides detailed
requirements for military ground platform electrical systems, including electromagnetic compatibility (EMC), starting
mode, normal operating mode, generator-only mode operation, and their associated spikes, surges and operating
limits.
It goes without saying that to ensure reliable system operation, a MIL-1275 compliant power supply needs to withstand
voltage transients as specified within the standard without causing component damage to itself or the electronics
powered by it. In addition to withstanding the spikes (see Fig. 1) and surges (see Fig. 2) without damage, it also
should continue to supply clean, well regulated power to its load(s) while“riding through” these spikes and surges.
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Power supplies evolve to meet military needs - Electronic Products#.U6T7pYXjLGE
Fig. 1: MIL-STD-1275 Envelope of spikes in normal operating mode of 28-Vdc systems.
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Fig. 2: MIL-STD-1275 Envelope of surges in generator-only mode for 28-Vdc systems.
In ground vehicles, the chance of input voltage being reversed is significant, a condition often caused by jumper
cables being reversed when jumping a dead battery. As a result, protection against reversed polarity on the power
leads needs to be incorporated. Using a diode for polarity protection is common, but wasteful in the amount of power
lost in the forward biased diode. If a shunt-diode is incorporated, there may be a danger of blowing a fuse or circuit
breaker. Some MIL supplies therefore use a FET to act as a series diode that is turned on in the presence of voltage.
This can result in less heat generated onboard.
Note: the limits stated in MIL-STD-1275 are typically determined while operating at an ambient air temperature range
of –32° to 52°C – or whatever the appropriate authority specifies. Many military-qualified supplies operate well beyond
that at–40° to 85°C ambient air temperature during full or partial/no load conditions.
Electromagnetic compatibility
In a military environment, computers must coexist with all other equipment, including powerful radio, radar, and
microwave transmitters as well as highly sensitive receivers. For that reason, MIL-STD-1275 embeds requirements
from the military EMI/EMC standard MIL-STD-461, which governs radiated and conducted emissions and
susceptibility. In this way, electrical systems are limited to the amount of noise they can generate and what types and
levels of interference they tolerate.
There is a very narrow range for permissible conducted EMI for a MIL-STD-461-approved device (see Fig. 3). There is
also a limit specified on the amount of line-to-ground capacitance to limit the currents through a vehicle’s ground
system — a major cause of potential interference.
MIL-STD-461 susceptibility requirements dictate the amount of conducted RF energy the device must be able to
withstand and still operate properly. For example, from 30 Hz to 5 kHz, the device must be able to withstand 126 dBµV
(about 2 Vrms) added on top of the dc supply voltage.
Radiated emissions levels (from 30 Hz to 40 GHz) are also specified in MIL-STD-461. Given that shields covering
signal lines are allowed, but none are generally allowed over the power leads, the largest source of radiated emissions
can be the power leads.
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Power supplies evolve to meet military needs - Electronic Products#.U6T7pYXjLGE
Page 3 of 4
Fig. 3: MIL-STD-461 CE102 conducted emissions limit for all applications.
Form factors and modularity
While many power supplies are customized for application-specific requirements, commercial-off-the-shelf (COTS)
dc/dc converters come in all shapes and sizes, including those pre-qualified to meet military requirements. Mechanical
form factors range from high density enclosed brick modules to various custom or standards-based board formats for
stackable or backplane-based subsystems. An increasingly popular choice for space-constrained embedded vehicle
applications is the modular PC/104 format.
Where size and weight are key considerations, PC/104 has become the COTS form factor of choice for an increasing
number of military uses due to its compactness (3.550 x 3.775-in.) and inherent ruggedness against vibration and
shock. Automotive-grade, stackable PC/104 dc/dc converters provide system architects with a wide range of choices
for input voltage, output voltage and power level, while remaining sensitive to cost. Generally, these supplies have not
conformed to MIL-STD-704 or MIL-STD-1275, however, so separate pre-filters or in-line power conditioners have
been used in tandem to ensure voltage input protections and appropriate EMI levels.
SWaP-constrained vehicle applications will generally prefer a more integrated solution that keeps physical size to a
minimum and eliminates the separate filter circuitry. The new ACS-5180 power supply (see Fig. 4) from Parvus is a
good example of a stand-alone PC/104-Plus board that does just that. It includes all the necessary input filtering and
conditioning circuitry to meet both military ground and aircraft power conditions (MIL-STD-1275, MIL-STD-704), as
well as MIL-STD-461 EMI and MIL-STD-810 environmental requirements, making it ready to drop into small formfactor unmanned and manned vehicle applications.
Fig. 4: Parvus ACS-5180 PC/104-Plus modular MIL-STD-1275 / MIL-STD-704 power supply.
Electrical isolation for military apps
Electrical (galvanic) isolation for power supplies is another important system design consideration. Galvanic isolation,
where the return path for power is not necessarily the same as for signals, allows system designers to avoid ground
loops. Ground loops in systems can be difficult to find and have been the cause of many sleepless nights
troubleshooting the cause and cure of spurious electrical noises.
To mitigate this concern in the ACS-5180, Parvus designed in 1,500-V galvanic isolation. This is done by transferring
the energy across the isolation boundary using the onboard transformer. This allows the systems designer to flexibly
select where the reference to ground is placed. Although not a common practice with commercial power supplies,
galvanic isolation is a necessary step for ensuring advanced electrical performance for military devices.
Keeping pace with military demands
With computing density a primary consideration, power supplies are a critical component as they directly impact the
size, cooling and mobility of a system. By implementing creative engineering solutions, today’s highly efficient,
compact power supplies are well equipped to help the military accomplish its objectives. ■
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