By William G. Standen
Power supplies and DC-DC converters are ubiquitous in all electronics systems, yet until recently they were among the last considerations in systems design. As a result, systems designers could almost always count on needing custom or application-specific power solutions. Aside from the cost to design the power supply, the complexity resulting from an application-specific device required extensive qualification testing. To complicate the situation, government-sponsored efforts to standardize power supplies and DC-DC converters, such as the SPSS or Sharps programs, never took hold.
Still, with a bit of up-front planning, today`s military systems are not necessarily prisoners to custom-designed power systems. Using distributed power architectures can help military systems designers eliminate the need for custom solutions. They can do so without sacrificing reliability or flexibility, and reduce overall system costs at the same time.
Distributed power: the first step
Traditional power systems use a "centralized" power architecture where a central power supply converts an AC or DC input to all the output voltages necessary for the application.
Yet "distributed" power uses an AC front-end that converts the raw AC current to a non-regulated DC. This current then feeds to DC-DC converter modules to provide precise, regulated output voltages. If a systems designer does not exceed the total power, he can add or delete modules as the application demands without changing the entire power supply. This reduces the cost of ownership over the long term.
In addition, designers can greatly enhance reliability by operating two converters in redundant mode. With this approach, one module failure will not cause a system failure. Engineers first widely applied this type of power architecture in the telecommunications industry, where a missing dial tone is catastrophic.
From a cost perspective a distributed system can be a strong alternative for systems requiring multiple output channels. Unless manufacturers build custom, centralized power systems in significant production volumes, they may not be able to recoup the cost of non-recurring engineering. In the commercial sector custom designs still dominate with typical volumes ranging from 10,000 to 20,000 units per year. In military and aerospace markets similar production volumes are not readily available. This makes a distributed approach, using off-the-shelf modules, very attractive.
The converter: the critical set
The first consideration in selecting the DC-DC converters is the available input BUS voltage. Most military systems must conform to either MIL-STD-704 or MIL-STD-1275 and provide 28 volts DC or 270 volts DC input to the converters. This is significant since the majority of converters on the market are intended for telecommunications applications where the primary BUS voltage is 48 volts DC. Other considerations are operating temperature range, size, converter reliability, and price.
Until recently, the field of available off-the-shelf DC-DC converters resided in two distinct camps. On one side commercial telecommunications manufacturers offer fairly compact products optimized to operate in temperatures from zero to 70 degrees Celsius at extremely low unit prices.
On the other side, hybrid microelectronics manufacturers offer very dense products, capable of -55 to 100 C, at very high unit prices. While the performance of these microelectronics products is clearly superior, the price difference is staggering. The obvious temptation is to take one of these commercial products and "make it fit" a military environment.
In some cases this means operating it beyond its specified limits. At Martek Power Abbott (formerly Abbott Electronics) in Los Angeles, we confronted this situation several years ago and began developing products that fill the middle range between these two extremes.
We knew we needed to build power electronics able to withstand tough military operating environments, yet that are available at affordable prices. We design our converters (NB and CB series modules) to operate throughout the military temperature range of -55 to 125 C — yet we do so using commercial components and manufacturing processes wherever possible. We attempt to cut labor and costly processes associated with microelectronic devices, for example, by using surface-mount construction techniques. This approach also enables us to achieve very high power densities.
To address temperature performance and mechanical integrity while using affordable off-the-shelf parts, we use a metal enclosure and fully encapsulate it with a thermally conductive compound. This construction technique enables Martek power devices to withstand severe thermal and mechanical shock levels. Unlike microelectronic devices, hermetic sealing is not required thus eliminating expensive enclosures and the possibility of contamination due to a cracked glass-to-metal lead juncture.
Our experience in mil-spec design led us to believe that conservative design guidelines are still the best insurance to long-term reliability, regardless or whether the component parts are plastic or metal. Martek`s design philosophy emphasizes reducing component stress. Component derating guidelines as defined in the NAVMAT power supply document (1981 Navy reference to power supply design) still provide a valuable road map to designing in reliability.
Input and output capacitors must operate at 50 percent to 70 percent of maximum rating while semiconductor junction temperatures are limited to a 40 C rise. A common but potentially dangerous practice involves using a commercial, telecom type converter at 50 percent of its rated load. The theory being that a 100-watt converter operating at 50 watts will guarantee reliability. This is not always the case. While the power stage of the converter will be at a 50 percent rating there is no way of telling what the voltage stresses are on the module without an in-depth circuit analysis.
Another area of concern is operation at temperature extremes. All DC-DC converters are specified over a given temperature range. However, the stated performance may or may not apply depending on the circumstances. A given converter may specify -40 C as the low-end temperature, yet this may apply only to an ambient operating state (modules energized) and not to a "snap" start with the unit stabilized at this lower temperature. Specifying performance at -40 C as an ambient (or even baseplate) operating condition is not meaningful. A DC-DC converter that is 80 percent efficient operating at 100 watts output power would dissipate 20 watts via the power stage. At -40 C ambient temperature and 20 watts dissipation the internal circuitry of the converter could be above 0 C. At Martek Power our engineers specifically design for the temperature extremes and qualify the converters at the design stage as well as production lot testing
In this day and age, any power company that seeks to supply devices to the military must consider the subject of product screening. All DC-DC converter microelectronic manufacturers offer some type of product screening. In all cases these tests are based on MIL-STD-883. This list of guidelines is tailored to uncover defects in the manufacturing process of these types of devices. Yet while it enhances reliability, 883 testing is as expensive as the devices it is intended to screen.
Commercial telecommunications manufacturers generally do not offer a standard production screening; some do not even perform design verification testing. Aside from acceptance testing the modules may not have received any significant run time. However, these manufacturers rightly point out that the cost of a replacement is far less than the cost of screening.
Martek experts develop the product screening profile along with electrical and mechanical design, which becomes part of the final design review. We use our expertise as the end item manufacturer to screen out marginal devices. Some end users have taken to screening commercial devices themselves. If this were the case then we would recommend a combination of thermal cycling and high temperature run time. Before selecting a DC-DC converter it is also advisable to qualify a sample device through a series of thermal shocks (five to ten shocks, non-operational from -55 C to 125 C) and parametric testing at the high and low temperature extremes.
Missing the BUS?
Of the two most common BUS voltages available for military systems — 28 volts DC and 270 volts DC — the simpler to derive is 270 volts DC that comes off of a rectified 220 volts AC. In some case designers must step this voltage down to provide a 28 volts DC BUS, which then feeds to the various system cards. Martek Power offers a series of modules (SM series) that convert 270 volts DC to 28 volts DC up to 300 watts total power per module.
These modules are fully isolated and can be paralleled up to 1 kilowatt. In addition, electromagnetic interference filtering is an option (companion modules) to enhance overall system performance. Off-the-shelf AC front ends are also available (HM series), that accept a wide variety of AC inputs; 155/220 volts AC, 47 to 440 Hz, single and three phase. These devices offer a wide operating temperature range and provide a 270 volts DC BUS voltage up to 1.2 kilowatt. EMI filtering should be considered on the AC front end as well as on the converters themselves.
William G. Standen is vice president of marketing and sales at Martek Power Abbott in Los Angeles. For more information, contact him by phone at 310-202-8820, ext. 273, by fax at 310-836-4926, by e-mail at wstanden@ abbottelectronics.com, or on the World Wide Web at http://www.martekpower.com/.
