Avoid conversion desertion

Are COTS converters viable in military dc-dc applications? Read on to look at the options

Defence equipment manufacturers continue to be driven toward commercial off-the-shelf (COTS) power supplies to meet time-to-market challenges, lower costs, better availability, and elimination of the expenses of custom power (see Figure).

But COTS dc-dc converters come with risks, too. Obsolescence, changes in specifications, and lack of supporting data for defence applications are just a few. Cost-effective selection demands an understanding of the key specifications for defence and aerospace applications.

The following guidelines are no substitute for a detailed study of MIL SPECs, but they provide a "quick start guide" when selecting COTS-based dc-dc converters for military applications.

1. Input voltage: The most common systems in military applications are 28V and 270V. These are defined in MIL-STD 1275A/B for Vehicles and MIL-STD 704A-F for Aerospace. A converter with a nominal input of 28V may be required to operate from 15V to 40V input. Dc-dc converters with an 18V to 36V input range are commonplace; those with wider input ranges less so.

2. Input immunity: Spike, surge, and ripple are demanding specifications for military systems. Standard COTS dc-dc converters don't meet these specs, but external filtering/conditioning modules can bring them up to standard. Look out for converters that come with complementary filter units—the overall performance of the power system will depend on both units being properly matched.

3. Output voltages and power ratings: Most standard output voltages—3.3V, 5V, 12V, 15V, and 28V—are available in COTS dc-dc converters, and some manufacturers may make changes if non-standard voltages are required. However, moving away from a standard product tends to increase the cost, lead-time, and risk of obsolescence. Sometimes, the problem is solved by using a standard part with a voltage trim function. However, care must be taken not to get too close to the overvoltage trip point when raising voltage, and that the resulting reduced power output is acceptable if lowering the voltage.

4. EMI (electromagnetic interference): The most common standard for military systems is MIL-STD 461E. The choice of dc-dc converter has a major impact on overall system performance in this respect, so selecting a converter that has filters to MIL-STD 461E reduces the risk of the overall system failing to meet EMI requirements.

5. Operating temperatures: Some data sheets show ambient temperatures for COTS converters, while others specify case temperatures or baseplate temperatures. Most military applications require converters to work within sealed enclosures with conduction being the primary cooling method. In these instances, the baseplate temperature is the most useful figure to extract from the converter vendor in order to calculate heatsink requirements. Operation down to -55°C is a common requirement in military applications. Industrial dc-dc converters are rarely tested below -40°C, and there can be a price premium of 40% to gain the extra 15° tolerance.

6. Cooling: Conventional heatsinks aren't needed if the system enclosure wall provides adequate thermal conductivity. The converter is mounted onto the enclosure using a thermal pad or paste. Some designs can use convection cooling. If so, the ambient and case temperature figures for the converter can help determine the most suitable cooling arrangements.

7. Hold-up and brown-out ride-through: Military systems are operated with input supplies that vary in quality. Thus, the ability to handle input voltage dips or short-term supply discontinuity is often essential. Most converters will have an undervoltage lockout function at their input to prevent damage by excessive input current. The converter is turned off during a voltage dip. A hold-up capacitor will delay the voltage dropping below the lockout point—the lower the undervoltage lockout point, the smaller the capacitor required.

8. Size: In most designs, it's desirable to keep the dc-dc converter footprint as small as possible to conserve pc-board space. An important consideration in selecting a converter is that some require external components to meet the datasheet specification, so this needs to be factored into the design from both a size and cost perspective.

9. Protection and control/monitoring features: Overvoltage and over-temperature protection can be implemented within the converter or externally. External protection is more easily customised, and some converters offer internal protection that can be disabled in order to use external circuits. The most common control and monitoring features are frequency synchronisation, an inhibit signal for switching the converter on and off remotely, voltage trimming, and a remote sense function that monitors the voltage at the point of load. The latter enables the converter to automatically adjust its output voltage with varying load levels.

10. Type of load. The type of load, whether resistive, capacitive, or inductive, will affect the performance of any dc-dc converter. Needless to say, this is a big consideration when determining average power levels and transient responses.

Taking these factors into account, the choice of dc-dc converter for military applications falls into 3 categories:

Category 1: Industrial COTS
These low-cost devices, which have few added features, are either baseplate-cooled or air-cooled. They will typically be part of a range of dc-dc modules that includes converters for telecom (48V nominal input), industrial (12V and 24V), and transportation (100V nominal input) applications. It's unlikely much supporting data will be available to provide compliance to MIL-STD for EMI or environmental performance—although there may be anecdotal evidence. These converters have high obsolescence risk because commercial products usually have shorter life cycles than military or aerospace products. It's also likely that their design or the manufacturing process will change over time.

Category 2: Sealed MIL-STD 883 products
MIL-STD 883 products, which run up to 10 times the cost of industrial products, are designed specifically for the military market. They're usually fully featured with respect to control and signalling, operate over a wide temperature range, typically -55°C to +125°C, and may be in a hermetically sealed case or use hermetically sealed components. EMI filtering and surge protection are included or available as options. There's low obsolescence risk if the vendor is well-established. Also, the converters are usually available on standard military drawings (SMDs), which means that customers are notified of design changes.

Category 3: Military COTS
This is a relatively new group of dc-dc converters designed specifically for defence and avionics applications, but also designed as standard products for off-the-shelf availability. They can be half of the cost of products in Category 2, but will be more expensive than industrial COTS devices. Military COTS converters meet defence specifications for EMI and environmental tolerance, have low obsolescence risk, and include all of the features found in industrial converters. They operate from -55°C to 100°C and are normally sealed, although not hermetically. This is generally acceptable if the final system enclosure is sealed. The vendor should have EMI filters and test data.

Category 3 military COTS is a relatively new group of dc-dc converters designed specifically for defence and avionics applications, but also designed as standard products for off-the-shelf availability.

Martin Brabham is military and aerospace industry director at XP Power.

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