Plastics reach cruising altitude with Aircraft Power Supplies

Power electronics are one of the last bastions of ceramic and metal-only packaging for high-reliability applications in aviation, but new manufacturing techniques and improved reliability of commercial-grade devices is inducing designers to take another look at plastics

Jul 1st, 1999
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Power electronics are one of the last bastions of ceramic and metal-only packaging for high-reliability applications in aviation, but new manufacturing techniques and improved reliability of commercial-grade devices is inducing designers to take another look at plastics

By John Haystead

The driving factors in the design of power supplies for commercial and military aircraft boil down to a short list of requirements — minimum size and weight, maximum reliability, and the lowest possible cost. In fact, more than any other factor, cost is inexorably driving aircraft system designers to use power components and supplies that are packaged and encapsulated in plastic, rather than in ceramic or steel, wherever possible.

Fortunately for aircraft electronics systems designers, the number of possibilities for plastics continues to grow as reliability and performance improves with each new generation of devices.

Size and weight

Engineers in the avionics industry continue to pack more and more power into smaller and smaller spaces, steadily pushing the power-density envelope of power converters. This trend places ever-increasing demands to pack electronic components into ever-tightening spaces. Power-supply technology has thus far been able to keep pace with shrinking avionics architectures. Yet at the same time, aircraft designers are rapidly switching their mechanical and analog systems, to "all-electric" configurations. This leads not only to a much greater number of power converters per airframe, but also adds to their cumulative size and weight contribution (see figure, page 26). The result: "the transition to all-electric aircraft often means you`re trying to stuff 10 pounds of stuff into a 5-pound sack," notes William Standen, director of sales and marketing at Martek Power Abbott in Los Angeles.

One approach to reducing power- system size and weight has been to move to higher voltage buses. Since high-voltage systems carry relatively low current and operate over thin and light wire, they represent a significant size and weight savings in aircraft design. The move to higher voltages also provides for more efficient, lighter power supplies in general, says Phil Thibodeau, director of advanced programs at Lambda Advanced Analog in Santa Clara, Calif. The higher the input voltage, the more efficient the conversion, he notes.

Military systems designers, in particular, favor high-voltage systems with a concurrent move away from traditional 400 Hz AC sources. In the past, military designers almost always used the 400 Hz AC bus to power all but flight and mission-critical systems (where battery-backup was essential), notes Andy Hilbert, senior director of product marketing at Vicor Corp. in Andover, Mass. Today, however, "this `rule` is more and more frequently being broken," Hilbert says.

One example of this trend involves STC Keltec in Fort Walton Beach, Fla. William Walker, the company`s director of business development and senior scientist, points to Keltec`s recent upgrade work on the AN/ALQ-99 jamming system aboard the U.S. Navy EA-6B Prowler electronic-warfare jet. This work included converting the ALQ-99`s AC power to 28 volts DC.

Designers have already implemented 270 volts DC power buses on the Lockheed Martin F-22 jet fighter, the Bell-Boeing V-22 tiltrotor aircraft, and the Boeing AH-64 attack helicopter. This power scheme also is part of the plan for the future Joint Strike Fighter (JSF). Though 28 volts DC is also still common in military aircraft, "applications are generally segmenting into distributed architectures with large high-power DC-DC `bricks` for system-wide 270 volts DC power, and lower-power 28 volts DC converters closer to the outboard systems where you really don`t want high-power sources too close," Martek`s Standen says.

As an example, Standen points to the Eurofighter`s ECR90 radar system, where designers first rectify AC power to 270 volts DC, then bring it to an intermediate conversion box upstream from the point of load where it downconverts and distributes as 24/28 volts DC board-level power. Once at the system, it further downconverts to 5 volt/3.3 volts DC for logic and 15/12 volts DC for linear circuits.

Some commercial aircraft designers are also moving toward 270 volts DC and even higher voltages, explains Simon Abel, vice president of sales and marketing at Interpoint Corp. in Redmond, Wash. He cites Europe`s Airbus Industries as an example, which, he says, is already moving to 380 volts DC. Expectations are generally mixed, however, as to the popularity of higher voltages for commercial aircraft. Commercial aircraft still commonly use 115 volts AC with follow-on AC/DC conversion, Lambda`s Thibodeau says, adding that he does not expect to see this change much in future. Says Thibodeau, "270 volts DC has issues relative to passenger and maintenance safety, and though a 28-volts-DC bus has developed for commercial systems, it provides a bulk source for low-power loads while higher-power systems have individual rectifiers built in."

Reliability/screening

The ever-widening distribution of power converters across entire aircraft platforms has had another direct influence on their requirements. With converters now located out in harsh operating environments on the wing or buried deep in the fuselage, there is concurrent demand for increasingly reliable and rugged devices.

Rigid product screening must be an integral part of the design of any new power-supply products, notes Martek`s Standen. For example, Martek engineers have incorporated thermal shutdown devices in their devices. This change enables the devices repeatedly to run up to their thermal maximum during the screening process. "We run them from 48 to 96 hours in this constant mode of climbing up in temperature and shutting themselves off, which allows us to easily weed out any marginal devices," Standen says.

Vicor experts also conduct post-production screening on their military product lines with full burn-in and electrical testing over temperature range. This process does not come cheaply, but in the end is worth it, points out Keith Nardone, Vicor military product manager. "Although we also have wider-input voltage ranges and derating guidelines on the components for our mil parts — which in some cases can add substantially to the cost of a pure-vanilla device — they`re still substantially cheaper than metal hybrids."

Interpoint engineers also are working on an in-house demonstrated-reliability program that Abel says will boost customer confidence. "Right now we do MTBF [mean time between failure] calculations, but we also want to provide our customers with demonstrated-reliability testing data," he says.

One word, plastics

With increasingly rigid environmental requirements for a broad range of power converters, it might seem that plastic packaging would become a less-attractive option for many applications. In fact, the opposite is true. There is no longer any question that plastics will play a big role in the future of aircraft power supplies.

Cost is leading the drive toward plastics. Although there are several new programs beginning, there is not a lot of money to pay for them, Martek`s Standen notes. "The military has always had a pretty good handle on power density, but cost is an increasingly difficult challenge, forcing people to be very creative in terms of using off- the-shelf hardware," he says. Today that creativity increasingly takes the form of plastic-encased devices. Says Standen, "You`re just not going to find the same level of devices anywhere else, and if you do, they will be horrifically expensive."

Perhaps the clearest indicator of the growing push toward plastic devices is the interest in them from the makers of hybrid devices. "Although our hybrid devices are already small, lightweight and reliable, we just can`t compete costwise against surface-mount, epoxy-sealed plastic devices," Interpoint`s Abel admits. "We`re also actively exploring ways to get into plastics either through our own internal development, partnerships, or acquisition. We already provide for the very high-end market, but we also have to look at how to get into the lower-cost market as well." Abel says he still expects Interpoint`s core business to be high-end, metal, hermetically-sealed devices.

Likewise Lambda`s Thibodeau also recognizes that cost has become much more of a determinant than it was in the past. Right now, Lambda`s product line is 100 percent hermetically sealed hybrids, but "this is going to change very quickly," Thibodeau says. "Our traditional hi-rel product lines are still growing, but at a slower rate. As a result, we`re exploring new technologies to bring our products more in line with customer expectations in terms of cost and reliability." Lambda leaders say they expect to have their first product in an injection-molded plastic package this summer. Though the first device will be a solid-state power controller — better known as an SSPC — Thibodeau says he also has a program in place to convert the company`s "AFL"-line of DC-DC converters to encapsulated, direct-bond copper construction.

Even company leaders whose business is almost exclusively custom high-rel power supplies are reacting to the cost pressures. At STC Keltec, reliability is still at the top of customers` requirements lists, yet cost has become a very close second, explains STC Keltec President John Cotunaccio. "Customers want both. Together with increasing pressure to bring prices down with non-developmental items, they also still want you to meet all the military specifications."

To deal with the situation, Keltec engineers are focusing on the use of industrial-grade plastic components for their high-voltage power supplies. At the same time, they also are implementing their own follow-on screening and full-mil-spec testing.

Keltec experts are working together with the Air Force Reliability Analysis Center in Rome, N.Y., to develop the required reliability and MTBF calculations. Walker says the approach is yielding some successes: "While not meeting the letter of the old mil specs, we are meeting or exceeding their spirit at a lower cost." In fact, he adds, "we`ve actually discovered that many of the industrial-grade components are not only superior in performance but also better as far as heat transfer within the device."

Show me the reliability

Aircraft systems designers are clearly loosening restrictions on the use of plastic-encased power devices. "Today, as long as you can demonstrate the reliability with upfront qualification testing, you usually won`t have a problem," Martek`s Standen, says.

Martek engineers are using commercial-grade devices wherever possible, Standen points out, even though they are designing power electronics from the outset with the same type of component derating and built-in reliability margins as they always have. The process involved tailored screening to provide the end-user with the confidence levels they`re looking for. "It`s not JAN `S,` but it is demonstrated reliability," he says.

As manufacturers and users gain experience with plastic-packaged devices, these confidence levels continue to build. "Today, the biggest difference we see between the reliability of plastic parts and full-mil devices is in Mil-Handbook-217`s calculated MTBF reliability figures," Vicor`s Hilbert says. "If you go to full metal, hermetically sealed, the calculated MTBF is much higher, but actual field reliability doesn`t seem to be that much different and the relative cost isn`t even close."

Certainly the best news, however, is the relative reliability of the devices themselves. For example, Standen says Martek experts found that encapsulation is actually superior to hermetic sealing in terms of vibration and shock. "Encapsulation gives you a good solid brick, and by using the right thermally conductive epoxies, the devices are also very robust in terms of heat dissipation, eliminating the need for heat clips and metal sinks."

Lambda`s Thibodeau says he agrees with this assessment, and takes the argument one step further. Hermetic devices, he notes, also retain any contamination such as moisture or residual epoxy gas that was originally present in the cavity at the time of sealing. "This can cause real reliability problems down the road which encapsulation avoids." Though Thibodeau says there is still a perception among many in the user community that hermetic is better, he believes it is time for the industry to re-examine the issue. "We`re pretty much convinced that given the right materials, plastic-encapsulated parts are the way to go."

Not a panacea

While the scope of opportunities for plastics in aircraft systems is clearly expanding, they may not yet be ready for all power-supply applications. For example, Thibodeau points to electric motor actuators as one area of potential concern. "If the power supply is co-located with the actuator, it can see extremely high temperatures, and we`re not yet convinced that plastics are a candidate there," he says. "Although if the actuator can be located outside the motor housing, it`s not a problem."

Down the road, Thibodeau says he sees plastic devices that address even these environments. "We believe the processes will come along in a year or two that will make 200-to-250-degrees-Celsius operation with plastics routine."

Electromagnetic interference (EMI) considerations may also require special attention. Although Keltec engineers provide metal- and plastic-encapsulated packages, "wherever there is an EMI requirement, we will still always use a metal case," Walker says.

Lambda designers also culls out EMI-sensitive applications by offering companion EMI filters for their line of DC-DC converters. "Given that very few converter manufacturers put internal EMI filters in their products, it`s really necessary to have this device available," Thibodeau says.

Meanwhile, other technologies offer promise for crossover environments and applications. For example, the "CERMOD" molded-ceramic hermetic enclosures from Omnirel in Leominster, Mass., fall somewhere in between hybrids and plastic packaging in terms of total reliability, ruggedization, and cost.

Omnirel designers do not make their own DC-DC converters, but supply high-voltage insulated gate bipolar transistor (IGBT) gate drivers and metal oxide semiconductor field effect transistor (MOSFET) components to several system manufacturers for use in their own custom power supplies. CERMOD devices offers higher reliability at temperature extremes than plastics and lower cost than hybrid technology, says Tim Hickey, Omnirel`s director of international sales and marketing. Experts are evaluating CERMOD technology and devices at the design-in phase of several new programs including the JSF.

Voltage up, voltage down

While voltage levels climb for aircraft power-delivery systems, they are concurrently dropping on the board and integrated-circuit side of the wire. Relatively low-voltage ICs appear finally to be making their appearance in aircraft systems. One of the challenges ahead for DC-DC converters will be to maintain conversion efficiency levels at 3.3 volts and lower voltages, Martek`s Standen notes.

Lambda`s Thibodeau agrees. "With the move to 3.3 volt, 2 volt, and lower voltages, any inefficiencies will show up as heat from the power supply which will have to be handled." Lambda experts are developing products based on a process called synchronous rectification, which actively rectifies the output AC waveform using gallium arsenide complementary heterojunction field-effect transistor logic MOSFETs — better known as GaAs CHFET MOSFETs — which provide very low conduction and switching losses at low voltages.

In addition, Thibodeau points out that even at high voltages, conventional diodes that rectify voltage outputs at higher frequencies have a reverse recovery time that wastes power. To address this, Lambda designers are implementing GaAs Schottky diodes that have very fast recovery times. Lambda officials say they expect to introduce low-voltage products ranging from 10 to 100 watts power output based on these technologies within the next quarter. Thibodeau says they have already demonstrated better than 90 percent efficiency on a 2-volt DC output power supply, up from 82 to 83 percent.

Interpoint`s Abel says he is already seeing customer requests for a wide variety of low-output devices including 1.8 volts and as low as 0.8 volts. IntelliPoint`s approach to meeting several low-voltage requirements is to provide variable-output converters. For example, through the insertion of a resistor, two of their newest devices allow customers to vary output voltage from between 60 and 110 percent of nominal output voltage.

Shrinking supplier base

One additional important trend in aircraft power is the dramatic reduction in the number of companies supplying high-density, high-reliability power supplies. "Today, there are so few companies with experience in these systems that some primes are actually encouraging other suppliers to participate," cautions Martek`s Standen.

Yet at the same time, STC Keltec`s Cotumaccio observes that "a lot of the big system primes are shutting down their own in-house power supply businesses, offloading their work to companies like us."

The same phenomenon is occurring at the component level. In fact, this may be another reason that the use of hermetically-sealed vs. plastic encapsulated components has become less of an issue, points out Vicor`s Hilbert. "There just aren`t many people making the former anymore and equivalent devices just can`t be found."

While this development bodes well for the bottom line of the remaining players, there are of course drawbacks. One obvious one is a reduction in high-reliability-specific technology innovation. For example the GaAs CHFETs that Lambda designers require for their new low-power converters are available from only a few suppliers. "In order to provide cost- competitive product, we need the market to generate adequate demand for these devices," Thibodeau says.

It is an old question, but one that still needs to be answered. Will the frenzy to obtain the lowest-cost products possible, and the concurrent widespread adoption of commercial technology, ultimately place limits on the capabilities and reliability of aircraft power supplies?

Right now, through the use of innovative design and thorough product screening, the answer from the power-supply industry appears to be no. Today, however, designers still also have a fallback option to specialized, full-mil qualified devices when needed. That may not always be the case.

Click here to enlarge image

This illustration represents the Air Force Research Laboratory`s More Electric Aircraft initiative, which aims at applying new and innovative electrically driven, smart-power subsystem technologies for aircraft secondary power. This approach traditionally uses hydraulic, pneumatic, mechanical, and electrical components.

Click here to enlarge image

The NH-series DC-DC converters from Martek Power Abbott are packaged in plastic, yet meet the temperature range of -55 to 100 degrees Celsius.

Magnetics are key to reducing power supply size

Product profile is receiving particular attention in the continuing drive to smaller-form-factor power supplies. Here, the limiting factor is primarily magnetics technology. The issues center on low voltages and high-frequency/current requirements combining to put new pressure on the relative profile/ performance of power-supply transformer technology.

Offering one promising new approach to the problem are engineers at BTC Power Electronics in Costa Mesa, Calif., a joint venture of Behavior Tech Computer in Taiwan and Flat Transformer Technology of Costa Mesa, Calif. BTC engineers are offering a line of commercial DC-DC converters that blend four "flat transformer" cores.

Flat transformers implement one winding on several cores, as opposed to conventional transformers that use several windings on one core. The result is a low-profile, high-density device with well-distributed thermal properties.

BTC designers are offering their IMT-series devices in standard- and half-size "bricks" with 200 watts output power. They plan products that can handle as much power as 960 watts by the end of the year. Input voltage ranges will be 200 to 400 volts DC and 37 to 72 volts DC with outputs of 3.6, 5, 12, 15, and 24 volts DC. The power limit of the technology in the standard brick size is 1,200 watts, says BTC President James Lau.

In addition to their performance characteristics, flat transformers also offer heat-dissipation advantages over conventional devices, which often suffer from hot spots from the build-up of windings. Noting that every 10 degrees reduction in internal temperature approximately doubles product mean time between failures, Lau says the temperature rise inside BTC units is less than 10 degrees above the base-plate temperature compared to the industry average of 20 to 40 degrees.

As elsewhere in the power supply market, cost plays a major role in the decision to use a new technology. In fact, "the reason many companies are not already using low-profile, brick converters is cost," Lau says. "Our goal is to provide more affordable products that can be used in a greater variety of applications." The cost of BTC`s 200-watt IMT-series converter is already roughly equivalent to 100-watt devices currently on the market.

BTC designers have also begun developing their own line of DC-DC converters using fully derated military components, which Lau says should be available this summer. — J.H.

All-electric power control

Power delivery systems are getting attention as part of the move to all-electric aircraft, as designers replace conventional electromechanical circuit breakers with solid-state devices.

These so-called Solid State Power Controllers (SSPCs) are already in use in the Lockheed Martin C-130J utility turboprop aircraft, Bell-Boeing V-22 Osprey tiltrotor, and the Boeing-Sikorsky RAH-66 Comanche scout-attack helicopter, as well as on the commercial Bombardier Global Express jetliner.

SSPCs offer several advantages over conventional devices. Because of their relatively small size, designers can package several SSPCs in one box and place them in-line between the generator and the load itself. This also provides system weight savings by reducing the amount of wiring between elements.

Officials of ILC Data Device Corp. in Bohemia, N.Y., are offering a line of 28 volts DC SSPCs, as well as 270 volts DC devices. The device has two innovative features. First, users can program the "STAR" series to provide real-time status and troubleshooting information. Second, users can have "thermal memory" that enables them to recognize how much heat the device dissipated to the wire during an initial trip, and the device will trip quickly if the fault remains after resetting. "Since the devices are guaranteed to trip within 145 percent of wire capacity, you don`t have to build in grossly-overcompensated wire sizes," says Steve Friedman, SSPC Product Manager at DDC.

Engineers at Lambda Advanced Analog in Santa Clara, Calif., are also developing a line of SSPCs. Reliability benefits for SSPCs over relays and circuit breakers include relative insensitivity to vibration and shock, rapid turn-on and turn-off times, and very low electromagnetic interference, says Phil Thibodeau, Lambda`s director of advanced programs. "SSPCs are the way to go to control large, complex electrical systems," Thibodeau says. Lambda officials expect to release their first SSPC products by the end of the summer. — J.H.

Who`s who in high-reliability aircraft power electronics

BTC Power Electronics

240 Briggs Avenue

Costa Mesa, Calif. 92646

Phone: 714-850-7320

Fax: 714-850-1163

Web: http://www.btcpower.com

Data Device Corp.

105 Wilbur Place

Bohemia, N.Y. 11716-2482

Phone: 516-567-5600

Fax: 516-567-7358

Web: http://www.ddc-web.com

Toll Free Customer Service 1-800-DDC-5757

Interpoint Corp.

10301 Willows Road

P.O. Box 97005

Redmond, Wash. 98073-9705

Tel: 800-822-8782 or (425) 882-3100

Fax: 425-882-1990

Web: http://www.interpoint-power.com

Lambda Advanced Analog

2270 Martin Avenue

Santa Clara, Calif. 95050

Phone: 408-727-0500

Fax: 408-988-2702

Web: http://www.lambdaaa.com

Martek Power Abbott

2727 S. La Cienega Blvd.

Los Angeles, Calif. 90034

Tel: 310-202-8820

Fax: 310-836-4926

Web: http://www.abbottelectronics.com

Omnirel

205 Crawford Street

Leominster, Mass. 01453

Phone: 978-534-5776

Fax: 978-537-4246

Web: http://www.omnirel.com

Signal Technology Corp.

Keltec Operation

84 Hill Avenue

Fort Walton Beach, Fla. 32548

Phone: 850-244-0043

Web: http://www.sigtech.com

Vicor Corp.

25 Frontage Road

Andover, Mass. 01810

Phone: 978-470-2900

Fax: 978-475-6715

Web: http://www.vicr.com

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