The squeeze is on for smaller and more rugged aircraft power electronics
Aircraft designers are planning for the day when electric power may replace hydraulic and pneumatic power, which is driving the analog power industry to make smaller, lighter components that are able to handle more current than ever before.
By John keller
Aircraft designers are planning for the day when electric power may replace hydraulic and pneumatic power, which is driving the analog power industry to make smaller, lighter components that are able to handle more current than ever before.
The large electro-hydrostatic actuator (EHA), pictured at right, from TRW Aeronautical Systems blends an electric motor with hydraulic pump in an effort to increae the use of electric control systems and reduce the need for hydraulics. This approach, demonstrated on a F-16 jet fighter research aircraft, will be part of the future U.S. Joint Strike Fighter.
Designers of analog power electronics certainly face vastly different challenges from those working on digital electronics, yet these two camps also confront some of the same issues. Analog power and digital systems integrators both press their suppliers to boost device performance efficiency and reliability, while reducing device size, weight, and cost. The ability to achieve these goals plays a big part in determining winners and losers in industries ranging from telecommunications to wireless appliances.
Yet perhaps no other systems engineers are exerting these pressures on their suppliers more than those who are designing the new generations of so-called "more-electric" and "all-electric" aircraft. These future aircraft, in which integrators seek to substitute electronics for hydraulics and pneumatics, will require power components that handle more current, are physically smaller, and more efficient than any power components available today, and that are at least as rugged and reliable.
This variable frequency generator from TRW Aeronautical Systems can output different kinds of electric current most necessary for new generations of electric aircraft.
This picture leaves power components manufacturers with a set of stark challenges, which each company addresses in different ways. In general, power component suppliers — from power semiconductors up to power-control subsystems — are seeking to increase the current their devices can handle, reduce power loss as much as possible, and reduce the heat their components generate. In addition, they are taking great pains to shrink device size and weight, and to bring out standard designs and modular architectures so they can bring down costs and maintain reliability.
These goals echo throughout the electronics industry. With applications such as more-electric and all-electric aircraft on the horizon, power component makers realize they have no time to lose, as they fall under increasing pressure from the systems integrators to develop new and better devices.
For all-electric aircraft designs "you better get real efficient," warns William Standen, vice president of marketing and sales for Martek Power S.A. in Torrance, Calif. "You need to get the power supply as close to 100 percent efficient as you can; less is intolerable. In power converters, everyone is trying to get new ways to make them more efficient. You just can't tolerate the heat." Martek offers a line of what its leaders call "military off-the-shelf" power supplies.
Martek's power supplies are designed into a wide variety of military and commercial aircraft for applications such as cockpit electronics, radar, electronic warfare systems, and automatic test equipment. Among the aircraft with Martek power supplies are the U.S. Army AH-64 Apache Long Bow attack helicopter, the European Fighter Aircraft — better known as the EFA — the U.S. Navy EA-6B Prowler electronic warfare jet, and on flightline test equipment for the U.S. Air Force F-16 jet fighter. Martek power supplies also are aboard the Boeing MD-90 and MD-80 commercial jetliners.
Today's military and commercial aircraft derive power from their engines in several ways. First and most importantly, they draw electric power from engine-driven generators — a source that comes under ever-pressing demands as airframe designers add electrically powered subsystems such as displays, computers, in-flight entertainment systems, and galleys for food preparation. Engines also provide compressed or, or "bleed air," that feeds air-powered pneumatic systems such as aircraft cabin heat, air-conditioning, and pressurization.
One of the most important uses of engine-generated aircraft electricity is to run powerful pumps that maintain pressure in the aircraft's hydraulic system. Hydraulic systems use pressurized fluid plumbed throughout the aircraft in tubes and pipes that moves flight-control surfaces such as rudder, elevator, aileron, and spoilers. The hydraulic pumps must run — and consume electricity — all the time to maintain steady system pressure, even when they are not moving fluid to control surfaces. Some aircraft designers consider this a waste of energy and are seeking more power-efficient ways to move control surfaces. This aim leads directly to the notion of more electric and all-electric aircraft.
In theory, "more-electric" aircraft would feature hybrid systems that combine electronics, hydraulics, and pneumatics. The "all-electric" aircraft designs, meanwhile, would have virtually no hydraulics or pneumatics at all. In fact, the ultimate goal of the all-electric aircraft, which probably will not come to fruition for another 10 to 15 years, is the elimination of hydraulic and pneumatic systems.
The idea behind the all-electric aircraft concept is to power everything aboard the aircraft with electricity. Instead of hydraulic pumps, plumbing, and hydraulic actuators, the all-electric aircraft would have power-conditioning subsystems located throughout the fuselage to step down and distribute power from engine generators. Powerful motors would move actuators on control surfaces to guide the aircraft. This approach will make essential not only the development of super-efficient power semiconductors, power supplies, and power conditioners, but also the development of new super-efficient power generators that are more tightly integrated into the aircraft engines than generators are today.
This approach offers several benefits, such as giving engine makers the freedom to redesign their power plants for increased efficiency, improving the ability of airframe designers to embed test and monitoring electronics to improve safety, and to get rid of the maintenance headaches of hydraulic systems that technicians must bleed periodically to keep them in top condition. Maintenance technicians, for example, could remove and repair aircraft subsystems without breaking hydraulic lines. Removing subsystems would simply involve unfastening electronic connectors.
In addition, design trends for aircraft electrical systems are moving toward increased use of so-called "smart power," which employees techniques such as microprocessor control of switching and power levels, built-in test and diagnostics, and automatic compensation for power spikes and dropouts. "Smart power can turn itself off when it sees an overload or overcurrent," explains says Tiva Bussarakons, technical product manager in the International Rectifier high-rel component subsystems group in El Segundo, Calif. International Rectifier specializes in power integrated circuits. In addition, today's power integrated circuits are more rugged than previous generations, and often can handle overvoltage situations without failing, Bussarakons says.
"Smart power is anytime you put intelligence with power," says Bob Pauplis, principle product line engineer at power supply designer Vicor Corp. in Andover, Mass. "It could be some kind of supervisory circuit that watches operation of DC-DC converter, or any time you have automatic sequencing with a power system that functions on its own. It could be a microprocessor-control system."
New power distribution
New power electronics enable aircraft designers to get rid of troublesome manual circuit breakers that most often are located in the aircraft cockpit. If a power spike causes an electromechanical breaker to pop off, pilots or other aircraft personnel must reset them by hand. Solid-state power controllers, solid-state relays, and microprocessor-controlled power distribution systems automate this task.
"Solid-state power distribution allows designers not to have to put all these breakers in the cockpit," explains Michael Pitka, product marketing manager for motor drives and remote control power at Data Device Corp. (DDC) in Bohemia, N.Y. "They can put in load-management centers that move power control into strategically placed points in the aircraft. They can put power in the galley instead of running wires to the galley," he says. DDC offers a line of solid-state power controllers and motor drives, as well as integrated power-control centers for aircraft use.
"Classically, a breaker will see a load, work off heat, and once it hits its load limit it trips, and you physically need to reset it to the on position," Pitka explains. "Breakers have bi-metallic strips that trip when they get to a certain temperature. But now everything is solid state," he says. "The trip is emulated in electronic circuitry. We sense the current, and convert it to what equivalent power dissipated would be. By sensing current we turn it into a voltage, and that tells us what the trip should look like. We can allow more current over a certain period of time. The processor simply calculates the amount of current over a given amount of time. With an algorithm it transforms current into the time it can stay on."
Among the chief technology enablers for smart power are steadily improving power integrated circuits such as metal oxide semiconductor field effect transistors — better known as MOSFETs — and insulated gate bipolar transistors — otherwise known as IGBTs — as well as in digital signal processors (DSPs), says Jeff Harman, analog engineer for brushless motor control at Apex Microtechnology Corp. in Tucson, Ariz. Apex manufactures switch-mode power supplies, power op amps, and brushless motor controllers.
"The enabling technologies are twofold: one is the advancements in the power MOSFETS and IGBTs, and the other is the DSPs, where the analog functions are replaced with software, Harman says. "Combining those two makes a smart and efficient power-handling system."
Yet on the other hand, the all-electric aircraft concept poses a host of difficult design challenges for power component manufacturers. The electrical systems in these aircraft must be able to handle vastly larger amounts of current than aircraft systems do today, and their power systems must be at least as small and lightweight. Some of the power electronics necessary for these systems exists today, and some has yet to be developed, experts say.
"You are talking about doubling the power and beyond, in an individual generator and what it might need to do," explains Mike Yates, chief engineer of systems integration at TRW Aeronautical Systems (formerly Lucas Aerospace) in Solihull, England. At the same time, however, aircraft designs cannot accommodate electrical components that are substantially larger and heavier than components are today — in fact, designers are demanding much more power generation from smaller and lighter components.
Increasing power loads
"As power requirements go up, loads go up, and you need more powerful generators," Yates explains. "You are talking about generating hundreds of kilowatts per engine in this new concept. You could need 40 kilowatts peak power for a rudder on a large aircraft. Loads could be quite large — not operating all the time, but large nonetheless." Engineers from TRW Aeronautical are developing electric-aircraft systems for the future Airbus A380 jumbo jetliner, and developed electric aircraft subsystems for the Bombardier Global Express long-range business jet.
Yates estimates the future A380 jetliner will require the equivalent of 150 kilovolts of power. That technology, in part, will come from developments for the Bombardier Global Express jet, which uses variable frequency generators. These allow the mechanical input speed to function independently of the frequency produced by the generator, and enables designers to eliminate complex gearboxes.
The SPS series DC switch-mode power supply from American Reliance, can output a miximum of 1,000 watts of power
Many power electronics components are available off the shelf to help systems integrators deal with the challenges of all-electric and more-electric aircraft. Yates points specifically to solid-state power controllers that offer a smaller, lighter, and more reliable alternative to mechanical electric contacts. Yet in some instances what is available off the shelf today simply would not be appropriate. Using today's power-conversion technology, for example, would require designs that are too large and too heavy for aircraft use. Something better clearly is necessary, Yates says.
So how much technology development is necessary before airframers can build the all-electric aircraft? "Most of the technology is available now," Yates says. "The computer is all there. The solid-state power control is new, but not different from other power switching." What is necessary, Yates says, is to scale existing power technology up a level in its ability to generate and handle power in small packages. Perhaps the chief technological breakthrough necessary to build all-electric aircraft, he says, is embedded power generation.
This kind of generator would be an integral part of the aircraft engine and would not run off a spinning shaft attached to the engine, Yates says. These power systems, although they represent a strong enabler, also would present their own challenges. "This technology wouldn't generate AC power; the frequencies would be all wrong," Yates says. You would end up with brushless to generate DC power. That technology requires power electronics to support the generator. It will have to be converted into AC, so you need electronics to do that. You could do that now, but it would require a large number of boxes, which would dissipate too much power."
Despite these technological challenges, Yates and other aircraft systems designers are moving ahead with the electric aircraft concept by using available technology in small stages. "This won't happen in one big bang; it will be incremental," Yates says.
Hybrid power system
One solid example of an incremental approach involves work at the Lockheed Martin Aeronautics Co. in Fort Worth, Texas. Lockheed Martin engineers developed a hybrid system combining distributed power and electric actuators on a test model of the F-16 jet fighter called the Advanced Fighter Technology Integration (AFTI) model, which they plan to design into their entry in competition to build the future U.S. Joint Tactical Fighter (JSF).
On the AFTI "We went from two centralized systems on the airplane to five distributed systems," explains Dennis Eicke, program manager of the JSF Integrated Subsystems Technology Program at Lockheed Martin. "The components we are using are commercial, with few military parts," he says. "For the JSF they will be predominantly commercial parts. The whole focus is to improve affordability of the airplane through systems integration."
The AFTI flight-control system, which will move to the Lockheed Martin JSF entry, first eliminated the F-16's original dual-redundant hydraulic pumps, Eicke explains. Each of the five distributed systems has one actuator with two hydraulic pumps. The tail surface on the F-16 test aircraft has two electric motors and two hydraulic pumps.
Power systems integration on the F-16 began at the aircraft's digital flight-control system, he explains. "The F-16 has digital flight control, but sends an analog signal to the servo actuator. We have three phases of the milliamp power going to the actuator, and then the actuator sums them to get the position rate command." Modifications on the AFTI aircraft involved transforming servo actuator control from analog to digital. Feedback signals from the actuator continue to flow back to the aircraft's flight-control computer as analog signals.
"The command from the computer goes to the actuator on the 1553 databus," Eicke says. "Feedback now is on a linear voltage differential transducer, which is embedded on a piston in the actuator. This LVDT generates voltage, and goes back to the computer in analog fashion. When the control surface moves, the computer closes that loop and sends the next command."
For the JSF and perhaps beyond, Eicke says a hybrid system is sufficient, and suggests that the "all-electric" aircraft may be further in the future than some experts believe. "I don't think we'll get to the point where we don't have hydraulics," which he calls "a very efficient energy-distribution system." Strictly in terms of size and weight, Eicke points out that sometimes hydraulics has advantages over electric.
"When you look at hydraulics vs. electric actuators straight up, the hydraulic actuator is less weight," Eicke points out. "What we are doing is reducing the amount of hydraulics. We are maximizing commonality and minimizing the hydraulics so we can reduce the size of the overall system and reduce the thermal load."
Among the electronic components necessary to design more-electric and all-electric power supplies are DC-DC converters, power conditioners, integrated circuits, and power controllers. These devices are integral links in the power chain that moves and processes raw AC power from aircraft engine electrical generators. This power chain delivers electricity in the amount and type necessary to run a wide variety of aircraft systems ranging from cockpit avionics and radar systems to galley microwave ovens and hydraulic pumps.
Power integrated circuits
Among the power integrated circuit providers are companies such as Advanced Power Technology of Bend, Ore., International Rectifier of El Segundo, Calif., and Omnirel LLC in Leominster, Mass.
Advanced Power Technology concentrates on metal oxide semiconductor field effect transistors — better known as power MOSFETs, says Gary Matthai, senior product marketing engineer at Advanced Power. "We are used in switch-mode power supplies and DC-DC power converters," Matthai says. "We focus on the high-performance end of the market with high current and high power. If you want a 500-, 1,000-, or 1,200-volt part, that is our niche." Advanced Power MOSFETs are on the U.S. Air Force F-22 jet fighter, the Boeing 777 jetliner, and the International Space Station.
Power MOSFETs switch the current in switch-mode power supplies. These devices switch the flow of power off and on quickly, in contrast with traditional DC power that is constantly on, Matthai explains. "Switch mode replaces energy that was taken out and used by the load," he says. It is much more efficient, and our devices are the switches that turn the energy on and off." What Advanced Power brings to the design table is efficiency, Matthai explains. "Designers want to minimize losses and resistance of the switch," he says. "Our products are very low on resistance, offer very fast switching, and have very low capacitance to minimize loss when they turn on and off."
Advanced Power offers a patented open-cell MOSFET design, which places sources and gates like interdigitated fingers on the die. "We use a metal gate rather than a polysilicon gate, which gives less resistance," Matthai says. Closed-cell MOSFETs, he says, place cells parallel on the die and concentrate on making cells smaller and smaller.
Competing with Advanced Power for power integrated circuit applications is International Rectifier, where "the majority of our bread-and-butter products are radiation-hardened MOSFET devices used for space applications, commercial and military satellites, and launch vehicles," says International Rectifier's Bussarakons.
Typically the company's MOSFETs can handle at least 100 kilorads total ionizing dose, and resist single-event upsets up to 82 mega electron volts — "the highest level there is right now," Bussarakons says. International Rectifier also offers hermetically sealed ceramic power integrated circuits for space use. "We have seen a shift to plastic parts, but in our business things are really booming for space use," Bussarakons says.
Enhancing efficiency as well as increasing automation are also what engineers at Micropac Industries Inc. of Garland, Texas, offer to aircraft systems designers with the Micropac line of solid-state relays and power controllers. These devices are designed to replace electromechanical power relays and remote-control circuit breakers. "Military and aerospace, systems designers try to drive smaller packaging, and lower weight. That is why they use solid state, rather than electro mechanical," explains Norm Dang, Micropac's engineering manager.
Dang says Micropac's power integrated circuits are built to be rugged enough for military and aerospace applications such as satellites and weapons. "Everything we work is for the aerospace market," Dang says. "We build and screen our parts for space applications in mind so we put design criteria to withstand harsh environments like radiation." Micropac power ICs operate in the military temperature range of -55 to 125 Celsius, can withstand shock and vibrations in the hundreds of Gs, and withstand radiation of as much as 100 to 300 kilorads total-dose radiation, Dang says.
Engineers at DDC also provide power integrated circuits, and specialize in providing integrated power-control center to distribute power from several places in an aircraft. In this way aircraft designers can retain tight control over power, and avoid long runs of wires directly from power generators. DDC has shipped at least 150,000 of these power centers to aircraft designers such as Boeing, Airbus, and Lockheed Martin over the past several years, says DDC's Pitka. DDC also supplies power-control centers to satellite and battle tank subsystems manufacturers.
These power stations use 8- and 16-bit low-power microprocessors that enable power systems designers "to put wire only where they need it," Pitka says. "DDC's big claim to fame is a lot of circuits in a small space," he says.
Perhaps the most crucial component in power electronic systems is the power supply. This device receives AC power from the aircraft generator, steps it down to the correct voltage, and converts the power where necessary from AC to DC, or from high-power DC to low-power DC, and vice versa.
American Reliance Inc. in Arcadia, Calif., provides two series of AC-DC power supplies — a linear power supply and a switch-mode power supply, says Javier Camarillo, sales engineer at American Reliance. The switch-mode supply is the most popular for aircraft applications, he says. The company also provides DC electronic power loads. Customers include airframer Boeing and the Lockheed Martin Reconfigurable Transportable Consolidated Automated Support System — better known as the RTCASS — for the V-22 tiltrotor aircraft," he says.
"Size of the unit was important," Camarillo says, pointing out that that the power supplies that they are supplying to Lockheed Martin are 1.75 inches high. "We can pack a lot of power in one unit; that was their main issue." The SPS units are primarily 1-kilowatt DC power, which can output a maximum of 1,000 watts of power.
In smart-power designs, the latest technology is not always the biggest advantage. In an era where designers also are looking for the low costs, modular open-systems approaches based on commercial off-the-shelf (COTS) components also are important. Taking advantage of modular designs are engineers at Arnold Magnetics Corp. in Camarillo, Calif.
"We make rugged COTS and severe-environment power supplies, and our forte is we are modular and reconfigurable," explains David Ferrari, vice president of sales and marketing at Arnold. "We have from one to ten outputs in one power supply, and handle any voltage from 2 to 300 volts, and output power up to 4 kilowatts," he says. "Our devices are for airborne, ground mobile, and Navy applications. We are part of hundreds of military programs."
At Datel Inc. of Mansfield, Mass., designers concentrate on DC-DC converters, which take DC input voltages and convert them to DC output voltages — most often at a different voltage than the input. These devices also provide noise isolation and power bus regulation. Datel's DC-DC converter line for military and aerospace applications is called the D-24, which inputs power at levels between 18 and 36 volts, says Bob Leonard, Datel's marketing manager. Most avionics power buses run at 28 volts, he points out.
"We make the 24-volt input DC-DC converter so avionics programs can use them," Leonard says. "These devices can be onboard the aircraft or in test in the air and on the ground." Datel's DC-DC converters aim at industrial environmental guidelines rather than the more-stringent military guidelines, Leonard says. They operate in temperatures between -40 and 100° C.
A primary driver for Datel designers is the trend toward ever-smaller system voltages, Leonard says. Where several years ago electronic systems particularly in the military operated at 5 volts, today they operate at 3.3, 2.5, 1.8, and even 1.5 volts, he points out. "Efficiency is king of the buzzwords there," Leonard says.
Datel's latest device designs involve synchronous rectification. "A lot of power supplies have inductors that smooth power output along with output capacitors," Leonard says. "As current goes into the inductor, the relaxation time has negative kick. FETs now have such low drain-to-source resistance that you can now use FETs for freewheeling diode. It lets us get away from 80 percent efficiency and push 90 percent."
Also pursuing DC-DC converter applications are officials of Interpoint Corp. in Redmond, Wash. "We are a hybridized DC-DC converter house with two product lines — a military/aerospace product line that from 1 watt to 200 watts, and then we have a space line that is rad-hard for space that goes from 5 to 120 watts," explains Steven Canzano, vice president of the Interpoint power and space group. "We treat those lines separately because our customers demand different things."
Interpoint offers three military/aerospace grades of DC-DC converters: industry standard, environmentally screened, and MIL-STD-883 compliant. "Basically we design the front end to handle higher voltages," Canzano says. "We use different capacitors, and sometimes different topologies. Generally we run two-switch single voltages in 100-watt ranges."
At Signal Technology Corp. - Keltec in Fort Walton Beach, Fla., designers are concentrating on shrinking the sizes of their power converters while increasing the current those devices can handle, says John Cotumaccio, president of Signal Technology.
"Looking down the road, the biggest change for the future is smaller and smaller size, and more and more power," Cotumaccio says. "When you get smaller, it changes how to get rid of the heat. Technologies we are driving toward are new cooling techniques, so we may be able to put out more power in smaller spaces. Cooling is one of the major thrusts we look for down the road. We are working with spray cooling for telecom applications, and military applications of the same concept."
The need for increased efficiency also drives designers at Martek Power. "What we do is embedded power, airborne wise," Standen says. "The big thing in aircraft is dropping voltages. Processors use 3.3 volts and lower all the way down to 1.5 volts. That is playing havoc on a lot of things now. How much loss can you afford? With one diode drop at 5 volts, who cares? But that becomes horrific at 1.5 volts."
Martek offers a family of power supplies called "bricks" on aircraft such as the Global Hawk unmanned aerial vehicle, the Tornado fighter-bomber mid-life upgrade in Europe, and upgrades to the U.S. Marine Corps AH-1 Cobra helicopter gunship, Standen says. The design trend in power bricks involves modular architectures that enable systems designers to buy widely off the shelf.
"Power bricks are not new, but the number of choices people have today is incredible," Standen says. "The DC-DC commercial world intersects the aerospace world. Both can stand very high temperature. Commercial guys did that as a byproduct by getting components in there to meet the size format. They can live in the military temperature range. It forced the designers to use parts that fit that format. Now there are so many manufacturers in the marketplace that you really have a wide selection. If you have a military application that is not exactly mission critical, you have a world of choice, with a difference in price by a factor of five."
Another player in DC-DC converters is Modular Devices Inc. of Shirley, N.Y. "They are converters built with hybrid packaging techniques for very small sizes and hermetic for high reliability," says Steve Summer, president of Modular Devices. The company's DC-DC converters are designed into the U.S. Air Force F-22 Raptor jet fighter and the Army CH-47 heavy-lift helicopter.
"Our products have built in EMI filtering as part of the construction," Summer says. "Instead of separate packages for EMI filter, we integrated it into the converter. It is useful to customers because it saves a lot of space and weight. We make both 28-volt parts and 270-volt parts. All the newer aircraft such as F-22 and even the tanks now are going to high voltage systems to save weight."
Engineers at Vicor, which specializes in DC-DC converters, are attempting to automate some power device tasks. "To a degree we provide smart power," says Vicor's Pauplis. "We put some intelligence in our systems. Our second-generation DC-DC converter units will arbitrate for control. One will control, and two will listen. If one fails, it would drop off the line, and the two would pick up the load, with one as a master." Vicor's DC-DC converters are on a variety of military aircraft, including the F-22 fighter.
Who's who in power electronics
Advanced Power Technology
American Reliance Inc.
626-303-6688, ext. 117
Analog Devices Inc.
Apex Microtechnology Corp.
Arnold Magnetics Corp.
Westlake Village, Calif.
Data Device Corp. (DDC)
631-567-5600 ext. 7381
Fairchild Semiconductor Discrete Products
Palm Bay, Fla.
El Segundo, Calif.
Santa Clara, Calif.
Lambda Novatronics Inc.
Pompano Beach, Fla.
Logitek, a subsidiary of NAI Inc.
Los Angeles, Calif.
Martek Power Abbott
Los Angeles, Calif.
Micropac Industries Inc.
Modular Devices Inc.
Nova Electric Corp.