Differentiation is the name of the game in today’s automobile business, and electronics continues to be a major part of differentiation. For the motors, solenoids, displays and lighting, semiconductor suppliers have rolled out several integrated circuits and discrete components to power each load as well as the circuitry that controls it. These products control vehicle loads in an increasingly efficient and highly reliable manner. Some of the simplest discrete devices, such as MOSFETs, IGBTs and LEDs provide the greatest potential to reduce fuel consumption and increase vehicle range.
The newest cars demonstrate why new semiconductor products are required. For example, the 2010 Lexus IS 250C has numerous examples of electrical and electronic technology. In the convertible, 15 electric motors raise and lower the three-piece hard top (Fig. 1). For quiet operation, a roof-speed brake-control system decreases the roof speed as the roof approaches the end of the closing operation in either direction.
In addition to the increase in motor controls, the 250 IS uses LED technology in the tail lamp cluster and brake lights. The car also has electric power steering, direct fuel injection, a hard-disk drive (HDD) navigation system, a premium 270-W surround sound audio system, and much more electronics. However, the 250 IS is just one example of electronics technology in vehicles.
The 2011 Audi A8 demonstrates LED technology applied to headlights where the technology can provide significant power reduction. Instead of consuming as much as 65 W for an incandescent high beam or 55 W for a low beam, an LED headlamp could be in the 5 W to 7 W range. In addition to full LED lighting, the A8 boasts 13% to 22% reduced fuel consumption compared to the previous model through brake energy recuperation and thermal management systems. The 3.0 TDI (turbocharged direct injection) also has a start-stop system.
Due to the growth of electronic systems, sales of power semiconductors are increasing at a compounded average annual growth rate (CAAGR) rate of 9.4% according to market research firm Strategy Analytics. "The two fastest-growing areas are HEV (hybrid electric vehicle) and alternator -- the alternator growth being driven by micro hybrid start-stop designs," says Ian Riches, director, Global Automotive Practice at Strategy Analytics. Today’s largest power semiconductor consuming systems include body and internal combustion (IC) engine control (Fig. 2).
The addition of greater electronic control has placed constraints on power consumption. ”We have seen increased interest in saving watts-per-module over recent years and months. However, the ever-increasing amount of electronic features fitted to cars means that the demands on the vehicle EE network are arguably greater than ever,” says Riches.
Changes for the Micro Hybrid
A closer examination of micro hybrids and alternators reveals the requirements and the driving forces behind industry developments to address increased efficiency for these and other systems. Automotive OEMs and their tier one suppliers continuously provide new requirements to semiconductor suppliers. This input can incrementally improve existing systems or enable new systems. Micro hybrid technology represents a fundamental change in the powertrain system providing stop-start (stop-go) and/or regenerative braking.
“Micro hybrids with stop-go functionality are going to become very big in Europe,” predicts Benjamin Jackson, product manager, Automotive Products Business Unit at International Rectifier (IR). “Some of our customers are putting huge forecasts out for up to 50% of all the new cars in Europe for the next few years to be micro hybrids,” he says.
An integrated starter alternator in a micro hybrid provides stop-go functionality. This allows shutting the engine off at idle to reduce fuel consumption, yet restarting within an undetectable timeframe for drivers. A short restart is just one system consideration. “When it re-starts, the entertainment system mustn’t cut out,” says Jackson. Other loads, such as air conditioning, need to stay on as well so a micro hybrid has to change from belt driven to electrically driven loads requiring a DC-DC converter. As a result, DC-DC conversion and other switching power usage such as Class D audio amplifiers are now becoming quite prevalent on vehicles as carmakers put more value on reducing weight, size and energy consumption.
Converting between different battery voltages is an issue in all hybrids. Stepping 300 V, 600 V or 900 V down to 14 V for cabin or so called ”hotel loads” also requires DC-DC converters. Product ratings for MOSFETs and insulated gate bipolar transistors (IGBTs) used in higher voltage systems can range from 300 V to 1200 V. The voltage specification for MOSFETs controlling 14-V loads also varies considerably depending on the application according to Jackson, but 40 V is an important and common rating.
Higher Efficiency Alternators
Another micro hybrid function is regenerative braking that involves operating the alternator at high output power levels to charge the battery during deceleration. In addition to micro hybrids, alternators in non-hybrid vehicles need more power output as well. With increasing electronic content, carmakers are limited by how much power they can provide from the battery or the alternator. At the North American International Auto Show (NAIAS) 2010, DENSO announced efforts to more effectively manage energy consumed in vehicles that included ongoing development to improve the efficiency of alternators.
To improve the alternator’s efficiency, synchronous rectification techniques used in higher-voltage hybrid starter-generators can also be applied to 14-V alternators. “We are now seeing people seriously looking at synchronous rectification,” says Jackson. “While a traditional alternator is maybe 50% to 60% efficient, with synchronous (rectification) you can be getting up in, I guess, the 80% range.” For a given amount of input power, the higher efficiency can easily mean 50 A or more of additional output power in a high-output alternator.
Packaging Power Semiconductor Technology
To help automotive suppliers more efficiently power the loads in micro hybrids, synchronous rectifier alternator designs and other systems, IR has improved the design and increased the size of its DirectFET package (Fig. 3). This surface mount technology (SMT) design has been used extensively in high-performance computer power supplies, but now meets automotive AEC-Q101 reliability requirements and has a Moisture Sensitivity Level rating of 1 (MSL1).
Basic design characteristics of the package that have not changed include lowest die-free package resistance, dual-sided cooling, low package inductance and simple package construction with no leadframe, no wire bonds and no molding. “We made changes to the passivation layer to improve its robustness and integrity,” says Jackson. Also, some of the materials used in the die attach were changed so the package is 100% lead free. In addition, the original maximum junction temperature of 150°C has been increased to 175°C.
With the MSL 1 rating, the automotive qualified DirectFET can address applications such as transmission-mounted controls. For that type of application, power density is obviously important, as well as ruggedness and reliability. “Power density is a term that five years ago was only used for servers and enterprise power,” says Jackson. “Now we visit automotive customers and they regularly use this term, power density and efficiency.”
The first two automotive products are the AUIRF7739L2 and AUIRF7665S2. The former is a 40-V, large can design with 700 µΩ typical (1000 µΩ max) RDS(on) and a maximum drain current rating of 270 A. “This is the lowest RDS(on) MOSFET in the world today at 40 V,” proclaims Jackson. Gate charge is about 220 nC. The large can is rated at 375 A maximum. Motor control, for example in electric power steering, DC-DC conversion, and battery switches in micro hybrids, are ideal applications for this device.
The AUIRF7665S2 is a 100-V, small can device with 51 mΩ typical (62 mΩ max) and 8.3 nC typical gate charge. This product targets the switching requirements in Class D audio, achieving 100 W/channel into 8 Ω without requiring a heatsink, as well as DC-DC conversion and fuel injection.
The medium size DirectFET is about the same size as the industry-standard PQFN but holds the same size die as the larger D-Pak. The large can DirectFET is about the same size as the D-Pak. It has a 60% smaller footprint than a D2Pak or 85% reduced volume but it holds a die that is 30% larger than a D2Pak. “You’re breaking this fundamental ratio between die size area and package footprint,” says Jackson. “This is what DirectFET does by eliminating these wire bonds and the leadframe.” The result is in a much lower RDS(on) in a given pcb area.
Within the DirectFET2 platform, IR can optimize the silicon in one of three ways: (1) low RDS(on), (2) low Qg, or (3) logic-level operation. The three options provide flexibility to customize the silicon for customers’ specific needs. The AUIRF7739L2 is an example of optimizing for low RDS(on) and the AUIRF7665S2 is an example of optimizing for low Qg. IR plans to release eight more products in early 2010.
Solderable Front Metal IGBT
IR’s solderable front metal IGBTs introduced just a month earlier target inverter modules used in electric vehicles and hybrid electric vehicles. The AUIRG7CH80K6B-M is a 1200-V chip with SFM technology that also allows dual-sided cooling. In addition to improved thermal performance, the solderable surface eliminates the need for wire bonds. “With wire bonding in a module, you do take a yield hit and you do have a great cost hit if you have a yield problem because you have to throw away the whole module,” says Jackson.
Efficient audio systems are among the more subtle approaches to reduce power consumption and make a vehicle more efficient. Although not designed specifically for the Volt, the Bose Energy Efficiency Series sound system is debuting in the Volt because General Motors’ engineers recognized the potential to improve the Volt’s all-electric range. “When you are making acoustic output, generally speakers and amplifiers are very inefficient,” says John Pelliccio, product planning manager, Automotive Systems at Bose Corporation.
Linear amplifiers usually run about 25% efficiency generating 1-W output for every 4 W they consume from the battery. Converting the electrical energy to cone motion in the speaker is very inefficient as well. “You have to pull a considerable amount of power from the battery in order to be able to make a sound system that is loud enough and powerful enough to overcome the wind noise and tire noise and give you the sort of concert-level volume that makes the music sound exciting,” says Pelliccio.
The Bose system that is going into the Volt is 30% smaller, 40% lighter and 50% more efficient than comparable automotive sound systems. “It actually uses about half the battery power that a regular sound system would use,” notes Pelliccio. “That is important in a car like the Volt because when you are talking about 40 miles of all-electric range, you want to be able to use every watt that you’ve got in that battery to move those tires a couple of revolutions forward.”
Since the system is proprietary, Bose engineers can’t say exactly what they are doing to improve the efficiency. However, the improvements are a combination of two aspects: Class D switching circuitry plus improved speaker design. “Class D audio will allow you to take that 25% efficiency that you get from the linear amplifier and move it into the 90 to 91% range,” says Pelliccio. Bose engineers worked on the magnetics of the speaker design to make the speaker much more efficient creating high motor force drivers. “We are taking the energy that is being very efficiently produced by the amplifier, and we are turning it into power that is being pushed out very efficiently by the speakers,” he explains.
The Bose speakers use neodymium instead of traditional ferrites so they are lighter, smaller and utilize the amplifier power more efficiently — users get a double benefit. “What Chevy [engineers] computed is that having that system in the car is the equivalent of shaving out about 50 pounds of mass,” says Pelliccio.
LED technology has found widespread acceptance in vehicles for interior and taillight applications. One of the highest energy savings can occur in headlights. The 2011 Audi A8 uses Osram’s OSTAR LED technology (Fig. 5). The OSTAR headlamp is a new product platform with up to five LED chips. Typical light values are 160 lm at 700 mA for each LED chip but values between 125 lm and 1100 lm are easily achieved. The easily controlled LEDs provide high and low beams as well as cornering lights, fog-lamps and daytime running lights. In addition to the reduced lighting power consumption, the LEDs can eliminate the need for a mechanical cornering systems used for conventional headlamps saving both weight and cost.
Turning on Less Power
Since adding more and more electronics has become a way of life for automakers, more efficiently controlling the power consumed by the loads and systems has also become a way of life for suppliers. Power semiconductors provide a starting point for more efficiently switching power. With more efficient techniques to reduce the loads such as LEDs and audio system design, the industry can continue to use 14 V for hotel loads and avoid higher voltage alternatives, such as 42 V, that were predicted over a decade ago.