As automakers rethink just about every aspect of their business, they will be asking for more technology and want it even sooner. In the area of power electronics, car companies appear to continue to require more of what has been requested for many years: higher efficiency, more integration to reduce cost and improve performance and, in some cases, special designs for new systems. Suppliers are obliging by continuing to deliver new products to solve these problems.
MEETING AUTOMAKERS NEEDS
In general, car companies need improved general-purpose power semiconductor and IC products for a broad range of applications. Specific requirements include improved high-voltage drivers, more efficient voltage regulators and circuits that require very low ignition-off currents. In many cases, the new product direction gets far more specific based on the automakers' newest strategies.
The changes that carmakers have in mind are easily seen in concept vehicles at major auto shows such as the North American International Auto Show (NAIAS) in Detroit[1]. Promoted as the place to see the future of transportation technology, global automakers strut their finest at NAIAS to get customer attention and confirm or revise product direction at this annual event. In perhaps the strongest future strategy statement, Ford demonstrated increased fuel economy plans for traditional internal combustion engines with its EcoBoost engine with direct injection technology and an electrification strategy for hybrid electric vehicles (HEVs), battery electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs). Both strategies require more power semiconductors.
Figure 1 shows Ford's Battery Electric Vehicle plan with Magna International. In addition to the electric motor, AC charger, high power DC-DC converter and high-voltage battery pack, the BEV replaces several traditionally belt-driven or engine-powered subsystems with electric versions. While none of these is new, the presence of these five electric approaches on the BEV will significantly increase their usage rate. According to Strategy Analytics' Mark Fitzgerald, full electric power steering (EPS) is already used on more than 25% of the 2008 North American vehicles. EPS can reduce the load on the IC engine to improve fuel economy or provide steering for HEVs at idle when the engine is off.
Electrification is only one obvious power challenge shown in concept cars at NAIAS 2009. In the Chrysler 200C EV concept, Chrysler promoted advanced connectivity for consumer electronics with its uconnect system. Featuring an onboard PC and a display much larger than Ford's Sync, the system, as all of the latest connectivity and navigation offerings, relies on continued audio and visual technology advancements. Figure 2 shows the dash display in the concept vehicle.
GETTING THE POWER
Increased electrification, audio and lighting for dashboard displays, especially LED lighting have driven the development teams at several suppliers. At the most fundamental level, more efficient power MOSFETs control most of the vehicle loads in both hybrids and IC vehicles.
In Japan, NEC's leadership in power products comes from partnerships with the large Japanese carmakers including Toyota. In recent years, NEC has brought this technology to North American tier one suppliers. “We are recognized as having strong products in a couple of categories in power, specifically because of our low RDS(on),” said Dave Stone, marketing director, NEC Electronics America (NECEA). “That particular specification is actually one of the lowest in the industry.”
High-level trends that NECEA has observed include low-voltage MOSFET (<100 V) requirements in the various hybrids for trench MOSFET technology. “We are seeing a huge growth in the electric motors in the vehicles not associated with the inverter or the high-voltage drive but other peripheral aspects,” said Dean Sullivan, staff product marketing engineer, NEC Electronics America.
One application driving the use of highly efficient MOSFETS is the micro hybrid. The stop-start system in these vehicles uses 14 V so 40 V MOSFETs are appropriate. However, in an urban environment 100 stop-starts per day may not be unusual. Due to the frequency of stop-starts, the MOSFETs are subjected to increased stress. Typically, the Automotive Electronics Council (AEC) qualifications include 1000 temperature cycles that correlates to 100,000 mile usage. “On a case by case basis, we are being asked for 2,000 cycle qualifications now,” said Sullivan.
To reduce the stress, tier one suppliers want to use the lowest on-resistance MOSFETs available to make the best usage of the available cooling. In the United States, NECEA sees strong interest in packaged MOSFETs as opposed to die that customers mount on ceramic substrates.
“The lowest RDS(on) device we have is 180-ampere rated,” said Sullivan. “It is 1.5 mÙ maximum, so the typical is 1.2 mÙ and that's both the die and the package combined, the device on-resistance.” The NP180N04TUG TO-263 7-pin MOSFETs use NEC's UMOS-4 trench technology for low on-resistance in the silicon. “Internally we use four wire bonds double stitched to the die,” explained Sullivan.
As shown in Figure 3, increasing the diameter and number of aluminum wires allows the package to achieve an 180 A current rating. Electric power steering could use a lower current device in this family. NEC is also seeing strong customer interest for this higher efficiency family in other electrification applications including the vacuum pump for brakes and an electric water pump for circulating coolant through the HVAC system.
The use of motors in vehicles is increasing in other applications. On average, a mid-range car today contains about 40 to 50 small motors while the premium car segment contains about 70 motors per car. More motors are being used across the entire fleet for increased convenience as well as safety features.
Seatbelt pretension systems are among the increasing motor loads. For its next-generation systems, called Active Seatbelts, Autoliv selected a power semiconductor from Infineon's NovalithIC family. Using chip-by-chip or chip-on-chip technologies, two power chips and a logic circuit to control and monitor the power stage are combined in the NovalithIC package. The resulting half-bridge can control a variety of motors as shown in Figure 4. In addition to saving space through the package-level integration, the design varies the power output continuously depending on application need. This feature reduces energy consumption by as much as 80 percent. With a typical on-resistance of 16 mΩ and a quiescent current of 7 µA, the device provides energy efficiency and low power consumption in the ignition key-off mode.
For supplying power in electronic modules, low drop out (LDO) linear voltage regulators continue to be an important solution. To avoid increasing the battery size, an added requirement is very low quiescent current draw to minimize overall current draw in the ignition-off mode. ON Semiconductor NCV86xx LDOs provide precision output voltages of 3.3 V and 5.0 V and output currents up to 350 mA with typical quiescent currents (Iq) as low as 22 µA. For automotive applications, the devices are qualified to AEC-Q100 and operate across a temperature range of -40 to 150°C. Onboard protection functions for the battery-connectable LDOs include devices with power-on reset and delay time select capabilities as well as reverse battery, short circuit and thermal overload protection depending on the specific device.
POWERING AUDIO
Whether the music is brought in, beamed in or built-in as Ford says about its Sync system, a great audiophile experience in the car depends on a high-performance audio system in the vehicle. To support and certainly not detract from the vehicle's fuel efficiency, add efficiency to list of “must haves” for this system as well. Voice amplification is also an essential aspect of hands-free cell phone use and navigation systems so the need for audio amplifiers is increasing.
International Rectifier's IRAUDAMP7 reference design for Class D audio amplifiers above 25 W per channel is one way to tackle the audio design challenge. The IRAUDAMP7S two-channel, 120 W half-bridge design provides a typical 91 percent efficiency at 120 W in a Class D stage driving eight Ohm loads. The typical total harmonic distortion of the design is 0.005% (THD+N) at 1 kHz, 60 W. A design using through-hole components uses the IRS2092 protected digital audio driver IC in a DIP16 package and another design uses surface-mount components and the IRS2092S in an SOIC package. In both designs, the IRFI4019H-117P digital audio dual MOSFET provides enhanced scalability. Protection features include over-current, overvoltage, under-voltage, over-temperature and DC protection.
Vehicle features that provide the driver an audible warning such as signal change, back-up warning and collision systems require a different type of audio amplifier. For these applications, National Semiconductor's answer is the LM48100Q monolithic amplifier. Called the Boomer audio power amplifier, the IC delivers 1.3 W and has features to ensure the speaker and amplifier operation in these safety systems. If a fault occurs, an open drain fault output flag is raised. In addition, output short circuit and thermal overload protection prevent damage during a fault condition.
POWERING DISPLAY LIGHTING
Dash displays provide the visual man-machine interface, functions such as touch-screen input for infotainment, HVAC and other controls, convey navigation and telematics data, and more. These systems require appropriate backlighting to cope with the vehicle's varying environment — another power application. To minimize the impact on fuel economy, LED lighting for this and other vehicle lighting applications is increasing.
Designed for automotive and other display backlighting applications, Linear Technology's LT3598 is a 45 V, 2.5 MHz DC/DC boost mode converter that operates as a constant current LED driver for up to sixty white LEDs. Delivering efficiencies up to 90 percent, the IC can drive up to six strings of LEDs with up to ten 30 mA white LEDs in series as shown in Figure 6. The chip's multichannel capability makes it appropriate for medium-sized TFT-LCD backlighting applications.
With an input voltage range of 3 to 30 V that meets the requirements of automotive and industrial applications, the chip offers 1.5 percent LED current matching to ensure uniform display brightness. Using the company's True Color PWM dimming technique, dimming ratios as high as 3,000:1 can be attained. A programmable 200 kHz to 2.5 MHz fixed frequency operation and current mode architecture provide stability over a wide range of supply and output voltages and minimize the size of the external components. The 4 x 4 mm QFN package with its center heat spreader has a junction to ambient thermal resistance (èJA) of 37°C/W providing a compact footprint for LED backlighting applications.
ROHM Semiconductor's BD8113EFV-E2 is another approach for backlighting TFT-LCD displays. The white LED driver provides a constant current source for white LED backlights in medium-to-large-format automotive TFT-LCD displays. The IC is a dual-channel device capable of supplying up to 150 mA constant current per channel with current-mode buck/boost DC/DC control. This ensures constant display brightness over the entire automotive temperature range (-40°C to 105°C), even with unstable battery input voltages. Capable of operating over a supply voltage range of 5 to 30 V (maximum input voltage of 36 V), the buck/boost configuration increases the number of LEDs that can be connected in series. Since ROHM also makes SMLK15W high heat dissipation white LEDs, the combination provides a complete subsystem for backlighting medium-to-large-sized automotive TFT LCD applications.
References
1. North American International Auto Show (NAIAS) http://www.naias.com/.
ABOUT THE AUTHOR
Randy Frank is president of Randy Frank & Associates Ltd., a technical marketing consulting firm based in Scottsdale, AZ. He is an SAE and IEEE Fellow and has been involved in automotive electronics for more than 25 years. He can be reached at [email protected].