On a per vehicle basis, hybrids certainly contain the greatest amount of new power semiconductors. However, other vehicle applications for power semiconductors continue to grow based on high-volume usage. Some applications such as ignition systems use high-voltage insulated gate bipolar transistors (IGBTs). However, the majority are low-voltage systems that use power MOSFETs. In addition to cost reduction, other key characteristics that customers are looking for include improved reliability, integration of additional features, application-specific design, packaging and more.
HIGH-VOLTAGE LOADS
Since the ignition system is present on all internal combustion engine vehicles, it represents the highest volume high-voltage vehicle load. IGBTs are the primary technology for firing the ignition coil in the United States, Europe and Japan. Low cost is the primary driving force for this application but not the only one. According to Gary Wagner, director of body electronics and smart switches, Fairchild Semiconductor, other factors include “the trends toward multifuel, the continuing need to improve fuel economy and emissions, as well as extending the ignition capabilities to emerging markets and some of the lower-value vehicles that may not have had the more sophisticated electronics associated with them.”
Wagner believes that coil on plug and technologies such as pencil coils have somewhat stabilized and become more mainstream. With this level of acceptance, the current trend is to add more intelligence to the drivers and to combine the driver to the switch that is mounted on the coil. For Fairchild, this means adding current sense capabilities as well as adding a control die in a multi-die configuration, typically side by side in a TO-247-type package. “These parts are in development right now,” said Wagner.
One of the high-voltage applications that is still emerging, or in the transition phase, is injector drivers. Direct injection with piezoelectric injectors for either gasoline or diesel engines is considered essential to meet stricter emission standards and could be required for future CAFE goals since it allows finer metering of the fuel than solenoid injectors. With direct injection and turbocharging, a six-cylinder engine can outperform a V-8 or a four-cylinder can outperform a six-cylinder. “This technology allows fun driving capability along with the fuel economy,” said Jim Spangler, field applications engineer, ON Semiconductor.
In high-pressure, direct-diesel injection, higher voltages are required to further atomize the fuel to get better burning and 150 V is a common voltage requirement. “In some cases, we are actually using small IGBTs as opposed to FETs for some of the even higher-voltage applications,” said Jim Gillberg, director of applications and strategic marketing, Fairchild Semiconductor.
“Many times customers that are asking for high-voltage boost converter circuits, the application is injector drivers,” said ON Semiconductor's Spangler. In some in-stances, the customers are reevaluating capacitive discharge circuits for spark discharge ignition, but more often than not, the target application is high-voltage injectors. Figure 1 shows one of the approaches used to drive high-voltage injectors.
ON Semiconductor supplies the UC3843BVD control IC and MBR-B20200 diodes used in the boost converter to develop the high voltage for the piezo drive circuit. In addition, the company's NCV7513 or NCV7517 are used as the pre-driver from the MCU to select the proper injector. MURS360 or MURD660 Ultra-Fast diodes are used for recirculation diodes.
It turns out that either IGBTs or FETs can drive the injectors. The choice depends somewhat upon the voltage. The higher the voltage, the more likely that the driver is an IGBT. However, if the system designer has not used IGBTs, they may be inclined to use power MOSFETs. With newer trench devices in the higher-voltage range, the MOSFET's size is reduced making it even more attractive.
Another high-voltage application is high-intensity discharge (HID) lamps. Small 300 to 600 V IGBTs are frequently used for these lamps. The application requires 175 °C operation temperature even though the housing mounts in the airflow at the front of the engine. One of the available products is Fairchild's FGD3N60LSD, an IGBT that can also be used for piezo-injectors. Housed in a DPAK, the unit has 600 V breakdown rating and conducts a continuous forward current of 3 A at a case temperature of 100 °C.
The power device in HID applications requires driver circuitry as well. IXYS' Micronix subsidiary recently introduced a full-bridge high-voltage integrated circuit (HVIC) for HID lighting applications. The MX6895 addresses low-cost, high-volume automotive Xenon headlight applications. Operating with an output supply voltage of -80 to -550 V with control inputs from 3.3 to 12 V, the MX6895 driver requires no external boost-related components. Features include soft turn-on level translators to minimize electromagnetic interference (EMI) effects when directly driving a MOSFET bridge network.
LOW-VOLTAGE LOADS
For low-voltage automotive applications, trench MOSFET technology is definitely here to stay according to Fairchild's Gillberg. Trench is the predominant choice for new designs instead of the previous planar MOSFET technology. “We are probably at a point that maybe we are 50/50 in production,” said Gillberg.
For many of the applications that Fairchild has recently addressed, the die size is not determined by the RDS(on) but by the energy capability of the part, typically for inductive flyback, either a clamped SCIS event or for an unclamped inductive switching (UIS) event. “Often when we are trying to size a MOSFET that is what dominates,” said Gillberg. “You actually get a lower RDS(on) than what you would think you'd need for the application.”
One of the newer areas for Fairchild is trench MOSFET designs focused on more linear applications, where the safe operating area (SOA) of the device in a linear mode is important. In the linear mode, wear-out mechanisms can occur because the device is continuously driven to high temperatures. “Typically, one of the wear-out mechanisms is in the wire bonding,” said Gillberg. Consequently, Fairchild is looking at improvements to those areas that keep the die from going through thermal extremes. A typical application is low-cost brush motors used in fans that do not switch but are controlled basically by the current passing through the MOSFET.
In some motor control applications, an application-specific driver and simple MOSFETs provide the design solution. Using simple N-channel MOSFETs, products such as Melexis' MLX81100 dc motor controller with bridge pre-drivers can control wipers, seat-belt retractors, the steering column lock, and electric brakes, as well as a variety of pumps, heaters and blowers. The integrated MOSFET gate-driver includes all the control, diagnostic, and protection circuitry required for a reversible dc motor applications.
Power supplies provide another application for MOSFET and driver IC designs. Designed to work over the automotive voltage range of 7 V to 40V and ambient temperatures from -40 °C to +125 °C, Allegro Micro-Systems A4401 is a quasi-resonant discontinuous flyback converter control IC. Shown in Figure 3, the unit includes an integrated transconductance amplifier, as soft-start, low-quiescent SLEEP mode, high-voltage enable (EN) pin, and more. The IC targets the multiple voltage rail requirements of vacuum fluorescent display (VFD) power supplies, but can be used in other automotive applications that require several regulated voltage rails.
ALTERNATE APPROACHES TO POWER ELECTRONICS
Since increased electronic systems and increased vehicle loads require more power from the alternator, alternative technologies and approaches that reduce power consumption to limit the alternator's size and the fuel required to drive the alternator are expanding. For example, the daytime running lights on the Audi A4 consist of 12 white OSRAM LEDs per headlight (see photo). The advanced power TOPLEDs produce a uniform white light with a brightness of 18 lumens and only require an operating current of 140 mA.
One of the main applications for Infineon's and other companies' protected high-side drivers is in lighting. “We are seeing the shift away from traditional bulbs toward LEDs,” said Rick Browarski, product marketing manager, switches, Infineon Technologies North America. In addition to passenger compartment use, external LED applications such as the headlights could provide the most significant power reduction. “In reality, outside of a few high-end models, it is still limited to brake lights, and maybe some side markers,” said Browarski.
Lower-end models continue to use bulbs but with LEDs proliferating in higher-end models. Infineon's SPI Power Controller (SPOC) provides a driver that can address both applications to have higher volumes and provide an easy transition to LEDs. The second-generation design, such as the BTS5662E (Figure 4), can handle up to six channels in a basic lighting or LED mode. This means that a tier one supplier can build a single module to address both applications.
The SPOC's operation is programmable through the SPI port. In the bulb mode, the nominal load is 27 W for turn signals and back-up lamps in the United States (21 W in Europe) for a steady-state current of about 3 A bulb compared to the LED mode where the typical current is 420 mA with a maximum of 700 mA.
As opposed to SPOC I that targeted lower power dissipation based on lower RDS(on) in a non-power package, the, SPOC II uses a smaller package with an exposed heat slug on the bottom and is optimized to be more thermally aggressive. Covering a right-hand or left-hand zone at the rear of the vehicle, the SPOC can handle other loads including the side markers (10 W) and license plate lamps (5 W).
One power electronics application that is increasing is audio amplification for entertainment systems. Toshiba provides a design that targets the automotive EMI issue. Using its BiCD technology, Toshiba's design of Class-KB (keyed bridge-tied load) audio power amplifier was a first in the car audio area. The TB2905HQ Class-KB design is a linear amplifier rather than a switched analog or digital type. This eliminates the switching noise found in designs such as the Class-D architecture. TB2905HQ addresses entertainment and infotainment systems packaged in smaller electronics enclosures. Technical capabilities include a JEITA maximum power of 47 W × 4 BTL (bridge-tied load) amplifier channels at 4 Ω and 14.4 V or high performance at 2 Ω with typical power of 55 W × 4 BTL amplifier channels at 14.4 V and 10% total harmonic distortion (THD).
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].