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 r.frank@ieee.org.