The ideal gas-powered car moves its driver and passengers from Point A to Point B in comfort while sipping gas and producing next to no harmful emissions. However, car buyers have sharply inflated their definition of “comfort” to the point at which many owners now expect their cars to provide satellite navigation, security, climate control, an Internet interface, and even beverage refrigeration. This escalation in features has caused a corresponding increase in the car-battery load when the engine is off and the car’s electronics are in a sleep state. Designers of electronics accessories for cars have to balance the power needs of car features and accessories with the finite power that a car battery contains when in a sleep state. In addition, rising gas prices make fuel economy increasingly important to consumers, causing automobile OEMs to turn to complex but efficient electronic-control systems for subsystems such as fuel delivery and air conditioning.

Back when car electronics were primitive and gas was cheap, there was little incentive to conserve the power that the car electronics used. However, designers of automobile accessories and subsystems must make designs that consume as little power as possible because today’s cars almost universally rely on a network such as the CAN (controller-area-network) bus and have a stringent sleep-state-power budget for electronic subsystems. Sleep-state-power budgeting prevents, for example, a weary traveler returning to the airport parking lot after a two-week trip from finding that the car’s always-on electronics had drained the battery in his absence.

An air bag is an example of a system that turns completely off when the car is parked; there’s no need for air-bag deployment in a parked car. Other subsystems that still have a function when the car is parked, such as a keyless-entry or alarm system, will remain on but in a sleep state, drawing a small “keep-alive” current. They are on a direct battery feed. Car OEMs define the car platform’s power distribution within the network architecture so that power is available to always-on systems, while other systems are asleep, waiting for a wake-up signal from the network. Although companies do not make public the sleep-state-power budgets for the accessories and subsystems, Tony Richardson, product-marketing manager of the power-products group at Linear Technology, estimates that [a constant drain of] 20 to 50 mA per day is a good range.

Network architectures and sleep-state power budgets have all but eliminated the dead-battery-at-the-airport scenario for cars with healthy batteries. However, after-market add-on electronics can still pull a battery down. After-market electronics can’t tap directly into the network and rely on a power connector, such as the cigarette lighter, for power.

“Each OEM’s messaging strategy is tightly controlled, and they don’t open it up to after-market guys,” says Kevin Anderson, systems and applications manager for the analog-products division of Freescale. “For purposes of data integrity, it’s not an open system.”

Even when the car’s engine is running and electrical power is plentiful, electronics accessories and subsystems still need to conserve power to manage car thermal hot spots and, increasingly, for fuel economy (see sidebar “Parasitic power losses”). Semiconductor and IC manufacturers continue to move into the automobile by adding low-power features and intelligence to even relatively simple electronics.

Solid-state devices are replacing relays, once common automotive-load switches, for increased reliability as well as lower on-resistance. Although a relay might have a resistance of 25 to 100 mΩ, an SSR’s (solid-state relay’s) resistance can be just 2 mΩ, roughly equivalent to a few feet of standard-gauge copper wire. Low resistance is important in subsystems such as the engine starter, in which current can reach 700A. Other circuits’ currents vary even as they remain on. For example, when an incandescent headlight first turns on, the bulb element is cold and allows a surge of initial inrush current of 100 to 150A for 20 to 50 msec. As the current heats the bulb, however, the resistance increases, and the current flow drops to 5 to 40A, depending on the bulb size. Intelligent SSRs, such as Freescale’s eXtreme Switch product family, allow for the large initial inrush current with a variable setpoint. As the bulb heats up and the resistance changes, the setpoint moves to a new threshold for the lower current. However, if the current suddenly increases at that point, indicating a short circuit, the chip acts as a fuse and turns off, protecting both the switch and the load. These intelligent switches, in addition to being more power-efficient, also increase reliability and lower BOM (bill-of-materials) costs.

Headlights are second only to a car’s HVAC (heating/ventilation/air-condition) system as an electrical-power load. HB (high-brightness) LEDs (light-emitting diodes) are finding use in city-lighting systems due to their lower power requirements, durability, and long lifetimes, and these same features will probably make them standard features in both interior and exterior lighting for some cars in the next few years. Taillights and daytime running lights use them; Cadillac has announced that they will be available in the Escalade Platinum’s headlights this summer (Figure 1).

In addition to power efficiency, LED headlights also make it easier to put the light where the driver needs it. Current high-end cars feature electric-motor-driven steerable headlights that track the car’s path around a turn. An array of LEDs that turn on and off to light the path can eliminate the need for less reliable tracking motors.

The main roadblock preventing solid-state lighting in cars is the cost. HB-LED-production numbers will have to grow to cause the price drop necessary to include them in Chevrolet models as well as Cadillacs. In addition, temperature changes strongly affect LED-light output, with output varying by as much as 50% over the automotive-temperature range of subfreezing temperatures in the winter to triple digits in the summer.

Headlight LEDs require sophisticated power-management ICs. “You don’t just drive one LED in a headlight,” explains Freescale’s Anderson. “You drive multiple ones in strings, and their brightness must remain constant, while the car-battery voltage can range from 8 to 16V in normal-run conditions. You need a switching power supply that will create a constant output with a variable input. Plus, LEDs themselves are not the same brightness universally, and they also age with time, so you have to make adjustments for each LED to make sure you get a constant output.”

HB LEDs in headlights are not the only subsystems that can justify the use of switching regulators in cars: Just about any module that the CAN bus controls can benefit from the high efficiency and variable output states of a switching regulator as opposed to the older and simpler but less efficient linear regulators. Although none of the car OEMs publish the power budget for modules the CAN bus connects, Richardson of Linear Technology says that a consensus exists among the OEMs that any power regulator to power the microprocessor in an end system needs a quiescent current in the sleep mode of less than 100 µA, giving the advantage to the more efficient switching regulators. For more on low-quiescent-current-switching design, see sidebar “Low-quiescent-current switchers target automotive-electronic design.”

Virtually all electronics subsystems include processors, and some core-based processors can conserve power in their system-sleep state by lowering their voltage, frequency, or both (Reference 1). For example, Texas Instruments’ ARM-based TMS470 family uses dynamic-voltage scaling, which allows you to drop the voltage from, say, a normal operating voltage of 1.2V to a sleep state of 1V, a corresponding current savings of 20%. Why not just completely turn off the processor? Even though the system is in a sleep state, it still must be able to respond to some simple commands, albeit at a reduced response rate.

Although such processor-based ICs have voltage management within the chip itself, these parts rely on external power-management parts to take the battery voltage, which is approximately 13V, down to the IC’s input voltage range of 2 to 3V. These power-management parts for the automotive world face a tough set of specifications for designs for future cars: They must drop their quiescent current from today’s 100 μA to 30 to 50 μA to support the sleep-state-power budget for power-management chips. Plus, they need to work more closely with the CAN bus for system-wake-up commands. Expect to see power-management chips for the automotive market in the coming year begin to integrate CAN transceivers into the power-management and power-regulation chips. In addition, power-management ICs must keep the switching-circuit EMI (electromagnetic interference) from affecting the communication-transceiver portion. These new chips will face tough design criteria to meet the needs of automobile OEMs and module designers.

As electronic controls creep further into car design, their potential for increases in efficiency goes beyond power-regulation opportunities. A motor-driven compressor can more efficiently perform many functions that mechanical subsystems traditionally performed, such as the belt-drive compressor for air conditioning. Similarly, electric-motor-driven, variable-assist power steering can improve fuel economy by as much as 0.5 mpg in reducing parasitic losses common in belt-driven hydraulic systems. Ford has announced it will be making its six-speed automatic transmission standard in future cars. This transmission, which electronically shifts gears, is more fuel-efficient than current automatic or manual transmissions.

There’s also opportunity for power savings in the scheme by which the alternator charges the battery itself. If it’s a nice day that requires no air conditioning, heating, or lights, the car requires less electrical power and puts a smaller load on the alternator. In most vehicles, however, the alternator provides an output to constantly charge the battery and is sized to provide a maximum load—for example, on a winter night when it’s snowing, and the heater and lights are on. A more efficient alternator will vary its speed based on the load.

EVs (electric vehicles) and PHEVs (plug-in hybrid electric vehicles) operating in their battery-only mode require the most power-miserly electronic subsystems (Reference 2). General Motors Vice President of Engineering Bob Lutz has talked about the importance of power efficiency in electronic accessories—even the mundane windshield wipers—for the Chevy Volt PHEV because any subsystem that draws on the battery reduces the range of the Volt in its battery-only mode, currently about 40 miles (Reference 3).

Tesla Motors is developing a high-end lithium-ion-battery-powered EV with a 240-mile range. As with conventional cars, cabin cooling is the largest accessory load for an EV, and Dan Adams, senior mechanical engineer for systems and integration at Tesla, estimates that it commonly takes about 2 kW of power in an EV. “Cabin heating comes next in power consumption, but for an ICE [internal-combustion-engine]-powered car, waste energy required to cool the engine is enough to heat the cabin and thus is 'free,’” he says. Because the electric power train in an EV is efficient, it takes awhile to heat, and the waste-heat power is much lower than the power available with a gas engine. The 3 kW to conventionally heat a cabin would be a serious drain on the battery’s energy. EV manufacturers, such as Tesla, are working on alternative methods for cabin heating, such as relying on seat heaters for localized heat, as well as PTC (positive-temperature-coefficient) heaters for general cabin heating.

Scott Brenneman, release engineer for vehicle electrics at Tesla, comments on how running accessories, such as a heater or a radio, affect the range. “In general, some simple math shows the impact on the range,” he says. “In rough terms, a given load would have a quantifiable effect on a 50-kWhr battery pack, assuming you also knew the other loads. Back of the envelope, let’s say the range over a given course is 240 miles, driven at an average speed of 30 mph, for a total time of eight hours. I suspect that the effect of most electrical subsystems on range is small except for a few items, such as the HVAC components. For instance, a 1-kW load running half the time would consume 4 kWhr, or 4/50=8% of the range. I think this is an extreme case, since 1 kW of heating or cooling is a lot. Typical 12V loads, such as exterior lights, are less than 0.2 kW in this scenario and thus would reduce range by about 1.5%.”

Adams provides some perspective on the quest to eke out relatively small amounts of power efficiency in the ubiquitous automobile. “Cabin comfort [and car transportation in general] requires great quantities of energy compared to the basic scale of human energy needs,” he says. “Thus, while a human can ride a bike and transport [himself] using only a fraction of a horsepower, a car driving 20 mph can easily consume 10 kW with another 2.5 kW for HVAC if cooling is on.” As a dedicated biker, however, Adams is a bit biased.