Design for Start-Stop

If you have driven a car in Europe or Japan recently, you may be familiar with an energy efficiency technique called stop-start. Drivers in the U.S. will soon begin to experience this fuel-saving measure as well. In automobiles, a start-stop system automatically turns off and restarts the internal combustion engine to reduce the amount of time spent burning fuel while idling. Hybrids, of course, have implemented this feature since their introduction. But stricter MPG regulations are bringing it to vehicles which lack a hybrid electric powertrain. For cars with ordinary internal combustion engines, the fuel economy gains from this technology can range up to 10%.

The process of turning off the car engine instead of idling has been likened to turning off electric lights in unoccupied rooms. The technology is relatively easy to implement and inexpensive compared to alternatives such as plug-in electric vehicles (EVs).

A powertrain for a conventional combustion engine can be adapted to start-stop operation by adding a start-stop optimized battery and starter motor, combined with suitable circuitry for power switching and control. Overall, such changes are expected to cost around $300. In some cases electric motors are also added to run A/C or other systems normally powered by the engine during idle.

Stop-start switching

The automotive industry has long used relays for switching high-current loads such as windshield wipers, HVAC controls and electric windows. A relay keeps high currents away from occupants and also lets designers use smaller, low-current switches in the dashboard and cabin consoles. New electromechanical relays for vehicular uses have been developed with better reliability and high current ratings. They come in small packages as well as in PCB-mount configurations. Their attributes can include low audible noise for in-cabin use and reduced coil power to save energy. They manage such items as fuel injectors, electric steering, traction control and ABS.

Now that major car makers in Europe and Asia are implementing stop-start technology, we can expect to see further improvements in power control and switching techniques. In the past, the driver actuated the starter motor via a high-current relay, using the column-mounted key or, in some models, a starter button. In start-stop applications, the vehicle must take over responsibility for starting and stopping the engine continuously throughout each journey.

The controls must be able to recognize the conditions that make it appropriate to turn off the engine and must also be able to restart the engine quickly. Designs are still evolving, as stop-start is a relatively new technology. Some systems restart the engine by sensing the positions of pistons in the cylinders and then ignite fuel in a selected combustion chamber. Others use stored energy to actuate a starter motor or integrated starter-alternator. One common challenge, of course, is that the engine must be turned on and off many more times throughout its lifetime than in a conventional non stop-start application.

Despite progress in automotive relay technology, these devices still have mechanical reliability and operating lifetime issues. So designers are considering solid-state switching with automotive-approved MOSFETs as an alternative.

Repeated stops and starts during each journey also call for changes to the battery and starter motor. An ordinary lead-acid battery will reach its end-of-life relatively quickly if used in a stop-start application. Similarly, the starter motor must be designed to withstand many more operations than has historically been the case. So designs for both batteries and starter motors are undergoing significant changes.

Automotive batteries have been evolving quietly but steadily as manufacturers have sought to reduce maintenance, extend reliability and raise typical energy delivery rates. Absorptive Glass Mat (AGM) battery technology satisfies these demands and is also well suited to start-stop applications. In an AGM battery, a fiber separator holds the electrolyte in place, helping to spill-proof the battery and make it resist vibration and impact. Sizing constraints are also relaxed; an AGM device can be smaller than a conventional flooded cell version, relative to the largest load in the vehicle.

The important properties of AGMs, as far as stop-start technology is concerned, include the ability to deliver energy at a higher rate than a conventional flooded-cell battery and an ability to maintain higher available capacity after having been deeply cycled. The advantages of AGMs are readily accessible to the automotive industry, because the units operate at almost the same voltage set-points as flooded cells and can effectively be regarded as drop-in replacements.

Automakers are making changes to the starter and alternator, in some cases combining the two functions in a single integrated unit. An integrated starter-alternator featuring intelligent control can recover energy normally lost during braking by charging the battery. It will also disengage from the engine when charging is unnecessary to reduce drag and thereby save fuel. This sort of integrated unit can coordinate engine shutdown when the vehicle comes to a halt and the restarting process when the driver releases the brake.

Protecting in-car electronics

Start-stop technology imposes several electronic design challenges because the main battery can fall as low as 6 V during engine cranking. This can disrupt the operation of in-car electronics such as the radio, climate control, GPS and interior or exterior lights. All these systems need a stable supply, or board-net voltage, of nominally 13 V to operate correctly. Hence the vehicle needs an additional subsystem, containing a power switch and associated control circuitry, both to disconnect the battery when the engine cranks and to let an auxiliary battery or dc/dc converter power the loads temporarily.

When the vehicle operates normally, the power switch in the subsystem must supply all the car’s electrical loads. At the same time, low conduction losses are imperative because the switch is on except when cranking.

IR’s 24-V AUIRF1324S-7L or 40-V AUIRF3004-7L MOSFETs are solid-state switches qualified for automotive applications. They combine a continuous drain current of 240 A (package limited) with low on-state resistance of 1.0 or 1.25 mΩ respectively to handle the maximum load current imposed by the vehicle while also minimizing system energy losses. Designers can connect several devices in parallel to further reduce on-state resistance. The MOSFETs can be controlled using a smart driver IC such as the AUIR3240S, which is designed explicitly for automotive applications.

Because the battery must remain connected when the car is parked, the combination of power switches and controller must have a low quiescent current to minimise drain on the battery. A quiescent current of around 50 µA is considered acceptable.

When the vehicle auto-start function activates engine cranking, the main battery voltage begins to fall. The power switch must turn off quickly to let the auxiliary supply maintain the board-net voltage at 13 V. This is important, because the low ohmic resistance of the power switch can let a high current flow to the main battery side with a voltage drop of only a few millivolts. The turn-on time is less critical because the current will flow in the body diode of the power switch.

It is challenging to design a driver having low quiescent current and that meets all these requirements, using discrete components. The IR AUIR3240S manages the turn-on and turn-off of the power switch. It contains a boost converter capable of operating from an input voltage in the range of 4 to 36 V, allowing continuous operation as the battery voltage falls during cranking. It also provides a MOSFET gate drive voltage of 12.5 V. The device integrates circuitry to monitor voltage and current, control on/off-time, diagnose faults and protect against thermal overload.

The IC architecture is optimized for intermittent operation, which effectively reduces current consumption to less than 50 µA. Only a few external passive components complete the design: an inductor and a capacitor for the boost converter and resistors for the diagnostic and measurement circuitry.

The internal block diagram of the controller IC includes the main dc/dc switch, K1, and freewheeling diode, D. Two comparators control the turn-on and turn-off of K1. One comparator monitors the gate voltage compared to Vcc to turn on K1. The other monitors the voltage across the shunt resistor connected to the Rs pin, which controls the turn off. The AUIR3240S can keep the MOSFET turned on while drawing very low current.

To meet automotive-industry requirements for safety-relevant systems, the AUIR3240S provides two diagnostic mechanisms to watch for excessive output current and system temperature. The IC supports thermal monitoring by providing circuitry enabling connection of an NTC device close to the MOSFET die.

Car users, pundits and legislators agree that electric cars will eventually be the norm. In the immediate future, however, start-stop technology can help reduce fuel consumption and CO₂ emissions at a relatively low cost. They entail only a few modifications to the powertrain, implementing improvements in batteries, starter/alternators and electronic components, and many of these are already well developed.

All in all, rugged, reliable solid-state power switches and innovative control ICs help stop-start technology let new cars meet tightening environmental performance demands while allowing the continued use of familiar, reliable, economical and easy to use internal combustion engines.