Power Electronic Modules Manage Alternator Systems

Automotive vehicles require sophisticated power management to support demands for higher fuel efficiency, improved performance and increased electric loads. The power electronics associated with these systems must be reliable over a wide temperature range. Also, the individual cost for these systems must be consistent with the automotive manufacturer's objectives. These requirements have been met with power electronic modules that manage the vehicle's alternator system.

Active Integrated Rectifier Regulator

Meeting this need is an Active Integrated Rectifier Regulator (AIRR) module now incorporated in DaimlerChrysler's Maybach. The AIRR was designed to meet Delphi Corp.'s specification for the Maybach's 14-V alternator, which produces 600 A at 6000 rpm under normal conditions. It produces a relatively high 200 A at idling speed, where most current-generation systems are underpowered.

The AIRR module developed for Maybach's alternator can handle 525 A for 20 sec. Module features include synchronous rectification and load dump strategy for centralized suppression. The module also provides controlled regulation, including communication exchange with the engine control unit. The configuration can be adapted to suit delta-wound and wye-wound alternator configurations. Fig. 1 shows a typical alternator installation.

AIRR Advantages

Compared with traditional passive rectifiers and alternator regulators, the AIRR generates substantially higher power levels in a smaller package. This new device integrates a voltage regulator with an active rectifier. The voltage regulator controls the alternator's output, while the rectifier converts the 3-phase output of the alternator into dc. That dc power then charges the battery and powers the automobile's electrical loads. The AIRR can improve output power by as much as 25% at idling speed vs. the passive rectifier approach.

To understand operation of the AIRR, we must compare the older generation “passive” rectifier with an “active” circuit. Fig. 2a shows the passive system and Fig. 2b shows the active system. The active system employs power MOSFETs whose gates are driven in synchronism with the 3-phase alternator. This rectifier consists of a patent-pending transistor bridge concept that can be adapted to customer-specific designs to allow better performance, higher efficiency and improved reliability compared with traditional solutions. Fig. 3 shows the AIRR assembly.

A controller board manages the AIRR's multifunction power stage, which sets the alternator modes. The controller board includes multiple diagnostic and fail-safe functions. The circuit is optimized for high efficiency, with a low inductance pin-out. Packaging is optimized for low thermal resistance, providing enhanced reliability in high-temperature environments and compact footprints. Special interconnection technology is used for high current handling.

A Different Version

Another version of the AIRR will manage power for an integrated starter-alternator (ISA) in General Motor's 2004 Chevrolet Sierra and Silverado “mild hybrid” trucks. The term mild hybrid denotes an internal combustion engine equipped vehicle using a 42-V alternator to give it fast start/stop capability. It allows the engine to shut down and avoid idling unnecessarily, which improves fuel economy.

The AIRR-controlled integrated starter-alternator replaces the conventional starter, generator and flywheel. This technique provides instant starts, high-efficiency 42-V electrical power and active damping of the powertrain system. An important feature in these trucks is the use of an ultracapacitor for regenerative braking.

At a stoplight, the gasoline engine stops running, but the accessories continue to work on stored electrical power. When the light turns green and the driver pushes the accelerator, the gasoline engine kicks in again, with little or no delay or disturbance.

To ensure full-accessory capability while the engine is temporarily stopped, an electrically driven hydraulic pump provides power steering. Also, an electric pump continues to circulate hot water if cabin heat is needed in the winter, and cold, dry air is supplied in the summer for an extended period through intelligent control of the conventional air-conditioning system.

This hybrid truck employs a 5.3-L Vortec V-8 engine, the same as the conventional versions of the truck. It can haul and tow just as much as its rugged gasoline counterpart. However, it gets 10% to 15% better fuel economy, thanks to a clever application of hybrid propulsion technology. It can tow boats, haul a full load, climb steep grades and still provide improved fuel economy. Extra fuel savings come from quickly shutting off fuel any time the truck is coasting or braking, and using an electric motor to smooth out any resulting vibrations.

The truck features a compact electric motor that is integrated in a patented, space-efficient design between the engine and transmission. The electric motor provides fast, quiet starting power and the ability to generate up to 14 kW of continuous electric power.

Electricity generated by the system has many uses. It may be stored in a 42-V lead-acid battery pack for future use, used to support on-board electric accessories, or employed to operate power tools or other appliances off the pair of 110-V, 20-A outlets in the cab and bed. As more 42-V accessories become available, the electrical architecture of this hybrid pickup truck can accommodate them.

Fig. 4 shows a simplified powertrain diagram that incorporates the AIRR for alternator power management. The AIRR in this system accepts the alternator input and produces 42 Vdc, which is then converted to 14 Vdc with a dc-dc converter. Although 42 V can be used with some automotive subsystems, 14 V must be used with other subsystems, such as lighting.

Additional Applications

In addition to its environmental advantages, GM market research has found that consumers would welcome the versatility offered by the hybrid's built-in 110-V electrical power outlets. Consumers typically mention using the 110-V outlets for running power tools, camping equipment and other recreational gear. Not only is this power available no matter where the truck goes, but it's also delivered quietly, cleanly and without taking up usable space in the pickup bed. Some consumers even indicated they'd like to connect the truck to their home to power essential appliances during power outages.

Thus, the advantages of ISA technology include:

• The ISA can start the engine in a few hundred milliseconds, so the engine can be shut off whenever the vehicle is stopped and then quickly restarted, saving fuel and cutting carbon-dioxide emissions.

• Starter-alternators make it feasible to significantly increase the engine's power-generation capability.

• The quick, powerful start minimizes emissions due to cold starts because the engine can be tuned to speed heating of its exhaust catalyst.

• By employing a 42-V architecture, the ISA can generate more electrical power, which can then be used to replace mechanical/hydraulic linkages with electrical power for the water pump, air conditioning and power steering.

• ISAs (along with the AIRR) also can provide several seconds of electrical boost as the vehicle accelerates. This allows the use of smaller, more-efficient internal combustion engines.

The ability to provide higher electrical power is particularly important in the winter in cold climates. The availability of additional electric power from the ISA makes it possible to handle these loads.

If you need more electric power than the ISA system can produce, the battery must make up the difference. This situation is exacerbated at low temperatures because the battery may not be rechargeable. During a short run, this places a life-reducing strain on the battery. However, the expanded power-handling capability of the ISA minimizes these problems.

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Alternator Technology

Extensive research in the automotive industry has led to the development of components for the switchover from 14-V to 42-V systems. The alternator is one of the key components of the 42-V system. Conventional alternators are rated at about 1.5 kW, whereas the new 42-V versions can produce up to 12 kW. The efficiency of a current 14-V alternator is, at best, about 72% and decreases as engine speed increases. Therefore, the 14-V alternator produces more like 1 kW at low speeds and as little as 0.5 kW at lower engine speeds. Some of the new 42-V alternators have efficiencies in the 80% or higher range and their output is more constant with engine speed.

Two types of 42-V alternators are now being developed. One is a belt-driven starter-alternator that basically replaces the present-day 14-V alternator. Its output is generally under 5 kW and can be air- or liquid-cooled. The other is a crankshaft-mounted starter-alternator (ISA) whose output is generally in the 5-kW to 12-kW range. It also can be air- or liquid-cooled. However, this alternator must be integrated into the engine's powertrain.

The Impact of 42 V on Power Electronic Systems

Increased electrical-power requirements were the reason for the switch from the 14-V to a 42-V automotive electrical system. In the 1950s, the 6-V system was replaced with a 14-V system. Actually, the battery voltage is 12 V but charging system voltage is 14 V; therefore, the 14-V notation. In a similar manner, the 42-V electrical system uses a 36-V battery with a 42-V charging system.

The automotive industry realized that the change in electrical system voltage provided potentially more electrical power and was also a means for improving fuel economy and reducing emissions. With volatile fuel prices and the desire for light trucks and sport utility vehicles, manufacturers are taking advantage of the switch to 42 V to achieve a possible 8% to 25% improvement in fuel economy through use of the stop-start capability.

In fact, the term mild hybrid has been used to denote an internal combustion engine equipped vehicle using a 42-V alternator to give it stop-start capability (also called idle stop by some manufacturers). By allowing the engine to shut down and avoid idling unnecessarily, fuel economy can be improved, along with a corresponding reduction in CO₂.

Today's typical vehicle can easily have more than 100 power loads. More than 80% of these loads have steady-state currents less than 10 A and power consumption levels below 500 W. Future vehicles could include electromagnetic valves and electrohydraulic brakes that will consume more than 1 kW. They would have steady-state current levels in excess of 100 A if they were placed on today's 14-V power bus.

With both the increasing incremental loads of less than 500 W and step-function increases in excess of 1000 W, vehicles will have to generate more than 5 kW in the future. The prospects of generating power in excess of 5 kW from a machine that's slightly better than 50% efficient has caused the auto industry to rethink how it will handle higher power. The answer has been a 42-V standard that limits the maximum overvoltage on the bus to 58 V. This voltage level addresses safety issues for higher voltages and minimizes the cost of components that will be required in these systems.

The change to 42 V will impact every aspect of the vehicle's electrical and electronic systems. Semiconductors and ICs used for power management are both enabling and affected technologies. For high-wattage loads, the amount of current being switched and the power dissipation will require power silicon and packaging technologies that are well beyond those used on today's vehicles.

An apparent discontinuity is the need for packaging that can handle kilowatt power levels. At the same time, when the voltage increases to 42 V, semiconductors that interface directly to the 42-V supply — including power MOSFETs, rectifiers, transient suppressors and especially power ICs — must not only be able to handle the higher voltage, they also must be optimized for the new 42-V systems. These changes can best be made when the overall system requirements are evaluated and understood.