Every new electronic feature on today's vehicles adds another nail in the coffin of the 14-V power supply system. While the nominal 14-V charging system for the 12.6-V battery has existed since the mid-'50s, automotive engineers have known for over a decade that this power system has a limited life expectancy. Some time in the early 21st century, the 14-V system will no longer provide the power for the features that automotive customers want.
Automakers will need to provide a higher supply voltage for higher-power automo-
tive loads to ensure reliable operation. Industry efforts have been under way to define the next steps toward a common architecture and are currently focused on a dual-voltage 14-V/42-V system with specified voltage limits. Changes in the vehicle's power supply voltage and overvoltage specifications will have a direct impact on semiconductors and consequently the electronics on cars.
The increasing need for electronic control systems and the loads that are in them is causing the drastic increase in power. Power train control, antilock brakes and air bags have become standard equipment. And, numerous motors are being added as every previous mechanical system in the vehicle is controlled. These new control systems and previously installed vehicle controls are being redesigned using a "mechatronics" approach-a method for designing subsystems with the simultaneous simulation of electronic, mechanical and hydraulic elements to ensure optimum performance.
In many cases, mechatronics combines the electronics, previously remotely located in a standalone electronic module, with the electromechanical actuator. This means the electronics can become an integral part of an electronically controlled motor, reducing wiring and connections. There are a number of mechatronic loads already common in today's vehicles, including motors, solenoids, relays, ignition coils, displays and lamps.
Meeting 5-kW demands
With the increasing use of electronic actuators, a rise in power consumption occurs. Today's high-end vehicle can require an alternator that provides 2 kW to maintain the battery's state of charge. The practical limit of the 14-V alternator, or Lundell machine, has been projected to be around 3 kW. The power requirements for future vehicles will easily be in the 4- to 5-kW range to power the actuators in systems such as active ride control, electromechanical engine valve actuators, electric water pumps, electric power steering and electric brakes.
Global activity may ensure that the next vehicle voltage is established as a standard before ad hoc variations emerge that would cause costly component and system variations. The MIT Consortium on Advanced Automotive Electrical and Electronic Components and Systems in the United States, as well as Forum Bordnetz in Europe, are comprised of automotive industry and supplier representatives pursuing standard voltage specifications for future vehicles. An architecture that provides a transition for today's 14-V system to a three-times higher sup-ply of 42 V is gaining acceptance.
A variety of semiconductors-power MOSFETs, smart power ICs, CMOS MCUs, memory and numerous discrete devices-are affected by a change in the vehicle's power supply system. However, those devices that interface directly to the supply have the highest impact on cost. Today's 14-V system has power devices with a 40-V minimum rating. The change to the dual 14-V/42-V system would require respective semiconductor ratings of 20 V and 5 V.
The starter and alternator are on the high-voltage side with a high-voltage battery. The possibility of combining the two into one unit is one of the potential advantages of the higher-voltage system. The dc/dc converter is a major cost increase to the dual-voltage system. However, it can be the means to reduce system voltage transients.
Reducing or eliminating the load dump transient can provide cost reduction, especially for power switching devices. The alternator load dump transient is the most potentially destructive transient in today's automotive system due to its delivered combination of high voltage and high energy. Load dump is a fault condition that occurs when the alternator is disconnected from the battery. Specified open-circuit load dump transient levels as high as 105 V can theoretically require suppressors to withstand over 100 joules of energy. Breakdown voltage levels of suppressors, while dissipating the energy accompanying a load dump pulse, vary significantly depending upon the current and the suppressor temperature. In the case of the Motorola MR2835S automotive transient suppressor, the breakdown voltage could vary from 24 to 40 V depending upon the current level and temperature of the device.
In the dual-voltage system, the load dump should be suppressed to less than 55 V. The maximum normal operating voltage of the 42-V supply is expected to be no more than 43 V (52 V including rectification ripple). Based on the characteristics of voltage suppressors like the MR2835S, maintaining such a tight suppression voltage will be a challenge.
Other high-voltage transients due to load switching also dictate a need for protection. These transients are much shorter in duration and deliver significantly smaller amounts of energy than the load dump pulse. In the dual-voltage specification, the potentially damaging transients can reach voltage levels as high as 100 V with a source impedance of 50 ohms. Survival of these local transients typically requires suppression at the drivers. Some power IC FET driver designs actively clamp and suppress these transients. Other designs rely on the avalanche capability of the FETs themselves. However, due to the uneven dissipation of the energy in an avalanched FET, the avalanche capability of a power FET is significantly less than the same FET when actively dissipating energy.
The proposed system voltage and overvoltage limits provide a narrower guard band of less than one and a half times the nominal system voltage for semiconductor devices that are specified at the proposed overvoltage limits. Power devices with 60-V ratings will be acceptable-if the maximum 55-V clamp voltage is maintained. Semiconductor manufacturers are concerned that the voltage requirements ultimately specified by automotive electronics designers could be 80 V or possibly higher.
Power processes that are used in vehicles include planar and trench power MOSFETs, and smart-power IC technology. Future improvements in all three of these semiconductor processes will benefit from voltage transient restrictions for the power supply system-even if the dual-voltage architecture is not adopted. Improvements in on-resistance based on historical and projected gains in semiconductor processing capability can be expected to provide a reduction of 30 percent within the next three to four years. This will improve the switching cost-performance within this time frame to allow even greater semiconductor power switching.
Packaging is a major cost constraint for power semiconductor devices. A larger die size means higher cost, but frequently the package cost can be more significant than the silicon cost-especially if a new package is required.
Vehicles with new power switching applications such as electrical power steering can benefit from the higher voltage. These loads (about1,000-W peak and 100- to 200-W average power) are controlled with discrete and hybrid power-module packaged semiconductors. Heavier loads such as electrical valve drive and electrical suspension will undoubtedly require custom hybrid power packaging. These same loads will also dictate the need for adapting the higher-voltage architecture. The increased packaging cost can be somewhat offset in these applications by more efficient silicon devices if the voltage is limited, as currently proposed.
The specification for dual architecture is receiving support from automotive manufacturers and electronics suppliers around the world. However, the implementation may not be universal, timing will vary considerably and acceptance is not necessarily 100 percent. Clearly, vehicle manufacturers that already have the highest number of loads and high power consumption need the higher voltage soonest. Manufacturers that can delay implementation until later may be able to go directly to a 42-V-only system and use smaller dc/dc converters for a limited number of 12-V loads.
Traditional automotive voltage faults have the added potential of connecting a 12-V to a 42-V circuit or vice versa. Automakers are looking for a creative way to prevent the traditional and the new problems. The solution could be a determining factor in how soon the dual-voltage system is available on production vehicles.