Increasing electrical power consump-tion is straining the existing 12-V sup-ply system of current vehicles. At some point in the not-too-distant future, manufacturers will have to change to a higher voltage to support improved emissions, safety, and new vehicle options. With the increased growth in vehicle loads (4%/year for the past two decades and about 100 W per year for the last five years), vehicle manufacturers are running out of time to define and then implement a standard.
Voltage effect on semiconductors
A variety of semiconductors - power field effect transistors (FETs), smart power integrated circuits (ICs), microcontoller units (MCUs), digital signal processors (DSPs), memory, analog ICs, and numerous discrete devices - are affected by a change in the vehicle's power supply system. Those devices that interface directly with the supply (without overvoltage protection or a secondary circuit) have the highest impact on cost. As shown in Figure 1, power/energy management involves the generation of power in the vehicle (typically by an alternator), the conversion of energy (from one voltage level to another), and the storage of energy (typically in the battery). Power distribution and protection requires semiconductors to withstand the full range of voltage variations that can occur on the vehicle. On the other hand, signal communication from power device diagnostics and communication protocols, inputs to the power-management system, and the embedded computing (logic) elements are also impacted both by a change in vehicle supply voltage. The technology road map changes in these products are occurring independent of vehicle manufacturers' need to define a higher voltage standard.
Experts believe there will ultimately be three different types of semiconductor classifications in 42-V systems (Figure 2).
A-type semiconductors are directly impacted by the system voltage, with the increased voltage impacting both their performance and reliability. Examples include power switches such as power metal-oxide semiconductor field-effect transducers (MOSFETs), transient suppression avalanche rectifiers, and smart power ICs.
B-type semiconductors are transitional devices between the voltage and a variety of lower voltages. The power-management dc/dc converters used in existing automotive electronics modules will increase in both number and complexity. The increase in complexity will occur since these devices will have to interface to the higher voltage supply and, in some cases, may control more than one voltage level.
C-type semiconductors such as microcontroller, DSPs, sensors, and memory devices will operate at today's levels and even lower voltage levels. Voltage levels of 3.3 V and less make these devices even more susceptible to voltage transients - system generated and electrostatic discharge (ESD). Protection will become even more critical in higher voltage systems, though voltage transients will be controlled in 42-V systems better than they have ever been in 12-V systems.
Controlling today's vehicle loads
As shown in Figure 3, today's vehicle loads are switched/controlled by different classes of semiconductor technologies. The exact point where one technology is both reliable and cost-effective is not a well-defined boundary. The proper technology choice varies depending on the semiconductor manufacturer and the specific process and design for the specific product. Since vehicle loads are causing the need for higher voltage, the differences between the semiconductors that switch these loads must be understood.
Power MOSFETs can handle about 96% of today's loads if only current-handling and thermal considerations are considered. The other 4% require a power module with multiple power switches in parallel. However, smart power ICs - those semiconductors that combine both the power device(s) and control circuitry in one package in either a single- or multiple-chip form - provide performance and protection features that are increasing their applications in many automotive systems. Since the circuitry takes available package space from the power portion, their current-handling capability is less than their power MOSFET counterparts'. Control circuitry requires multiple pins in the package that limits the area for the high-current power portion. Also, the base technology is different, which leads to inherently less efficient power devices in smart power ICs.
The solid bars in Figure 3 identify bidirectional motors - those that use four or more power devices in bridge configuration to control the motor in both forward and reverse directions. Brush dc motors use an H-bridge that requires four power devices. Brushless dc motors or three-phase motors require six power devices. Stepper motors use at least four power-switching devices. The cross-hatched bars are for a single power switch, but higher current levels may require multiple parallel power devices. With increased system voltage, all of the semiconductor technologies in this figure will require higher voltage capability.
Dual-voltage applications
The advantages of a dual-voltage system with restricted voltage transients are revealed by analyzing specific control points within the vehicle. In Figure 4 the lighting controls are on the low (14-V) voltage side of a dual-voltage architecture. Today's low on-resistance power MOSFETs, combined with smart power control, can switch the lamps in a taillight assembly with minimal heating in an IC package. Additional circuitry can sense and react to lamp failures that allow the brake and taillamp functions to be maintained in multiple bulb assemblies even if the brake or other lamp(s) fail. This safety application is even more cost-effective with a restricted low-voltage supply. If light emitting diode (LED) lamps are used, the reduced current levels would reduce the size of the power-switching devices. The reduced size could mean lower cost for discrete power switches or allow the output device to be cost-effectively integrated into a power IC.
Vehicles with new power-switching applications such as electrical power steering can benefit from the higher voltage. These loads - about 1-kW peak and 100-200 W average power - are being controlled with discrete and hybrid-power module-packaged semiconductors. Heavier loads such as electrical valve drives and electrical suspensions 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.
MOSFET gated power
Low-voltage power MOSFETs are used in vehicles for switching solenoids in antilock brake systems or controlling pumps and motors. Frequently, several power devices are in the same module requiring either a large amount of heat sinking or highly efficient switches. Continuous improvement of power MOSFET processing and design has allowed the on-resistance to be reduced from one generation to the next and achieve other application targets. Continued on-resistance reduction can be expected by MOSFET manufacturers for automotive applications. Table 1 shows the areas for improvement that can be addressed both by semiconductor and system designers. However, even more significant is the on-resistance reduction that will occur if central transient suppression is used in vehicles.
An additional advantage to a dual-voltage system is in the transient voltage suppression of the 14-V rail. With a single output 42-V alternator design, a dc/dc converter to create the 14-volt rail will be required. This alleviates the transient issue that exists today. As a result, the 14-V load applications could use 30-V or possibly even 20-V MOSFETs. Figure 5 shows the proposed voltage limits on the 14- and 42-V bus in a dual-voltage architecture.
Initially the dual-voltage system was proposed as a bridge to an all 42-V system, though some designers feel that a dual system is more trouble than going directly to a single 42-V system. One major issue with a single 42-V system is that lamps, which have longer filament length with increased voltage, would have reliability issues. Researchers are working to alleviate this by providing pulse width modulation (PWM) to reduce the voltage seen across lamp loads.
Although the standard set for load dump in the 42-V system is 58 V, most system designers are inclined to use 70- to 100-V MOSFETs until confidence is gained in the transient voltage suppression system. Following the introduction of the first models, an evolution towards 60 V will probably occur. Available products include ON Semiconductor's 80-V power MOSFETs.
A three-times reduction in current from increasing the voltage three times and dissipating the same power in the load results in a nine-times reduction in power dissipation in the power switch. This means improved efficiency with the same die size or dramatic reduction in die size and possibly a reduced package to meet cost requirements and board space.
Another method for reducing losses is improved package technology. Two of the most promising methods for MOSFETs are solderable front metal and solderable epoxy. These methods allow the use of a clip with a large surface area to be mounted from the package to the source of the die rather than the smaller wire bonds. This dramatically reduces junction-to-case thermal resistivity.
With 42-V systems, relays become more expensive because of increased complexity to avoid arcing. This means increased usage of semiconductors to replace relays due to improved cost situation.
Automotive designers are increasingly specifying integrated protection for fault conditions including short circuits. The lamp and motor loads being driven by the power devices range from less than a few watts to a fraction of a kilowatt. The smaller loads are addressed by monolithic smart power ICs. The larger loads make this approach too costly. One solution is modifying the MOSFET process by adding a few masking layers to allow the design of simple protection circuitry. Temperature sensing, thermal shutdown, and current limiting are obtained by this approach. Frequently, standard high-volume packaging is used, which increases cost-effectiveness. These lower-level integrated power devices have been proven in the vehicle environment and are expected to see increasing usage with new systems such as electric power steering.
Transient suppression
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. Specified open-circuit load dump transient levels as high as 105 V can theoretically require suppressors to withstand over 100 J 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 ON Semiconductor 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 a 42-V system, the load dump is expected to be suppressed to levels less than 58 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). In light of the characteristics of existing transient suppressors like the MR2835S, maintaining such a tight suppression voltage will be challenging. However, new approaches are being investigated to meet this specification.
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 (up to 100 mJ) 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 power FETs themselves. However, the avalanche capability of a power FET is significantly less (as little as 1/10 less) than the same FET when actively dissipating energy, due to the uneven dissipation of the energy in an avalanched FET. The power density in those cells that break down during avalanche is much higher than in an actively clamped device.
Smart power ICs for 42-V applications
Several of the functions performed by a variety of electronic control units (ECUs) in the modern automobile involve the actuation of electromagnetic solenoids or motors with the 12-V power supply. In most cases, these loads are controlled by a microcontroller and an interface circuit. For some applications, the interface circuit consists of an integration of several drivers and the related diagnostics and distributed protection features in one power IC.
Efficient system designs consolidate a number of circuit elements (e.g., discrete components, standard logic, and specific function ICs) into single-chip solutions. Precision linear regulators, comparators, and operation amplifiers are optimized using bipolar technology. Logic, active filters, active loads, and current mirrors are efficiently implemented using complementary metal oxide semiconductor (CMOS) technology. Power FET devices are the dominant choice for power switching. ON Semiconductor's Powersense process is an evolving, mixed-mode process that combines bipolar, CMOS, and power FET technology and enables this type of integrated, direct microcontroller interface capability. The availability of devices with minimum breakdown voltage characteristics in excess of 60 V makes the Powersense process a good choice to design system power ICs for conventional 14-V automotive systems as well as the dual-voltage systems of the future. Table 2 shows the array of capability that can be designed in a smart power IC.
Higher-voltage processes will be required for the 42-V system, such as an 80-V minimum breakdown voltage, to be able to drive the inductive loads in 42-V systems. The higher breakdown voltage is needed due to the 58-V maximum load-dump specification for the 42-V system. To drive inductive loads (solenoids, relays, and motors), an active clamp at the output is often required for improved energy handling capability of the power device. This is becoming more of an issue as technology advances continue to shrink power device area to the point where energy is the most difficult design parameter. The breakdown voltage of this active clamp must be higher than the load-dump value, yet lower than the power device's minimum breakdown with allowances for margin, resulting in the need for an 80-V power device. Also, a higher clamp voltage is advantageous when driving certain inductive loads, such as in direct-fuel-injection systems, due to fast turnoff requirements of the solenoids. The larger the voltage across the coil during flyback, the faster the energy in the coil is removed.
Transitional-voltage semiconductors
Transitional-voltage semiconductors are the link between the 42-V system and applications that will still require 14 V or less for many years to come. These applications are centered around body electronics and will primarily include lighting and Type C semiconductors. There are over 100 lamps in today's vehicles that require a 14-V system due to limitations in tungsten filament technology. Without a changeover to high-intensity-discharge or neon technologies, transitional-voltage semiconductors will be required to isolate lamps from the 42-V supply.
Figure 4 displays a dual-battery architecture with a large dc/dc bi-directional converter that controls and charges both voltage buses and batteries. This architecture becomes both costly and impractical as additional high power loads are implemented in the vehicle. It is believed at this time that a decentralized architecture will provide the most benefits to a 42-V system. Figure 6 shows an example of decentralized architecture with voltage from a 42-V integral starter alternator damper (ISAD) providing 42 V directly to 42-V loads and indirectly to distribute 14-V and lower voltage loads. Many medium-power step-down converters (from several watts to a few hundred watts) will be used to supply power to 14-V loads located throughout the vehicle. These devices can be located either remotely, at either the load or the control electronics, or located in a central module, providing power conversion for several loads. A controller, along with Power FET and other discrete components from ON Semiconductor, can be combined to create a medium-power point-of-use dc/dc converter such as the one demonstrated in Figure 6. Lower power loads (less than 10 W) can be powered by integrated supplies. This architecture exhibits the additional benefit of providing a tightly controlled 14-V bias, which will extend the lamps' lifetime. Figure 7 shows the circuit diagram for supplying 14 V from the 42-V bus for automotive loads up to a few hundred watts.
Semiconductors indirectly impacted by higher voltage
Semiconductors that are not directly connected to the primary supply voltage on current and future vehicles are the products potentially affected the least by the change to a higher voltage. These are typically at the signal communication levels shown in Figure 1. However, computing semiconductors are progressing to higher frequency operation and higher levels of integration through smaller and smaller geometries, and subsequently require lower voltage.
The Semiconductor Industry Association road map calls for significant voltage decreases for the highest level of integration of both portable and desktop computer applications. Operating voltage will decrease in portable computers from 1.5 to 0.3 V, and combinations of voltages will be required between now and 2014. Desktop voltage will decrease from 1.8 to 0.6 V to keep up with Moore's Law and the computing/memory forecast. For automotive electronics to take advantage of these technologies, which are growing at a much greater rate that automotive semiconductors, their supply voltage and associated voltage limits must be adhered to. In some cases, this poses much tighter constraints on power-management circuitry.
Controlled voltage supply
Two to four years from now at Convergence 2002 or 2004, presenters will be comparing higher-voltage production architectures. Next-generation vehicles can benefit from power and smart-power semiconductors if system voltage is restricted to allow lower minimum breakdown voltages. This level of improvement will be even greater with advances made in future semiconductor processing and packaging that will allow automotive customers to turn on the power to all the options they want.
