Voltage Specifications

25.7.2 Voltage Specifications

power consumption integrated circuits operate at 3.3 V, mak- ing the regulators more inefficient. This inefficiency also means

A major issue when implementing a high or dual voltage sys- that larger heat sinks are required to remove the heat from tem is the nominal voltage of the high-voltage bus, and the the ECUs.

operating limits of both buses. While there are many possibil- ities, there is a growing consensus in the automotive industry for a nominal voltage of 42 V for the high-voltage bus (cor-

25.7.1.2 Higher Fuel Efficiency

responding to a 36 V lead-acid storage battery) [39, 56, 60].

A secondary motivating factor for the introduction of a higher This voltage is gaining acceptance because it is as high as pos- system voltage is the challenge of achieving higher fuel econ- sible while remaining within acceptable safety limits for open omy. The average fuel economy of present-day automobiles in wiring systems (once headroom is added for transients) and it the United States is in the vicinity of 30 miles per gallon (mpg). provides substantial benefits in the power semiconductors and There is little market incentive for automobile manufacturers wiring harness [61]. Furthermore, this voltage is sufficient to to increase the fuel economy of vehicles for the US market implement starter/alternator systems and “light” hybrid vehicle where the price of fuel is relatively low. The price of gasoline in designs [62, 63]. While no vehicles equipped at 42 V are in pro- the US ($1.70 per gallon) is less than the price of bottled water duction at present, availability of 42 V components is rapidly ($4.00 per gallon when bought by the quart). Although market increasing and 42 V equipped vehicles may be expected early forces have not been a driver for the development of fuel- in this decade. efficient vehicles, a number of new incentives have emerged

The permissible static and transient voltage ranges in over the past few years. One of these is the fine imposed on the an electrical system are important design considerations automakers by the US government if the average fuel economy for power electronic equipment. At present, no universally of their fleet falls below the mandated standard. The man- accepted specification exists for high or dual voltage auto- dated standard for cars has increased from 24 mpg in 1982 to motive electrical systems. However, the preliminary spec- its 1997 level of 27.5 mpg, and will continue to increase. In ifications proposed by the European automotive working Europe, the German Automotive Industry Association (VDA) group, Forum Bordnetz, are under wide consideration by the

662 D. J. Perreault et al. TABLE 25.7 Voltage limits for 14 and 42 V buses proposed in [56]

Voltage

Value V 42,OV −dyn

Description

55 V V 42,OV −stat

Maximum dynamic overvoltage on 42 V bus during fault conditions

52 V V 42,E −max

Maximum static overvoltage on 42 V bus

43 V V 42,E −nom

Maximum operating voltage of 42 V bus while engine is running

41.4 V V 42,E −min

Nominal operating voltage of 42 V bus while engine is running

33 V V 42,OP −min

Minimum operating voltage of 42 V bus while engine is running

Minimum operating, voltage on the 42 V bus. Also, lower limit operating voltage for all non-critical 33 V

loads (i.e. loads not required for starting and safety)

V 42,FS Failsafe minimum voltage: lower limit on operating voltage for all loads critical to starting and safety 25 V

on the 42 V bus

V 14,OV −dyn

20 V V 14,OV −stat

Maximum dynamic overvoltage on 14 V bus during fault conditions

16 V V 14,E −max

Maximum static overvoltage on 14 V bus

14.3 V V 14,E −nom

Maximum operating voltage of 14 V bus while engine is running

13.8 V V 14,E −min

Nominal operating voltage of 14 V bus while engine is running

12 V V 14,OP −min

Minimum operating voltage of 14 V bus while engine is running

Minimum operating voltage of 14 V bus. Also lower limit operating voltage for all non-critical loads 11 V V 14,FS

9V

Failsafe minimum voltage: lower limit on operating voltage for all critical loads on the 14 V bus

automotive industry [56]. These specifications, summarized in loads Table 25.7, impose tight static and transient limits on both the

42 and 14 V buses. The upper voltage limit on the 14 V bus is

ac dc

G far lower than in the conventional 12-V system. The allowed loads dc dc

upper limit on the 42 V bus is also proportionally tight. These strict limits facilitate the use of power semiconductor devices such as power MOSFETs and lower the cost of the protection circuitry needed in individual functions. However, they also

FIGURE 25.19 Dual-voltage architecture based on a dc/dc converter. require much more sophisticated means for limiting transients

(such as load dump) than is found in conventional systems, which imposes a significant cost. Appropriate voltage range specifications for dual/high voltage electrical systems are thus In this implementation, an alternator and associated battery

a subject of ongoing investigation by vehicle manufacturers, provide energy to one bus (typically the 42 V bus), while the and will likely continue to evolve for some time.

other bus is supplied via a dc/dc converter. If a battery is used at the dc/dc converter output, the converter needs to be rated for slightly above average power. Otherwise, the converter needs

25.7.3 Dual-voltage Architectures

to be rated a factor of two to three higher to meet peak power requirements [61]. The architecture of Fig. 25.19 has a number

Conventional automotive electrical systems have a single alter- of advantages. The dc/dc converter provides high-bandwidth nator and battery. Dual-voltage electrical systems have two control of energy flow between the two buses, thus enabling voltage buses and typically two batteries. Single-battery con- better transient control on the 14 V bus than is available in figurations are possible, but tend to be less cost effective [61]. present-day systems or in most other dual-voltage architec-

A variety of different methods for generating and supplying tures. Furthermore, in systems with batteries on both buses, the energy to the two buses are under investigation in the automo- dc/dc converter can be used to implement an energy manage- tive community. Many of these have power electronic circuits ment system so that generated energy is always put to best use. at their core. This section describes three dual-voltage electrical If the converter is bidirectional it can even be used to recharge system architectures that have received broad attention. In all the high-voltage (starter) battery from the low-voltage battery, three cases the loads are assumed to be partitioned between the thus providing a self jump start capability. The major challenge two buses with the starter and many of the other high-power presented by this architecture is the implementation of dc/dc loads on the 42 V bus and most of the lamps and electronics converters having the proper functionality within the tight cost on the 14 V bus.

constraints dictated by the automotive industry. Some aspects The dc/dc converter-based implementation of Fig. 25.19 is of design and optimization of converters for this application perhaps the most widely considered dual-voltage architecture. are addressed in [64].

25 Automotive Applications of Power Electronics 663

ac

Field Current

loads

dc Regulator

ac G loads

dc

field

FIGURE 25.20 Dual-voltage architecture based on a dual-wound alternator.

The dual-stator alternator architecture of Fig. 25.20 is also often considered for dual-voltage automotive electrical sys- tems [65, 66]. In this case, an alternator with two armature windings is used along with two rectifiers to provide energy to the buses and their respective batteries. Control of the bus volt- ages is achieved via a combination of controlled rectification and field control. Typically, field control is used to regulate one output, while the other output is regulated using a controlled rectifier. Figure 25.21 shows one possible implementation of this architecture. It should be noted that to achieve sufficient output power and power steering from the dual-wound alter- nator, the winding ratio between the two outputs must be FIGURE 25.21 Model for a dual-wound alternator system. The two out- carefully selected. For 42/14 V systems, a winding ratio of 2.5:1 put voltages are regulated through field control and phase control. For a is typical [66]. Advantages of this electrical architecture include 42/14 V system, a winding ratio between the two stator windings of 2.5:1 low cost. However, it does not provide the bidirectional energy is typical. control that is possible in the dc/dc converter architecture. Furthermore, there are substantial issues of cross-regulation and transient control with this architecture that remain to be

ac loads fully explored.

G dc dc

In a third architecture, a single-output alternator with a dual-output rectifier is employed. This approach is shown

loads schematically in Fig. 25.22. As with the dual-stator alternator

configuration, this architecture has the potential for low cost. One widely considered implementation of the dual-rectified alternator is shown in Fig. 25.23 [65, 67–69]. Despite its FIGURE 25.22 Dual-voltage architecture based on a dual-rectified simplicity, this implementation approach provides less func- alternator.

tionality than the dc/dc converter-based architecture, generates substantial low-frequency ripple which must be filtered, and has serious output power and control limitations [66]. An Nevertheless, the low energy storage density and the high alternative implementation, proposed in [37] and shown in cost of suitable batteries makes pure electric vehicles non- Fig. 25.24, seems to overcome these limitations, and may competitive with internal combustion engine vehicles in potentially provide the same capabilities as the dc/dc converter- most applications. An alternative approach that is generat- based architecture at lower cost. Clearly, this architecture has ing widespread attention is the hybrid electric vehicle (HEV). promise for dual-voltage electrical systems, but remains to be An HEV combines electrical propulsion with another energy fully explored.

source, such as an internal combustion engine, allowing the traditional range and performance limitations of pure electric vehicles to be overcome [70]. Alternative energy sources, such as fuel cells, are also possible in place of an internal combustion

25.8 Electric and Hybrid Electric

engine.

Hybrid electric vehicles can be classified as having either a parallel or series driveline configuration [71]. In a series HEV Battery-powered electric vehicles were first introduced over all of the propulsion force is produced from electricity; the one hundred years ago, and continue to incite great pub- engine is only used to drive a generator to produce electricity. lic interest because they do not generate tailpipe emissions. In a parallel hybrid, propulsive force can come from either the

Vehicles

664 D. J. Perreault et al. Field Current

required power levels, the electrical driveline must operate at Regulator

hundreds of volts, necessitating the electrical subsystem to be sealed from access by the user. The engine, on the other hand, need only be rated to deliver the average power required by the

field vehicle, which is much lower. In a system that does not require utility recharge of the batteries (i.e. can drive indefinitely on fuel alone), the engine size is set by the power requirements of the vehicle at maximum cruising speed. If utility recharge of the batteries and a battery-limited driving range is accept- able, engine power requirements can be reduced even further. Because the engine does not provide tractive power, it can be designed to run at a single optimized condition, thus maximiz- ing engine efficiency and minimizing emissions. Furthermore, the need for a transmission is eliminated and there is a great deal of flexibility in the engine placement.

In a parallel HEV, traction power is split between the engine and the electrical driveline. One possible approach is to uti-

FIGURE 25.23

A dual-rectified alternator with a phase-controlled lize a single machine mounted on the engine crankshaft to rectifier.

provide starting capability along with electrical traction power and regeneration [72–75]. This approach can be replaced or

Field Current complemented with other approaches, such as use of a power-

Regulator splitting device such as a planetary gear set [70, 76], or using

different propulsion and generation techniques on different sets of wheels [71, 77, 78]. In all parallel hybrid approaches,

field some form of transmission is needed to limit the required speed range of the engine. A wide range of divisions between engine size and electrical system size is possible in the parallel hybrid case, depending on structure. Depending on this split, the necessary electrical driveline system voltage may be as low as 42 V (which is safe for an open wiring system) or as high as 300 V. Also because the electrical subsystem, the internal combustion engine subsystem, or both may provide tractive power under different conditions, there exists a wide range of possible operating approaches for a parallel hybrid system. Consequently, the control strategy for a parallel hybrid tends to be substantially more complex than for a series hybrid.

One parallel hybrid approach that is receiving a lot of atten-

FIGURE 25.24

A dual-rectified alternator with a switched-mode tion for near-term vehicles is a “light” or “mild” hybrid. In rectifier.

this case, a somewhat conventional vehicle driveline is com- plemented with a relatively small starter/alternator machine mounted on the crankshaft [62, 63, 72–75, 79]. The electrical

engine or the electrical drive. In both cases, batteries or other drive power is typically below 10 kW average and 20 kW peak. electrical storage devices are used to buffer the instantaneous The starter/alternator can be used to provide rapid, clean difference between the power needed for propulsion and that restart of the vehicle so that the engine can be turned off generated by the engine. The selection of a series or parallel at idling conditions and seamlessly restarted. This so-called driveline depends heavily on the performance requirements “stop and go” operation of the engine is valuable for fuel and mission of the vehicle.

economy and emissions. The starter/alternator can also be In a series HEV, all power delivered to the wheels of the used to implement regenerative braking, to provide engine vehicle must be delivered through the electrical driveline. The torque smoothing (replacing the flywheel and allowing dif- electrical driveline components, including the batteries, power ferent engine configurations to be used) and to provide electronics, and machine(s), must all be rated for the peak trac- boost power for short-term acceleration. At the low-power tion power requirements, making these components relatively end, such systems can be integrated directly into the open large and expensive if performance (e.g. acceleration) compa- wiring configuration of a 42 V electrical system, simplifying rable to a conventional vehicle is to be achieved. To achieve the the vehicle electrical architecture. System-level control remains

25 Automotive Applications of Power Electronics 665

a major challenge in realizing the full benefits of such systems. of their introduction. It is safe to say that power electronics Starter/alternator-based hybrids are expected to be a signifi- will continue to play an important role in the evolution of cant near-term application of power electronics and machines automobiles far into the future. in automobiles.