Automotive Applications Maximize Available Power
General Motors introduced the first 42-V vehicle for the North American market in 2003. However, don't expect to see a lot of additional 42-V vehicles announced for a few more years. Car-makers are finding alternatives — smarter ways of using the 14-V power systems they already have — and ways of eking more power out of new 14-V components. Without high-volume opportunities to drive down the cost of components and end products, even hybrid electric vehicles (HEVs) that are growing in popularity will use high voltage only for the traction motor and 14 V for all the legacy electrical/electronic equipment. (Note that 14-V systems are sometimes referred to as 12-V systems because that's the more-familiar battery voltage.)
The recently published report, “Power Management in Today's and Future Automotive Systems”[1] discusses several design issues in the vehicle's power-supply side, such as batteries, super capacitors, flywheels, fuels cells, alternators and starter-alternators. It also examines demand-side issues, including vehicle loads and architectures, and provides significant detail on high-efficiency auxiliary motors, switched reluctance motors and materials selection.
Increasing Power Demand
Fig. 1 shows the potential trend for average power consumption in the current decade. The current trend shows that 150 W will be added each year and a step increase of 2 kW will occur at some point based on vehicle loads, such as electric air conditioning, electromagnetic valves, electrohydraulic brakes and even electrical power steering (EPS) for larger vehicles. These and other loads that typically have peak power requirements of 1 kW or more are the real drivers of 42-V systems. Another factor has been the 42-V integrated starter-generator (ISG), which also is referred to as the integrated starter-alternator. The ISG provides a stop-start function for reducing fuel consumption. Such stop-start systems have been implemented on at least two production vehicles.
Loads that have implemented recently with the existing 14-V bus include power running boards, superchargers, and electric power steering. However, the challenges for efficient operation and adequate cooling require advanced power MOSFET technologies and, in the case of electric power steering, are limited to smaller vehicles.
Table 1[2] shows an example of why the increased power-steering load on larger vehicles dictates a higher voltage supply. Since the motor size increases with the additional load and its subsequent torque requirements, the 14-V system eventually becomes inadequate for certain loads.
As the data in Table 1 illustrates, the motor design is not acceptable for a 10,000-N power-steering load on a 14-V system for future middle-class to luxury-class cars or large SUVs; the peak current, controller and harness losses would be excessive. The peak motor current at 14 V would actually be even higher to allow for the controller and harness losses.
To implement a 42-V EPS system on larger vehicles without implementing a 42-V charging system, a dc-dc converter and possibly a supercapacitor will be used. The result is a localized 42-V system to address a specific load problem. Localized dc-dc boost conversion has already been used to provide supplemental electrical heat in at least one European vehicle.
Impact of Improved Efficiency
Improved switching technology may allow some systems to be implemented on a 14-V bus that would have previously required 42 V. In addition, choosing the right voltage rating significantly impacts the on-resistance for a given MOSFET die size and the power dissipation in the system.
Today's 14-V vehicles typically use 40-V rated MOSFETs, but 55-V, 60-V and even 30-V devices are not uncommon. The overvoltage occurrences in these systems are well understood, and system designers can choose the voltage rating that's right for the particular system.
The safe operation of a power MOSFET requires operating below the maximum junction temperature. Factors that will cause the junction temperature to increase include the impact of steady-state losses caused by on-resistance, switching losses and operation in the avalanche mode. The maximum current that can flow through a MOSFET under steady-state conditions can be calculated by:
Where:
TJ is the maximum allowable operating junction temperature
TA is the maximum ambient temperature
*RDS(ON) is at maximum (175°C) junction temperature RT (or RN) is the total steady-state thermal resistance for the ambient to junction path via the heatsink.
Typically, RT includes:
R Ht/sk (Heatsink)
R sub (Substrate)
R Mtg (Interface thermal resistances often included with a substrate figure)
R JC (Junction-to-case)
*RDS(ON) is the effective resistance of the MOSFET corrected for maximum operating temperature by multiplying the room temperature rating by a factor shown on the data sheet based on maximum junction temperature.
Power MOSFET improvements have not advanced at the same pace as the high-level computing chips, but they have made remarkable continuous improvements since their first usage in vehicles. The equivalent cell density has increased by about 100 times since 1979, and the R*A product for automotive 40-V N-channel MOSFETs has been reduced to about one-fourth of its 1997 value. Note that the 42-V specification was still being defined in the 1997 time frame.
To understand the sensitivity of a power MOSFET in today's 14-Ω system, the impact of a 1-mΩ reduction with a trench MOSFET design (Fairchild Semiconductor's FDP022AN03LDO) to a theoretical (3.2-mΩ) MOSFET is shown in Table 2 using the equation for IDSS shown above. The maximum ambient temperature (TA) of 125°C for engine compartment components is used in this example. The following notes are applicable:
-
RTH(CS) is the typical value achievable using available isolating/lubricating pad material.
-
Range of typical achievable values is variable. A value of 1°C/W is typical.
-
IDSS = √(ΔT/RTH(JA)*R*RN).
-
All power MOSFET data are taken from the data sheet.
-
This assumes only static power loss due to RDS(ON).
The 1-mΩ reduction means that an additional 16.7 A (= 79.7 A - 63.1 A) can be conducted through the lower-on-resistance (2.2-mΩ) MOSFET. Increasing the amount of the heatsink to reduce the RTH(SA) to 0.5°C/W only allows the higher on-resistance part to conduct 8.8 A (= 71.9 A - 63.1 A) more — less than half of the impact of a 1-mΩ reduction in on-resistance. (Note: This analysis has neglected switching and avalanche losses but provides a starting point for evaluating improved performance capability.)
Changes in Power Sources
The limits of the 14-V alternator were around 3 kW when initially analyzed by members of the MIT Industry Consortium more than five years ago. The MIT Consortium has been the driving force for establishing a global 42-V standard. More recently, using many of the techniques that will be implemented in 42-V systems, such as power MOSFETs instead of diodes and liquid cooling, the belt-driven 14-V alternator's capability has been increased to 5 kW without decreasing the belt or bearing life. Delphi designed this system for the Mercedes Maybach, a vehicle loaded with electrical and electronic equipment.
For a quick calculation, the 6 kW required from the engine to drive a 50% efficient alternator provides 3 kW. The same 6 kW with an alternator that is slightly more than 80% efficient can provide 5 kW. As a result, even with the current trend of 150 W added each year and a step increase of 2 kW at some point, an advanced 14-V alternator may be able to handle the power until the end of the decade. This advanced 14-V alternator is more expensive than the existing Lundell design but a much lower-cost alternative to a transition to 42 V.
Recently, companies such as Visteon and Denso have introduced 14-V alternators that are rated at 3 kW and aimed at more mainstream vehicles. Fig. 2 shows Visteon's SpeedStart 12 belt-driven integrated starter-generator. This system uses a design methodology similar to designs for 42-V systems with MOSFETs controlling the phases instead of rectifiers to reduce losses and liquid cooling. The system is designed to work with the latest valve-regulated lead acid (VRLA) batteries. VLRA batteries are an improvement over the traditional flooded lead acid battery that provide a near-term alternative to costlier solutions of NiMH or Li-ion technologies for vehicles.
Future Battery Requirements
To meet future vehicle requirements, several system changes could occur that will impact the vehicle's power supply, especially the battery:
-
In Europe, new low-emission direct-injection diesel engines scheduled for the 2004-2005 time frame require 800-A peak current to give higher cranking speeds for a quicker cold start.
-
Higher current peaks for electric steering, braking and suspension require increased leveling loads on the battery.
-
Critical safety and security loads require a back-up battery for keyless entry, electric handbrake, electric braking (first stage is electro-hydraulic brake-by-wire) and electric power-assisted steering.
At least one vehicle manufacturer (Mercedes) has already added a second 12-V battery to cope with the peak loads on a particular vehicle. While this has doubled the battery cost, it's a cost-effective alternative to the transition to 42 V.
42-V and Hybrid Vehicles
The transition to 42 V is frequently discussed with hybrid vehicle technology. However, 42 V was initially viewed as an issue separate from vehicle propulsion. Using the 42-V portion of an integrated starter-alternator as a motor to supplement the vehicle's internal combustion engine occurred as a solution to a different problem.
As John Miller, formerly of Ford Motor Co. and now with J-N-J Miller Design Services and the outside representative for the MIT 42-V Industry Consortium, notes, “The mild hybrid terminology came about once the capability of integrated 42-V starter-generator could be tested on vehicles.”
Subsequently, belt-driven 42-V ISGs and even 14-V belt-driven ISGs, such as the Visteon and Denso machines, have been developed. Restarting the internal combustion engine within a time frame of 400 ms or less is a key system requirement.
Presently, the higher-voltage hybrids do not use 42 V for vehicle loads. The two 42-V vehicles use a 42-V ISG as a mild hybrid to improve fuel economy. Toyota's Crown model was the world's first 42-V vehicle, but it's only available in Japan. GM's Sierra and Chevrolet's Silverado 42-V vehicles were introduced in 2003 for commercial fleet purchases.
The GM vehicles have 10% to 12% better fuel economy than a vehicle with a standard gasoline engine due to the use of the stop-start function provided by the 42-V motor.
The Honda Insight and a Civic version using the same powerplant and Toyota's Prius have been in production for a few years but are smaller vehicles. The 2004 Prius (or Prius 2) brings hybrid performance to a mid-size vehicle and is sold outside of Japan. To handle the larger size vehicle, the bus voltage was increased from 288 V to 500 V (max) to power a 50-kW motor.
The Ford Escape and GM's Saturn Vue, which have yet to be introduced, will use 300-V bus for the traction motor. Until cost-effective 42-V equipment is available, these full hybrids will continue to use traditional 14-V electrical and electronic equipment.
References
-
This article is based on Intertech's “Power Management in Today's and Future Automotive Systems.” Visit www.intertechusa.com/studies/PowerManagement/PM_Study.htm for more information.
-
Sunil Murthy, Tomy Sebastian and Buyun Liu, “Implications of 42-V Battery Power on the Design of Motors for Electric Power Steering System,” Future Transportation Technology Conference & Exposition, Costa Mesa, Calif., August 2000.
For more information on this article, CIRCLE 345 on Reader Service Card
Supercapacitors
Supercapacitors have been in development and evaluation for solving critical vehicle power requirements for several years. One of their first implementations may be for EPS systems. EPS has a high peak load (100 A) when a front wheel is parked against the curb (AKA curbing) and needs to be turned to exit a tight parking space. This situation can occur at engine idle with low alternator output and high start-up loads (window de-icing) that would reduce the alternator's ability to maintain charging voltage.
Since battery power could be down after a cold start, its power would also be limited. A more fail-safe solution is required. In at least one case, supercapacitors will be used in 2005 to provide distributed power for an electrically assisted power-steering system.