[Technology Report]
A Power Shortage Is Driving Automotive Applications To 42 V
Changes in the electrical system alter the landscape under the hood as well as foster the development of power semiconductors.
Looming under the hood, a power shortage is about to transform the automotive electrical system. The traditional 14-V power system is being taxed to its limits as car makers design-in a host of electronic features to improve the rider's comfort and convenience, while also boosting the car's performance in terms of handling, safety, and energy efficiency. Inside the cabin, sophisticated entertainment systems, climate controls, and other devices raise electrical power consumption. Meanwhile, the shift from mechanically controlled functions to electronic versions within the engine compartment poses an even greater threat to existing 14-V power budgets.
Currently, functions like antilock brakes and engine controls rely on power electronics tied into the car's electrical grid. Plus, drive-by-wire systems, which trade mechanical linkages for electronic controllers, sensors, and actuators, are now starting to implement power steering, braking, and throttle. Electronically controlled suspension systems that sense and adapt to drive conditions are in the works as well.
These new controls not only offer new ways of enhancing fuel efficiency and handling, but also eliminate some of the mechanical constraints of conventional car designs. Electronic actuators can be positioned more easily than mechanical assemblies that must be placed near the engine, where they derive power from the front-end accessory drive. Power-steering hydraulic pumps are an example of such assemblies.
With all of these systems coming on board, however, power demands are expected to rise sharply. Average power levels are presently around 1 kW with peak power demands in the vicinity of 2 kW. But, these levels are expected to rise significantly in the near future. Within five years, we may witness peak power levels of 12 kW.1 These levels are beyond what current 14-V systems can bear. Batteries and alternators won't be able to keep up. Even existing wiring will be inadequate as rising current levels produce unacceptable voltage drops.
The solution is to move to a higher system voltage, which boosts the power generating and handling ability of alternators, batteries, and cabling. That higher voltage is 42 V—a value selected by the MIT Consortium on Advanced Automotive Electrical/Electronic Systems and Components. Comprised primarily of automakers and their suppliers, this association is working to develop standards for a 42-V automotive electrical system. The consortium calls this system the 42-V PowerNet.
The consortium aims to define system requirements in a way that allows maximum flexibility. As Thomas Keim, director of the consortium, says, "There's no interest in standardizing the architecture or layout of components," because car manufacturers want to decide on this individually. Instead, the goal is to standardize critical system parameters, such as voltage range and battery termination. So, component vendors can develop a pool of components from which manufacturers will order.
Also, the consortium is addressing issues relating to safety. For instance, there must be some industry consensus on jump-starting requirements, particularly when the introduction of 42 V initially means a dual 14- and 42-V electrical system within cars.
Progress On A Voltage Range So far, the greatest progress toward standardization has been in defining the voltage range. A German organization known as Forum Bordnetz, consisting of German and other European car makers and suppliers, has developed a specification that establishes undervoltage and overvoltage limits, maximum allowable ripple, etc. (see the table). The MIT Consortium is endorsing this specification. The voltage limits determined by this proposed standard will have a direct impact on the development of the power components for emerging automotive applications.
Although there's still discussion on various approaches, it appears, at least in the beginning, that the 42-V system will coexist with 14 V. This will be a dual-voltage system where high-power loads operate off of the higher voltage and medium- to low-power loads run off of the lower voltage. Among the architectures being discussed are dual- and single-battery systems. In both systems, power that's generated by a 42-V alternator is rectified and stored in a 36-V battery (42 V when fully charged). A dc-dc converter is then used to generate the 14-V power bus.
In a dual-battery system, a 12-V battery (14 V when fully charged) follows the dc-dc converter, providing energy storage for the 14-V loads. In the single-battery approach, the 14-V loads run off of the dc-dc converter. Each approach has pros and cons. With a dual-battery system, 14-V loads that are active when the engine is turned off won't drain the battery used to start the car. The dc-dc converter also can be sized smaller because the 14-V loads can rely on the battery to provide peak power.
On the other hand, the 12-V battery adds weight and cost to the vehicle. Also, it's possible that isolation of 14-V loads from the 36-V battery can be accomplished through power management circuits rather than a second battery. When the 12-V battery is eliminated, there's an additonal issue: whether to generate 14 V with a single, centralized dc-dc converter or multiple point-of-use converters configured in a distributed power architecture.
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