Power Electronics

The Industry's Transition to 42-V Electrical Systems

The automotive industry knows it needs 42-V electrical systems, and the OEMs realize how painful the costs of such a transition will be. Can the present 12-V battery-based architecture continue to meet the demands of the future? This becomes more uncertain with each incremental watt of electrical load. We know that automotive OEMs made a big switch in the mid-1950s from 6-V to 12-V electrical power plants in vehicles. That change was primarily driven by a single, pressing need — that of a robust ignition system that could keep pace with the upward crunch of engine compression ratio.

Historically, the continual piling on of electrical load pushed the industry to identify alternatives to the now-burdened 12-V electrical distribution system. In 1988, a grassroots effort, led by a team of concerned engineers operating under the auspices of SAE, was established to tackle this impending ground swell in electrical loads. That effort was called the dual- and higher-voltage committee, and by 1994, it released a guideline recommending a move to four-times voltage, or 48 V. This was a rational choice given that a lead-acid battery under charge would require a float voltage of some 56 V. Four-times battery voltage is still within the 60-V limit that signifies entry into the realm of globally recognized hazardous voltage.

The group began disbanding in 1995, because the anticipated tidal swell of loads in North America simply didn't happen. It was expected that the electrical burden on the 12-V system would sharply drive up installation cost, cause the 12-V network to falter, or become unable to maintain regulation or stability; this didn't happen either. It's as if the 12-V world had taken on the form of a self-organizing system and orchestrated a move to push the acknowledged 3-kW limit to 5 kW.

Interestingly, at about this time in Europe, automotive OEMs facing the same dilemma decided to go before the global automotive community and make the case for a three-times battery voltage, or 42-V electrical network. A series of workshops was coordinated by MIT, which evolved into the Consortium on Advanced Automotive Electrical and Electronic Components and Systems. The consortium began activity to standardize to 42-V globally, among other tasks. This led to the preparation of an ISO standard now known as the 42-V PowerNet draft standard.

It has been 10 years since the 42-V standards activity began. In this time, the ISO draft standard has been globally accepted and endorsed by SAE and JSAE. Suppliers have shown that all existing electrical components can be recast into 42-V versions, and in many cases, particularly for those containing power electronics, at lower cost. So just what is holding up this transition? Did the 12-V world somehow self-organize, thereby postponing the onset of the relaxation mechanisms (factors forcing the changeover to 42 V) to somewhere at or beyond 5 kW?

Or maybe the relaxation mechanism lurks in the guise of electrified chassis and power-train systems, such as electric assist power steering or electrohydraulic brakes or belt integrated-starter-generator (ISG) that are being implemented at 12 V. With utility power grid brownouts and blackouts, we can determine the criticality level (or breaking point) in transmission power level. When loads exceed that critical level, the system relaxes by crashing catastrophically. With automotive 12-V systems, are we teasing the criticality limit with electrified ancillaries when we really don't know where the limit is?

We may soon see early signs of the impending relaxation mechanisms. With 12-V electric-assist steering, the telltale signs are loss of performance under high assist when the alternator output is low (engine idling) and so on for the other systems. Another sign may be fatiguing of the cable termination on the alternator or belt ISG or other high-power component, and its eventual fracturing under continuous exposure to vibration because of its heavy gauge. The point of this is that we may not know the exact criticality limit of 12-V systems, but what we do know is that for a 42-V system, it will be three times higher.

John M. Miller, Ph.D., is founder of J-N-J Miller Design Services, PLC, where he is principal engineer. He is currently outside representative on the MIT-Industry Consortium on Advanced Automotive Electrical and Electronic Systems and Components. Miller is a fellow of the IEEE, the recipient of the Henry Ford Technology Award for the development of the starter-alternator system for hybrid vehicles, the recipient of IEEE's Third Millennium Medal, and the editor-in-chief of the IEEE Power Electronics Society newsletter.

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