Several documented standards, including MOST Cooperation, SAMSON or AMIC, IEEE1394, CAN and LIN, are doing a superb job defining APIs and other requirements for communications on an open-bus architecture. However, the common-bus architecture in these standards doesn't provide a clear definition for power distribution. The automotive environment, as outlined in SAE standards J1455 and J1211, is a harsh environment for electronic circuits.
Concurrently, the power generated within an automotive electrical system is subject to multiple transient conditions. Without circuit protection, most electronic circuits in today's voltage converters wouldn't survive. And with more features being added to vehicles, the power demands on the power source and distribution continue to rise. Consequently, maintaining the 12-V bus with the increasing power demand will require increased current levels. Thus, larger diameter is needed to accommodate the losses in the copper wire if current levels increase. Automotive wire routing can exceed 30 ft, which may result in voltage drops of up to 3 V. This voltage loss may not be acceptable for units specified to receive 12 V. This is of particular concern as the regulation of the 12 V varies from 16 V to 9 V, depending on battery conditions. Lowering the bus voltage further aggravates this condition. Also, the trend with digital silicon devices is toward smaller geometries and lower voltages. Of equal concern are the dc-dc converters providing regulation at 1 V or below. A 1-to-14 duty cycle is needed to regulate a 1-V output from a 14-V source.
Distribution architectures have undergone an accelerated evolution in the past few years. The telecom industry, which was dominated by isolated products that converted the point of load (POL) voltage from a 48-V bus has evolved into intermediate voltage or intermediate bus architecture (IBA). Often, the driving force behind this evolution was cost.
While an IBA approach has a multiplying effect on the losses of the two conversion stages, the impact is minimal compared to the cost advantages of a single isolation barrier. Simply put, the architecture is more cost effective for multiple voltage requirements in a given system. To further the trend of eliminating redundant processes in the conversion stage, limited-regulation isolated converters, or bus converters, reduce the cost by providing a ratio-metric output voltage to that of its voltage source.
This form of nonredundant power sculpting can be extended to the automotive industry and can offer even more in a 42-V system. Within the 12-V automotive scheme, an intermediate converter provides a central location for suppression of transients and avoids the need for tertiary components to address transients on an individual basis, reducing the cost of those devices. A 12-V bus converter also provides a more tightly regulated 12 V (or slightly lower) to avoid the poor duty cycles associated with low-voltage regulation from a nominal 14-V system.
The IBA approach may have its biggest impact as automotive manufacturers move toward a 42-V system. In line with the section on losses associated with increased power, the 42-V system is proposed to increase voltage and reduce current. This minimizes I*R losses.
With a 42-V system, the IBA architecture of the data communications industry is a direct fit with a loosely regulated 42-V bus allowing for a ratio-metric bus converter providing an intermediate bus. From the intermediate bus, POL converters provide the final regulated voltage requirements. Also, the bus converter can have a central location for transient suppression. The result is an architecture designed to have suppression and regulation performed only once within the power distribution.
The intermediate bus approach for “autocom” power leads to cost reductions on each device added to the common bus. These cost savings let manufacturers offer more comfort and communications solutions for consumers. The power solutions for the end applications will receive a transient-free voltage and can be outfitted with a low-cost (POL) converter solution. Also, the bus converter will provide a ratio-metric voltage from the 42-V source and provide central transient suppression. The end result is a high-performance autocom power system at a reduced cost.
Eric Wilcox is responsible for Datel's dc-dc product development. He received the BSEE degree from the University of Connecticut and the MBA degree from Northeastern University. Wilcox has more than eight years of experience in the high-tech industry.