COVER STORY
48V/12V dual battery automotive systems require bidirectional DC/DC controllers for optimum performance
With fuel economy regulations tightening and autonomous-driving capability with connectivity proliferating, the old-fashioned 12-volt automotive electrical system has reached its usable power limit. Furthermore, a vast increase in automotive electronic systems, coupled with related demands on power, has created an array of new engineering opportunities and challenges. As a result, the 12V lead-acid battery automotive system with its 3kW power limit has been supplemented. Bruce Haug, product marketing manager, Controllers, Analog Devices, Inc. explains
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newly proposed automotive standard, LV148, combines a secondary 48V bus with the existing 12V system. The 48V rail includes an integrated starter generator (ISG) or belt start generator, a 48V lithium-ion battery and a bi-directional DC/DC converter for delivery of up to 10kW of available energy from the 48V and 12V batteries combined. This technology is targeted at conventional internal combustion automobiles, as well as hybrid electric and mild hybrid vehicles, as auto manufacturers strive to
meet increasingly stringent CO2 emissions targets. Typically, the 12V bus will continue to power the ignition, lighting, infotainment and audio systems. The 48V bus will supply active chassis systems, air conditioning compressors, adjustable suspensions, electric superchargers/turbos and also support regenerative braking. The decision to use an additional 48V bus, which is expected to be available across production model ranges soon, can also support starting the engine, which would make stop-start
operation smoother. Moreover, the higher voltage means smaller cable cross-sections are needed which reduces cable size and weight. Today's high-end vehicles can have more than four kilometers of wiring. Vehicles will become more like PCs, creating the potential for a host of plug-and-play devices. On average, commuters spend nine per cent of their day in an automobile. Thus, introducing multimedia and telematics into vehicles can potentially increase productivity as well as providing additional entertainment.
The key components for autonomous driving include a computer, cameras, radar and LiDAR sensors, all of which require additional energy. This additional energy is required to improve vehicles’ connectivity, not just to the Internet, but to other vehicles and buildings, traffic signals and other structures in the environment. Furthermore, drivetrain components, power steering, oil and water pumps will switch over from mechanical to electrical power. The future for the 48V battery system is much more near-term than the fully autonomous car, although many automotive suppliers see strong demand for the technological building blocks ultimately needed for self-driving vehicles
Figure 2: LTC3871 typical applications block diagram
over the next few years. According to some auto manufactures, a 48V based electrical system results in a 10-15 per cent gain in fuel economy for internal combustion engine vehicles, thereby reducing CO2
emissions.
Moreover, future vehicles that use a dual 48V/12V system will allow engineers to integrate electrical booster technology that operates independently of the engine load, thereby helping to improve acceleration performance. Already in its advanced development phase, the compressor is placed between the induction system and intercooler and uses 48V to spin-up the turbos. Nevertheless, the implementation of an additional 48V supply network into vehicles translates into major design challenges for suppliers across the value chain. In particular, providers of semiconductors and Electronic Control Units (ECUs) will be affected – they will need to adjust their operational range to the higher voltage and in part re-design their products. Correspondingly, the manufactures of DC/DC converters will need to develop and introduce specialised ICs to enable this high power transfer. Accordingly, the Power by Linear Group has designed and developed a number of DC/DC converters which are able to facilitate this energy transfer with very high efficiency to both conserve energy while also minimising the thermal design required due to their much lower power loss.
It is clear that there is a need for a bi- Figure 1: LTC3871 bidirectional schematic 12V output from a 26V to 58V input, delivering 30A of current 8 November 2018 Components in Electronics
directional step-down and step-up DC/DC converter that goes between the 12V and 48V batteries. This DC/DC converter can be used to charge either battery and
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