The chassis is a common ground potential for all 48-volt ECUs in the car. As the chassis has a non-zero impedance, a significant return current will be conducted through it, and a portion of this return current will find its way through a parallel path: the copper cables‘ shielding. An OEM has stated that the shield of Shielded Twisted Pair (STP) cables can conduct more than 8A of return current due to the 48-volt jump start effect.
In addition, the need for a ubiquitous communications network within the vehicle, and particularly between ECUs belonging to different voltage domains, represents a source of potential hazards. Thus, it imposes the additional requirement of galvanic isolation between the communicating nodes. Any event that could cause the 48-volt to cross into the 12-volt might destroy the ECUs in the 12-volt domain, provided the line transceivers do not provide sufficient galvanic isolation.
With regulations driving car companies to reduce Greenhouse Gas (GHG) emissions further by 2021, a new hybrid architecture concept based on a two-voltage power line (12-/48-volt) is already in the advanced marketing announcements of OEMs and Tier-1. As a further example of this new industry-wide technological trend towards 48V power supply and the handling of it, the German VDA published recommendation 320, which covers electric and electronic components in vehicles for the development of a 48-volt power supply. It defines requirements, test conditions and tests performed on electric, electronic and mechatronic components and systems for use in motor vehicles with a 48-volt on-board power supply.
48 Volt Jump-start Parasitic High Energy Pulse
48-volt-based energy networks or mixed 12/48 volt topologies are and will keep being the mainstream of HEV and Plug-in Hybrid Electrical Vehicle (PHEV) powertrains. The electrical ground, which is connected to the vehicle chassis and is common to high and low voltage ECUs, creates problems on start-up events that are continuously taking place in such powertrains. For example, the infotainment system shares the electrical ground with the energy generation and control systems. The high return currents flowing through the chassis on start-up couple into the infotainment low-voltage system through the cable shielding, which is connected to the same electrical ground of the vehicle. Copper-cable shielding provides a parallel return path (alternative to the chassis) for the currents of the diverse ECUs. Due to this, currents higher than 8 ampere can be measured in the cable shielding during a typical jump start. If the communications link between the ECUs in the low-voltage systems like infotainment or ADAS is optical, then the native galvanic isolation will isolate them from the high voltage/high energy systems and their associated events, thus preserving their reliability.