48V architectures

When optimizing wiring harnesses for 48V architectures, OEMs will need to evaluate different architectures. Figure 4 through Figure 6 show three options when implementing a 48V low-voltage rail: 48V primary distribution and 12V local, 48V distribution and 12V distribution, or 12V distribution and 48V high current loads only.

Figure 4 48V architectures (48V primary distribution, 12V local).

Figure 5 48V and 12V distribution – ZCM 48V & 12V.

Figure 6 12V primary distribution, 48V high current loads.

The least disruptive approach to 48V design is to use the 48V rail to power high-current loads and keep everything else at 12V. 48V and 12V can be distributed to zone control modules or other ECUs; however, this approach poses some challenges. The distribution of two different voltages makes the routing of the wire harness a factor, because routing both 12V and 48V in the same wire harness could result in a potential short from 12V to 48V. Functional safety considerations will also add costs, as there may be a need for redundant 12V and 48V supplies.

A more drastic design change is to move directly to a 48V power distribution architecture and create a 12V rail locally as needed. 48V distribution with local 12V is the best architecture to achieve the full benefit of moving to 48V because it provides the greatest reduction in wire harness size and cost.

In a 48V distribution with 12V local, there are many different options for creating a local 12V rail at the ECU, or selecting a different voltage entirely (25V, 16V, 5V, 3.3V). Figure 7 provides two possible power architectures, distributed and central 12V, for 48V systems.

Figure 7 Voltage conversion from 48V at the ECU.

In a distributed architecture, multiple DC/DC converters with lower power requirements can create a 12V rail for different load groupings. This approach enables the use of DC/DC converters with integrated metal-oxide semiconductor field-effect transistors, along with the freedom to choose the voltage (such as 48V to 3.3V) and better thermal spreading across the PCB. If OEMs want to reuse existing 12V designs, a central 12V rail is the easier approach. In this architecture, an always-on DC/DC converter provides power to functional safety-critical loads, while a DC/DC converter with high power requirements supplies the rest of the 12V system. Another option is to use bidirectional 48V-to-12V DC/DC converters to allow back-electromotive force from the motors, or positive transient voltage energy from the 12V rail to flow back into the 48V rail.