Zonal automotive power distribution can dramatically improve efficiency, weight, and cost: part 3

If something can’t go on, it won’t; supplying the automobile’s voracious electrical demands from a 12-V battery is an example of this reality.

If you haven’t already, check out Part 1 and Part 2 of this discussion.

Under the zonal architecture, many legacy loads will still be supported by a 12-V battery or its functional equivalent derived from a 48-V battery. In the mild-hybrid system, a 48-V battery is used alongside the traditional 12-V battery; in a full-hybrid design, the 12-V battery is eliminated, and 12 V is derived from the 48-V battery.

However, it may be possible to eliminate the 12-V battery in EVs/HEVs and the 48-V battery. All the lower-voltage rails can be derived from the higher-voltage vehicle battery packs (presently either 400 V or 800 V).

Some insight on the advantages of going “full zonal” by eliminating the 12-V and even the 48-V batteries comes from Vicor Corporation, a leading proponent and DC/DC converters manufacturer, which are critical to the implementation. While there are various sets of numbers depending on the installation specifics, even one example of replacing a 12-V PDN with 48 48-V clearly indicates the magnitude of the benefits with respect to cabling weight, PDN efficiency, and other aspects of this new approach (Figure 1).

Figure 1. A distributed 48-V power distribution system makes it easy and economical to derive 12-V power for “legacy” systems at the point of consumption (Image: Vicor Corp).

Eliminating the separate 12-V battery and relying solely on the 48-V battery achieves substantial, quantifiable savings (Figure 2).

Figure 2. Moving to 48 V enables lighter, lower-cost wiring harnesses with significantly reduced power losses due to resistive heating (Image: Vicor Corp).

Key to this battery elimination is the availability of small, efficient DC/DC converters, which regulate the 48-V output down to 12 V and can be located closer to the load rather than at the 48-V battery. This reduces cabling losses while supporting legacy 12-V loads if needed.

A good question is: Why not try to eliminate the 48-V battery? That would save further space, reduce weight, and cut costs (Figure 3).

Figure 3. It is possible to eliminate the 48-V battery in EVs/HEVs and derive the needed 48 V from the 400/800 V battery pack (Image: Vicor Corp).

If this idea is so attractive, why hasn’t it been adopted sooner? There are several reasons:

Long-established legacies are difficult to displace, especially when the supply chain, manufacturing, costing, and post-sales support considerations are well known. Eliminating lower-voltage batteries is a significant shift in PDN strategy and execution.

The need has increased dramatically in the last few years and looks to be getting more intense, but it hasn’t yet reached an insurmountable limit.

The new solution must be electrically superior and offer lower volume, weight, and cost than the batteries and power regulators it replaces.

Finally, the technical capabilities of the available DC/DC converters and regulators were not adequate.

The last point most immediately affects designers, as the proceeding is useless if the essential building blocks are not available and credible. Of course, these converters must be cost-effective and reliable.

In one analysis, Vicor shows a centralized PDN with losses of 180 W that has its losses reduced to 45 W with a 94% efficient converter using a decentralized system. While that is impressive, it may not be enough to justify the transition. However, using their 98% efficient converter, the losses are cut to just 15 W, which is nearly an order of magnitude improvement. This is in addition to weight and volume savings.

In addition to efficiency, there’s another DC/DC converter consideration related to transit response. Batteries respond fairly well to load transients (primarily sudden surges), which are a natural and unavoidable aspect of automobile operation. Their transient response is limited by their internal resistance and the resistance and inductance of the cables.

Until recently, the transient response of the DC/DC converter in dynamic load conditions was not fast enough. Unlike batteries, they are closed-loop designs that rely on feedback to slew properly and crisply. New converter designs, however, have overcome this limitation (Figure 4).

Figure 4. High-efficiency, compact 800 V (and 400 V) to 48 V DC/DC converters and 48 V to 12 V DC/DC converters are the building blocks that make new PDNs and eliminate 48-V and 12-V batteries possible (Image: Vicor Corp).

Another good question is this: why even support 12-V at all? Wouldn’t it make more sense to run everything on 48 V except, of course, for the traction motors? Again, the answer is mainly tied to legacy considerations, but that factor does not constrain some auto companies to the same extent. For example, with their just-announced Cybertruck, Tesla plans to switch to 48 V for all non-motor power and eliminate the need for a 12 V source or PDN entirely.

Conclusion
Zonal power for automotive power distribution topology is a concept that is coming of age. The increasing electronic power demands within autos and advances in DC/DC technology make it necessary, achievable, and doable.

The classic solo 12-V battery is on the way out, with 48-V supplanting it and even taking its place. In some cases, there will be a tangible 48-V battery, but for EVs and HEVs, it may be a 48-V rail derived from a 400 V/800 V battery pack. Many rules of thumb, guidelines, legacy components, and approaches to powering the car will change dramatically, and new thinking and perspectives will be needed.