Tesla recently announced that 48V low-voltage systems will be used in all future Tesla electric vehicles. As the industry moves in this direction it poses an opportunity and a challenge for OEMs and Tier 1 suppliers to adapt. Adopting a zonal architecture that is decentralized, where 48V is being converted to 12V at the load, is the most efficient way to architect this type of system. Small, power-dense Vicor modules make it easy to design and build a zonal architecture to support xEVs.
Cars, trucks, buses and motorcycles makers are rapidly electrifying their vehicles to reduce CO2 emissions. OEMs are taking multiple approaches to electrification, hybrid powertrains, plug-in hybrids (PHEVs) and battery-electric vehicles (BEVs) are the primary paths of electrification. While hybrid and PHEV powertrains retain an internal combustion engine and are tightly linked to an alternator-based 12V PDN, BEV platforms give the OEM a clean sheet in designing the PDN for fully electric vehicles. However, there’s a general hesitancy to modify the long-standing 12V power delivery network (PDN). Changes often require new technologies that need extensive testing and may require new suppliers that can deliver the automotive industry’s high safety and quality standards.
Maximizing a 48V PDN using power modules In a BEV platform the power source is a high voltage (HV) (400 or 800V) battery, and that high voltage needs to be reduced to a safety extra low voltage (SELV), which is under 60V. The first workable level of SELV is 48V, or the OEM can step down power to 24V or 12V for the vehicle PDN. Now there is a choice of adding systems that can deal directly with a 48V input, or to retain legacy 12V electromechanical loads such as pumps, fans and motors and instead convert the 48V to 12V via a regulated DC-DC converter. In order to manage change and risk, existing BEV power delivery systems are slowly adding 48V loads but still use a large centralized multi-kW HV-to-12V converter that feeds 12V around the vehicle. However, this centralized architecture does not take the full advantage of a 48V PDN, nor does it utilize the benefits of available advanced converter topologies, control systems and packaging.
The vast majority of these centralized DC-DC converters (Figure 1) are bulky and heavy, since they use older low-frequency PWM switching topologies. They also represent a single point of failure for many critical powertrain systems. These centralized systems also concentrate the thermal load at a single point, requiring a significant cooling system (Figure 3).
A different architecture to consider is zonal power delivery (Figure 2) with modular power components. This power delivery architecture uses smaller, lower-power 48-to-12V converters, distributed throughout the vehicle close to the 12V loads. The simple power equations P = V • I and PLOSS = I2R explain why 48V is more efficient than distributing 12V. For a given power level, the current is four times lower at 48V than in a 12V system and has 16 times lower losses (I2R). At ¼ of the current, the cables and connectors can be smaller, lower weight and cheaper. The zonal power architecture also has significant thermal management and power system redundancy benefits (Figure 4). It’s another way of delivering kilowatts of power around the vehicle without the weight, thermal concerns and volume of a traditional DC-DC converter.
Modularity for zonal architectures optimizes efficiency
A modular approach to a decentralized power delivery (Figure 4) is highly scalable.
The 48V output from the battery is distributed to the various high-power loads in the vehicle, maximizing the benefits of lower current (4x) and lower losses (16x), resulting in a physically smaller and lighter PDN. Depending on a load power analysis of the various distributed loads, one module can be designed and qualified for the right power granularity, and then it is possible to scale the power level of the system upwards by using it in parallel arrays.
In this example, a 2kW module is shown. As noted, the granularity and scalability are system dependent. By using dispersing the modules to the endpoint zones instead of a large, centralized DC-DC converter, N+1 redundancy is also possible at a much lower cost. This approach is also advantageous when load power changes during the vehicle development phase. Instead of implementing changes to a full ground-up custom power supply, engineers can either add or eliminate modules. In addition, the module is already approved and qualified, reducing development time.
Implementing a scalable zonal modular 48V architecture
In the case of pure electric vehicles or high-performance hybrid cars, high-voltage batteries are used due to the high power demands of the powertrain and chassis systems. A 48V SELV PDN still has significant benefits for OEMs, but now the power system designer has an additional challenge of a high-power 800V-to-48V or 400V-to-48V conversion.
This high-power DC-DC conversion also requires isolation but because using a regulated converter over this range is very inefficient and requires a large thermal management problem, this conversion should not include regulation. By using downstream regulated point-of-load converters, the high-power upstream converter can use the more efficient fixed-ratio topology. This is extremely beneficial due to the wide input-to-output voltage range of 16:1 or 8:1 for 800/48 and 400/48, respectively (see Figure 5). OEMs often locate this efficient step-down solution inside the battery pack itself, and in some cases even eliminate the battery. Vicor fixed-ratio high-voltage bus converters deliver rapid current delivery at fast slew rates, enabling OEMs to lose the 12-14kg of unnecessary 48V battery weight.
It is very difficult and costly to decentralize the high-voltage isolated converter due to safety requirements in distributing 400 or 800V. However, a high-power centralized fixed-ratio converter can be designed utilizing power modules instead of a large silver box DC-DC converter.
Power modules are highly scalable and easily paralleled for a range of vehicles with differing powertrain and chassis electrification requirements. Vicor BCM® fixed-ratio bus converters are also bidirectional, which supports various energy regeneration schemes. Due to the Sine amplitude Converter (SAC™) high-frequency soft-switching topology, BCMs achieve efficiencies over 98%. They also feature power densities of up to 2.6kW/in3, which significantly reduce the size of the centralized high-voltage converter.
Tesla has thrown down the gauntlet. They are committed to moving to 48V, the next essential step in the global electrification of automobiles. Others will follow suit. The race to develop the best BEV will require pushing the boundaries and introducing new technologies to stay ahead of the pack. Upgrading the power delivery network to 48V is an obvious next step. The most efficient way to adopt 48V and maximize the PDN is to move to a zonal architecture using power dense power modules. In addition to the benefit of lighter 48V cable weight, power modules enhance thermal efficiency and deliver the highest 48V-to-12V conversion efficiency throughout the vehicle. Furthermore, compact power modules easily scale and are a seamless complement to moving to a zonal 48V architecture.
Source : Vicor
