Development direction of vehicle power supply system: 48V and distributed

Tesla's stock price continues to rise, proving the prospects of electric vehicles/hybrid vehicles and the market's need for greener/energy-saving travel methods. In order to reduce system power consumption, both traditional fuel vehicles and electric vehicles are trying to directly add a 48V DC bus system to the power system.

What are the benefits of using 48V? According to Ohm's law, the higher the voltage, the smaller the current, the smaller the loss and heat dissipation of the wire, which is why the power grid uses high-voltage transmission and distribution. For automobiles, the current 12.6V lead-acid battery is a long-standing compromise that takes into account factors such as size, capacity, cost, life and safety.

However, today's cars need to have more electronic equipment than cars a few years ago. In addition to basic functions such as engines, lights, radios, electric sunroofs, electric power steering, networks and infotainment, many other safety and convenience functions are now integrated, especially high-power ADAS systems. If 12V power supply is continued to be used, it will cause heating and loss of the wiring harness. If thicker wiring harnesses are used, it will not meet the lightweight and cost requirements of the car.

The birth of 48V power supply system

Unlike traditional 12V lead-acid batteries, the 48V battery uses a lithium battery system with higher weight and volume energy density.

In some ways, the 12V/48V mild hybrid system is a compromise solution, but unlike other compromise solutions, this solution has many advantages, reducing costs and improving efficiency and return rate (Source: Delphi Technology)

In this approach, a 48V battery - as specified in automotive standard LV148 - is used in conjunction with a 12V lead-acid battery (which is rated to provide 3KW of power) as a supplement. The 12V and 48V batteries will be used together with their distribution harnesses, with both batteries providing energy output when power output is needed. For example, the 12V bus can provide power for the ignition, lighting and infotainment systems, while the 48V bus will be responsible for chassis control systems, air conditioning, adjusting the suspension, recovering braking energy, driving electric superchargers and turbines, etc.

A dual voltage mild hybrid approach will effectively distribute the voltage and available power, which makes the most sense from an efficiency and operation perspective (Source: Delphi Technologies)

Consider a basic component: the starter. How does a 48V battery change the basic architecture of the engine starter, thereby better enhancing the start-stop effect, which is conducive to reducing fuel consumption when waiting at traffic lights and parking, and further enhancing fuel efficiency. In most cars with only 12V batteries, the battery provides power for the starter and the alternator serves as a charger. However, the combination of 48V and 12V can provide up to 10KW of power. The starter motor and independent alternator are replaced by an integrated starter generator (ISG) or a belt starter generator. The 48V lithium-ion battery and bidirectional DC/DC converter will be enclosed in a rugged box. This is actually a mild hybrid system that works in conjunction with the internal combustion engine. It can provide two-thirds of the benefits, but only one-third of the cost, and the economic benefits are increased by 15% to 20%.

Why 48V and 12V

More than 10 years ago, the industry's discussion on 24V was very intense, and some major manufacturers have even begun to produce 24V connectors, switches, relays and other basic components. In addition, the debate on whether cars should completely eliminate 12V batteries and only use 24V batteries, or let them coexist, has not been resolved. However, 24V voltage cannot better reflect the advantages brought by high bus voltage.

At the same time, in most areas, DC voltages exceeding 60V are considered unsafe and require special wiring precautions, safety locks, the use of conduits and physical shielding, etc. Therefore, 48V has become the mainstream standard, and the peak value of the 48V system will be lower than 60V, which means that safety requirements are met, thereby reducing steps such as safety certification.

The power management challenge

In addition to 48V, electric vehicles also have 800V/400V power battery systems. These high-voltage systems bring more challenges to power electronics design.

“Today’s challenges with electrification are: reducing costs, reducing CO2 emissions and the changing demands of power management, powering traditional 12V loads, increasing power levels for lighter, higher performance vehicles, accelerating charging times, and managing 800V and 400V battery systems,” said Patrick Wadden, global vice president of automotive business development at Vicor.

Manufacturers of cars, trucks, buses and motorcycles are trying to adopt more power electronics to improve the fuel efficiency of internal combustion engines and reduce carbon dioxide emissions. There are many electrification options, but most manufacturers choose 48-volt mild hybrid systems instead of full hybrid systems. In a mild hybrid system, a 48V battery is added in addition to the traditional 12V battery.

“When there is an 800V or 400V battery in the car, Vicor can directly convert 800V or 400V to 48V to power loads such as electric turbines, windshields and cooling pumps. By creating a 48V power bus, the 48V lithium battery power system can be completely eliminated, providing OEMs with higher power density while reducing weight and size, and these solutions are fully scalable to support a variety of needs from entry-level to luxury levels.

Figure: Enabling a virtual 48V battery (Source: Vicor)

Figure: System switching from 12V to 48V (Source: Vicor)

48V technology distributes power efficiently

48V technology can increase power capacity by 4 times (P=V*I), which can be used for heavier loads such as air conditioning and catalytic converters at startup. To improve vehicle performance, 48V systems can power hybrid vehicles, achieving faster and smoother acceleration while saving fuel.

“The biggest challenge is people’s hesitancy in the face of new technology,” said Wadden. “For the automotive industry, 48V mild hybrid systems offer a way to quickly introduce new vehicles with low emissions, long range, good fuel consumption and practicality. It also provides new and exciting design options for higher performance and functionality while also reducing CO2 emissions.”

Most centralized DC-DC converters are large and heavy because they use the old PWM low-frequency switching topology, so there is little demand in the industry to replace them. A more advanced architecture uses power modules and distributed power rails.

“The benefits of using a decentralized model can be realized at the system level, with lighter cables around the vehicle,” Wadden said. “There are benefits to placing the converter closest to the load, which minimizes impedance and resistance, simplifies cooling methods, and in some cases even eliminates cold plates or liquid cooling. This allows for more options and flexibility in achieving functional safety.”

This power delivery architecture uses smaller, lower-power 48V-12V converters, so the distributed power structure provides significant thermal management advantages in the power system.

“Let’s look at a diagram of a centralized system versus a decentralized system. On the left is a centralized traditional architecture with a 3.3KW onboard charger with a 400V input to a 12V output to power the 12V loads in the car. On the right is a distributed power architecture: the converter is placed at the load point, and the distributed architecture eliminates the onboard charger and distributes power around the vehicle as needed. This also allows for the use of redundant power supplies to achieve ASIL FUSA. As power demand increases, it becomes increasingly difficult to manage, and these older onboard chargers need to be eliminated,” Wadden said.

The new 48V PDN supports traditional 12V loads and can support new high-power drive, steering and braking systems using traditional wiring harnesses. High-density modules have advantages over larger, bulkier discrete solutions as loads continue to increase. Vicor offers a variety of modules for 48V power delivery, these devices include fixed ratio and adjustable conversion solutions, supporting 48V and 12V loads in buck or boost mode. These converters can be housed in a single housing or distributed throughout the vehicle using smaller and lighter 48V PDNs.

Figure: The difference between centralized and distributed architecture (Source: Vicor)

Figure: Comparison of power consumption loss between traditional mode and distributed mode. In comparison, the power consumption loss of centralized mode is four times that of distributed mode (Source: Vicor)

Figure: Vicor solution (Source: Vicor)

Vicor’s NBM family of products are the choice for OEMs when they need to place voltage converters around the vehicle closest to the load, either stepping down 48V to 12V or stepping up 12V to 48V.

When using 400V and 800V charging stations, the compatibility of the vehicle with any charging station requires the conversion solution to be as simple as possible, but most importantly efficient. The NBM6123 provides 6.4kW fixed ratio 400V and 800V conversion in a 61 x 23mm CM-Chip package, enabling a scalable, efficient, high-density solution for compatibility between roadside charging stations and different vehicles. The bidirectional capability of the Vicor solution allows the same module to be used for step-up or step-down conversion. The NBM6123 can also be used to deliver air conditioning to the vehicle during charging, minimizing battery balancing circuits.

in conclusion

Today, the electrification of vehicles is evolving in many forms and the power delivery systems are very complex. A vehicle has many different systems, each of which may have different power requirements. A modular power approach is inherently more flexible and scalable to meet a variety of challenges. Vicor's high-performance solutions are small and lightweight, designed to solve power conversion, charging and delivery problems in any system.