48V automotive systems for mild hybrid vehicles

By: John Grabowski, System Design Engineer, ON Semiconductor

Preface

As the world becomes more and more concerned about environmental conditions, especially air quality, CO 2 emissions from vehicles need to be reduced by reducing the average fuel consumption of vehicles. One way to achieve this is to use hybrid engines instead of pure internal combustion engine (ICE) powered vehicles.

在典型的汽车中，牵引动力系统必须能够在非常宽泛的功率和速度条件范围内运行，这通常以“转矩-速度”范围来表示。混合动力系统的采用让系统设计者能够自由地在不同的转矩-速度范围内，以优化每项系统性能的方式，对多重动力源进行布置。电动力源可提供非常高的转矩，且在加速汽车时非常有用，但它仅在有限的时间段内可用。所涉及的具体时间取决于电池大小和电机的转矩输出。借助这种可产生动力源的高转矩，ICE尺寸得以显著缩小。这使其燃油能效也大大提升。然而，添加电动力源当然并非简单的工程问题，需要一种有关许多汽车系统设计考量的方法。

Traditionally, electrification has been achieved by adding a high voltage ( ~ 350V) battery and a high performance electric motor directly coupled to the ICE powertrain. These 'full' hybrid vehicles have been the defining category of fuel efficient vehicles and are attractive from an efficiency perspective. However, they also add significant cost and weight to the vehicle.

48V automotive system architectures have received considerable attention recently. These systems can be considered a step towards full hybrid vehicles. Typically, they are called "mild hybrids", but when designed at lower power levels, they can also be classified as "micro hybrids". They use a relatively compact 48V battery, a high-performance electric motor, and at least one additional 48V electrification subsystem. The lower cost of 48V systems makes them attractive to many automotive OEMs, and they will soon become part of most automakers' product portfolios.

48V Architecture

The options for 48V architectures are wide and growing. The most basic system includes a battery, a starter generator, a 48V to 12V converter, and usually at least one 48V load. Since 48V cars still retain a 12V battery and multiple 12V loads, these systems may exist as dual voltage systems for now.

Figure 1. Typical 48V mild hybrid system electrical topology

With these dual voltage systems, a host of new configurations become possible. Since the 48V system is fundamentally capable of delivering higher power levels, it will enable new higher power peripherals such as 48V E-Turbo and 48V E-Roll stabilization systems. Additionally, the higher power availability will drive power hungry 12V loads to migrate to the 48V bus to take advantage of the higher energy efficiency.

Initially, the 12V system side of the dual voltage system will remain intact, minus the 12V alternator. Since there is no source of generation for the 12V power, a converter is required to transfer the power generated at 48V to the 12V side. Although the converter needs to be highly efficient, it still imposes a loss penalty on all 12V loads that need to draw power from the converter. Between the increased losses and power limitations, there is a strong incentive to move 12V peripherals to 48V operation.

These converters are bidirectional in design, allowing both batteries to be used simultaneously during periods of high demand. Bidirectional converters are able to transfer power from either battery to the other and may exist for some time in the future.

There is no technical reason to keep the 12V starter other than redundancy, and removing it may be a trend in the future. If it is no longer needed, the size of the 12V battery can be significantly reduced or even removed completely. But this would be a bold move and would require very careful design of the converter.

On the 48V system side, the starter generator is the main component. It is responsible for all power generation of the vehicle, as well as starting the vehicle. It also performs regenerative energy recovery during vehicle braking. In this mode, the machine acts as a generator to provide negative torque to the powertrain, slowing the vehicle and restoring battery charge. Starter generators are available in a variety of configurations and power levels, each with very specific implementation goals.

The hybrid community has adopted a shorthand code to identify the location of the electric motor within the chassis subsystem. This system generates labels using a set of Px indicators, indicating each location where the electric motor is coupled to the powertrain. The indicators (P0 to P4) increase in value as the power insertion point passes through the rear of the vehicle.

Figure 2. Px hybrid system terminology and example power levels

The power level of the BSG is minimal, as its insertion point is at the front accessory drive (FEAD) of the engine, where the transmission of torque must be connected via a steel belt. The remaining insertion points of the chassis (P2-P4) are all capable of higher power levels, as they are coupled via steel gears. In addition, the higher power machines have the additional function of being able to provide traction assistance to the ICE. This means that in addition to the power provided by the ICE, the electric power source can also improve the acceleration of the car. In some configurations, it is conceivable that the car can be driven by the electric power source alone, with the ICE turned off. This depends on the amount of traction assistance available from the electric machine and its position in the chassis subsystem.

48V Subsystem

The dual voltage system requires the addition of a 48V to 12V power converter. This component is required to power a 12V system in the absence of a 12V alternator. The need for bidirectional behavior and high efficiency requires a specialized design approach. The typical power range of these converters is in the 1kW to 3kW range, and in order to maintain high efficiency in such a large power range, the multi-level buck-boost converter is the most popular topology currently. The buck topology allows power to flow from the higher voltage side to the lower voltage side. Similarly, the boost topology allows power to flow in the reverse direction. The multi-level design allows many individual converter sub-circuits to be shared to combine into a high power design. When the converter output is heavily loaded, all sub-circuits are able to operate. When the converter output is lightly loaded, many sub-circuits will be turned off, providing lower losses and higher efficiency.

There are many different conceivable 48V loads, and many of the higher power loads cannot be achieved with a 12V system. The highest of these is the electronically controlled supercharger. Since the supercharger needs to accelerate to very high speeds in a fraction of a second, it requires quite high transient power. A typical supercharger drive consists of a low inertia three-phase electric motor driven by a three-phase inverter. Although the average power is relatively low, the peak power can reach over 8kW. This wide power range configuration is a perfect match for the 48V system. Many other automotive subsystems are also well suited to the 48V architecture, whether in single-phase or three-phase configuration. A list of possible 48V loads is shown in Figure 3.

Figure 3. Summary of mild hybrid subsystem components

Other 48V systems

The 48V battery system consists of lithium-ion cells, which require more attention and handling than lead-acid batteries. For this reason, 48V cars require a battery management system (BMS). This system is responsible for monitoring the battery voltage and battery temperature so that the battery can be charged safely. This situation is also further complicated by the fact that the 48V system has regeneration capabilities. When the remaining charge in the car battery is low enough, regeneration can be commanded, but the control of the BMS needs to be very careful, which is crucial to prevent overcharging or overheating.

48V circuits also have more complex requirements for fusing and contacts. It is not certain whether 12V blade fuses will provide adequate arc protection if used in 48V systems. Also, since the relay contact distance required for 48V systems will be greater than that required for 12V systems, fuses and relays will need to be redesigned. Since the requirements of these components can be easily met by using semiconductor devices, these problems are likely to be solved by electronic solutions .

in conclusion

Adding 48V systems to 12V cars will give designers the opportunity to achieve the fuel efficiency improvements needed in today's cars. It will also greatly increase the need for new and innovative power electronics circuits, and while many variations of 48V architectures will emerge, the ultimate judgement will be made by automotive customers after they weigh the feature benefits against cost.