To meet these standards, automakers are increasingly focusing on mild hybrid electric vehicles. These vehicles, in addition to a standard 12V battery, also utilize a secondary high-voltage battery. Against this backdrop, the 48V and 12V power rails in dual-battery systems have become a key research and application area in the automotive technology field.
German automakers pioneered the design and construction of systems based on 48V batteries. Compared to traditional 12V batteries, 48V batteries offer numerous advantages. They can deliver higher power at lower current, meaning that while meeting the vehicle's power needs, losses during current transmission can be reduced. Furthermore, due to the lower current, the required wiring harness specifications can be reduced, thereby lightening the harness and optimizing the overall vehicle weight without compromising performance. In this development process, the LV148 standard has gradually become an important starting point for dual-battery vehicle systems.
In the architecture of a dual-battery vehicle system, the 12V and 48V power rails each play different roles. Typically, the 12V bus provides power for the vehicle's ignition, lighting, infotainment, and audio systems—systems crucial for basic vehicle operation and driving experience. The 48V bus, on the other hand, primarily serves high-performance or high-energy-consuming components such as the active chassis system, air conditioning compressor, adjustable suspension, and electronic supercapacitor/turbocharger, and plays a key role in supporting regenerative braking. Furthermore, the 48V bus assists in engine starting, making soft-start operations smoother and improving the comfort and reliability of the vehicle's starting process.
However, building such a dual-battery system is no easy task, and engineers face numerous challenges. Many automotive original equipment manufacturers (OEMs) explicitly require bidirectional energy transfer between the 48V and 12V rails. This is because in practical use, when one battery is depleted, energy needs to be drawn from the other for charging; and under certain heavy loads, both batteries need to work together to provide additional power to the opposite voltage rails. To ensure safe and efficient battery charging, the controller must be able to precisely control the charging current. In most automotive applications, power transfer requirements are not low, typically ranging from 2kW to 3kW. Furthermore, the voltage fluctuations between the 48V and 12V rails are significant. According to the LV148 specification, the normal voltage range for the 48V rail is between 36V and 52V, while the voltage of the 12V rail can vary between 6V and 16V. In addition, robust protection circuitry must be included to handle various fault conditions that could damage the system. Therefore, designing a DC/DC converter that can connect to 48V and 12V rails is by no means a simple task.
Despite the challenges, the problem can be effectively simplified through clever design. The lack of significant overlap between the 48V and 12V rail voltage ranges facilitates circuit design. When power needs to be transferred from the 48V rail to the 12V rail, a buck converter can be used, which reduces the higher 48V voltage to a level suitable for the 12V system. Conversely, for power transfer from the 12V rail to the 48V rail, a boost converter is the appropriate choice. Considering the system's kilowatt-level power requirements, synchronous MOSFETs should be used in each converter instead of traditional freewheeling diodes to improve system efficiency. Buck and boost topologies are common and mature technologies in power electronics. However, designing two separate converters not only occupies valuable board space and increases system complexity but also raises costs. In-depth research reveals that the power chains of buck and boost converters are highly similar; both contain at least two power MOSFETs, an inductor, and a certain amount of output capacitor. The main difference lies in the controller. In a buck converter topology, the controlled switch is a high-side MOSFET; while in a boost converter topology, the controlled switch is a low-side MOSFET. Therefore, by selecting a suitable controller and simply changing the controlled switch, it is possible to change the direction of current flow in the inductor while using the same powertrain components, thus enabling the evolution from a two-converter solution to a single-converter solution.
In high-current designs, while synchronous switching is necessary, it is insufficient to solve all problems. For example, a 2kW power supply would draw approximately 166A on a 12V rail. Such a large current often necessitates multiphase operation to achieve design goals in practice. Multiphase architecture offers numerous advantages, effectively reducing component size and simplifying thermal management. To facilitate parallel connection of each power phase, current-mode control should be used in buck or boost operation. Furthermore, multiphase operation allows for staggered switching of each phase, meaning not all phases switch simultaneously at any given time. This reduces output ripple, thereby helping to lower electromagnetic interference (EMI) and improve system stability and reliability.
In any automotive electrical system, the design of protection circuits is crucial, affecting operator safety and stable system operation. Common protection functions, such as undervoltage lockout (UVLO) and overvoltage protection (OVP), ensure that the battery is not overcharged or undercharged during charging, preventing damage from abnormal voltage. Peak inductor current limiting helps prevent excessive stress on each power phase, avoiding inductor saturation and extending component lifespan. In dual-battery vehicle systems, circuit breakers are also required to disconnect the electrical connection between the 48V and 12V rails when necessary, preventing the propagation of fault currents. Monitoring circuits are also indispensable; for example, by monitoring the current in each channel during energy transfer, fault conditions can be detected promptly, providing real-time assurance for the safe operation of the system.
Digitally controlled DC/DC converters were once considered a potential solution, but this approach has several significant drawbacks. First, it requires a large number of discrete components, including current-sense amplifiers for each phase, power MOSFET gate drivers, protection circuits, and monitoring circuits, which occupy considerable valuable space on printed circuit boards (PCBs). Second, it requires high-end microcontrollers to implement the converter's current and voltage control loops, increasing system cost and complexity. Furthermore, microcontrollers can introduce delays in protection circuits, which can lead to catastrophic damage at high power levels. Additionally, digital control has a long design cycle, typically requiring several years, and demands in-depth knowledge of switching power supplies and digital control technologies from designers. However, digital control is not without its advantages. From a system-level perspective, it offers greater flexibility, allowing dynamic changes in control parameters and regulated voltages, and the ability to share information with other subsystems, thereby improving overall system performance.
To address these challenges, Texas Instruments (TI) introduced the LM5170-Q1 synchronous dual-phase bidirectional buck/boost controller. This controller integrates a current-sense amplifier, a high-current gate driver, and system protection features, including integrated circuit breakers and channel current monitoring. These integrated features eliminate many discrete components required in digital solutions, significantly simplifying circuit design. By stacking multiple LM5170-Q1 controllers in parallel, kilowatt-level power delivery can be achieved, while its proprietary average current-mode control scheme optimizes current control of the rechargeable battery, ensuring efficient and safe charging.
With the continued development of automotive electrification and intelligence, the 48V and 12V power rail technologies in dual-battery systems will continue to evolve and improve. In the future, we can expect to see more efficient, intelligent, and integrated power management solutions, providing stronger support for improving vehicle performance, energy efficiency, and safety, and driving the automotive industry towards a more environmentally friendly and intelligent direction.
