Simplifying dual-battery power systems, 48 ​​V/12 V automotive applications are just around the corner

This means that the vehicle's internal systems will use either a 48 V lithium-ion (Li-Ion) battery or a 12 V sealed lead-acid (SLA) battery, but not both at the same time. However, due to the chemical properties of each battery, in addition to designing two independent charging circuits for them, a mechanism must be used to allow charge to flow between them without causing any damage to the batteries or any system in the vehicle. In addition to this, using two batteries provides redundant power in the event that one of the batteries fails during operation.

While this will certainly complicate the design of the various electrical subsystems within the vehicle, there are some advantages. According to some automakers, 48 ​​V-based electrical systems can improve the fuel economy of internal combustion engine vehicles by 10% to 15%, thereby reducing CO2 emissions. In addition, future vehicles using a 48 V/12 V dual system will allow engineers to integrate electric booster technology that operates independently of engine load, thereby improving acceleration performance. Such electric superchargers are already in advanced stages of development and will be located between the bleed air system and the intercooler, using the 48 V voltage rail to spin the turbine blades.

Fuel economy regulations are becoming increasingly stringent around the world, while connected, autonomous driving features are becoming more common in new vehicles. As a result, 12 V automotive electrical systems have reached the limits of their available power. At the same time, as if these changes were not enough, the demands on automotive electronic systems have grown significantly. Together with the associated power demands, these changes have created a whole new set of engineering opportunities. Clearly, the 3 kW power limit of 12 V lead-acid battery automotive systems must be supplemented.

In addition, there are new automotive standards that affect how these systems work. The newly proposed automotive standard, called LV 148, combines a secondary 48 V bus with the existing automotive 12 V system. The 48 V rail includes an integrated starter generator (ISG) or belt starter generator, a 48 V lithium-ion battery, and a bidirectional DC-DC converter that can provide energy in the 10 kW range from the 48 V and 12 V batteries. This technology is suitable for traditional internal combustion engine vehicles, hybrid vehicles, and mild hybrid vehicles, helping automakers meet increasingly stringent CO2 emissions targets.

This new standard requires that the 12 V bus continue to power the ignition, lighting, infotainment and audio systems. The 48 V bus will power active chassis systems, air conditioning compressors, adjustable suspensions, electric superchargers/turbochargers, and even support brake energy regeneration.

Adding a 48V power supply network to the vehicle is not without significant impact. Electronic Control Units (ECUs) will be impacted and will need to adjust their operating range to the higher voltage. This requires DC-DC converter manufacturers to use dedicated ICs to enable this high power transfer.

As a result, ADI Power Products has designed and developed DC-DC converters that can perform this energy conversion with very high efficiency, thereby saving energy while minimizing thermal design issues.

Obviously, this requires a bidirectional buck and boost DC-DC converter between the 12 V and 48 V batteries. Such a converter can be used to charge either battery and allow both batteries to power the same load simultaneously if the system requires it. Traditionally, these initial 48 V/12 V dual-battery DC-DC converter designs used different power components for buck and boost.

ADI Power recently introduced a bidirectional DC-DC controller, the LT8228, which uses the same external power components for step-up conversion as for step-down conversion.

The LT8228 (shown in Figure 2) is a 100 V bidirectional constant current or constant voltage synchronous buck or boost controller with independent compensation networks. The direction of power flow is determined automatically by the LT8228 or controlled externally. Input and output protection MOSFETs are used to prevent negative voltages, control inrush currents, and provide isolation between terminals during fault conditions such as a switching MOSFET short. In buck mode, MOSFET protection at the V1 terminal prevents reverse current. In boost mode, the same MOSFET controls output inrush current and protects itself via an adjustable timer circuit breaker.

In addition, the LT8228 features bidirectional input and output current limiting and independent current monitoring. Masterless, fault-tolerant current sharing allows any paralleled LT8228 to be added or removed while ensuring current sharing accuracy. Internal and external fault diagnostics and reporting are available through the fault and report pins. The LT8228 is available in a 38-lead TSSOP package.

The LT8228 is a 100V bidirectional peak current mode synchronous controller with MOSFET protection. In buck mode, the controller provides a buck output voltage V2 from an input voltage V1, or in boost mode, provides a boost output voltage V1 from an input voltage V2. The input and output voltages can be set up to 100 V. The operating mode can be externally controlled through the DRXN pin or automatically selected. In addition, the LT8228 provides MOSFET protection for the V1 and V2 terminals. The MOSFETs prevent negative voltages, provide isolation protection between the input and output ports in the event of internal or external faults, and provide reverse current protection and inrush current control. In applications such as battery backup systems, the bidirectional function allows the battery to be charged using a high voltage or low voltage power supply. When the power supply is not available, the battery can provide a boosted or bucked power supply.

To optimize transient response, the LT8228 uses two error amplifiers: EA1 in boost mode and EA2 in buck mode, with independent compensation pins VC1 and VC2, respectively. When reverse inductor current is detected under conditions such as light load operation, the controller operates in discontinuous conduction mode. The LT8228 uses the following four pins for input and output current limit programming in buck and boost modes: ISET1P, ISET1N, ISET2P, and ISET2N. In addition, the controller provides independent input and output current monitoring using the IMON1 and IMON2 pins. The current limit programming and monitoring functions are valid over the entire input and output voltage range (0 V to 100 V).

In addition, the LT8228 provides masterless, fault-tolerant output current sharing that can be used for multiple parallel LT8228s to achieve higher load currents, better thermal management, and redundancy. Each LT8228 can be adjusted to the average output current without the need for a master controller. When a single LT8228 is disabled or under fault conditions, it stops outputting current to the average bus, making the current sharing scheme fault-tolerant.

The LT8228 improves the performance, control functions and simplifies the design of 48V/12V dual-battery DC-DC automotive systems by using the same external power components for both buck and boost. It can operate in 48V bus to 12V bus buck mode or 12V to 48V boost mode as needed. When starting the car or when additional power is needed, the LT8228 allows both batteries to power the same load simultaneously. With this versatile bidirectional converter, power conversion designers can easily configure the 12V and 48V battery systems required for future fully autonomous vehicles.