48V mild hybrid system is the best choice for transition, and performance should be the priority in selecting bidirectional DC/DC converters

Market research firm IHS predicts that by 2025, the number of vehicles equipped with 48V mild hybrid systems worldwide will reach 11 million, accounting for more than half of hybrid vehicles; and in the Chinese market, as new energy subsidies accelerate their decline, 48V mild hybrid systems will become the mainstream of hybrid systems even earlier.

Figure 1: IHS: 48V mild hybrid system market size 2016-2025

Strictly speaking, the 48V mild hybrid system is actually a more in-depth electrification transformation of traditional fuel vehicles, that is, on the basis of the original power, a 48V lithium-ion battery, an integrated starter generator (ISG) or a belt starter generator (BSG), and a bidirectional voltage controller (DC/DC) for converting between 48V and 12V voltages are added. In order to improve system compatibility and reduce the difficulty of promotion, cars equipped with a 48V mild hybrid system will be equipped with a 48V high-voltage battery system and a 12V low-voltage battery system at the same time, which can provide up to 10kW of available energy. The 12V bus will continue to be responsible for processing traditional loads such as lighting, ignition, and entertainment information systems. The 48V bus will support active chassis systems, air conditioning compressors, adjustable suspensions, and electronic superchargers/turbochargers. It also supports regenerative braking.

Compared with the 12V system, the 48V mild hybrid system has functions such as brake energy recovery, acceleration assistance, hybrid starting, gear stop, coasting, etc. in addition to the fast start-stop function; in terms of versatility, the 48V system can be compatible with more different electrical devices, such as the increasingly common large entertainment screens and complex central control systems; the 48V system can also load the air-conditioning compressors and cooling water pumps on traditional internal combustion engine vehicles that rely on engine loads, which not only effectively reduces the engine load, but also allows the use of these high-power devices when the engine is turned off. In addition, higher voltage means reduced operating current, and the required cable cross-sectional area is smaller, which means reducing the cable size and weight, which is in line with the trend of lightweight vehicles and has lower costs.

Figure 2: Schematic diagram of a Tier 1 manufacturer’s 48V mild hybrid system

The bidirectional DC/DC converter is the core component of the 48V mild hybrid system. It is the bridge between the 48V and 12V electrical networks and manages the 48V and 12V batteries. It is an important part of ensuring the safe operation of the vehicle system, and its performance requirements cannot be ignored. Analog chip manufacturer ADI, which is known for its high performance, is very suitable for this application with its bidirectional two-phase synchronous buck or boost controller LTC3871.

The LTC3871 brings new levels of performance, control and simplified circuits to 48V/12V dual-battery DC/DC automotive systems by allowing the same external power components to be used for both buck and boost purposes; and can parallel up to 12 phases to meet the needs of high-current and high-power (3KW-10KW) applications. According to data from the ADI official website, when starting the car or requiring additional power, the LTC3871 allows two batteries to power the same load at the same time, achieving up to 97% efficiency; it operates at a user-selectable fixed frequency of 60~475kHz, or it can be synchronized to an external clock in the same range, and can select continuous operation or pulse skipping mode at light loads.

The LTC3871 also provides overload and short-circuit protection, independent loop compensation for buck and boost modes, EXTVcc for improved efficiency, +/-1% output voltage regulation accuracy over temperature, undervoltage and overvoltage lockout, etc. The automotive-grade version has an operating temperature range of -40°C~150°C. The LTC3871 has been verified to meet the AEC-Q100 specification requirements and is designed to meet the diagnostic range in functional safety standard (ISO26262) systems.

Figure 3: ADI’s bidirectional multiphase synchronous buck/boost controller solution for 48V/12V dual battery applications.

For the application of redundant battery system in autonomous driving vehicles, ADI has recently launched the bidirectional buck-boost switching regulator controller LT8708/-1 with an efficiency of up to 98%. It is powered by an input voltage that can be higher, lower or equal to the output voltage. It is very suitable for two batteries of 12V, 24V or 48V, as well as 48V/12V and 48V/24V dual battery systems. The device runs between two batteries to avoid system shutdown in the event of a failure in one of the batteries. Similarly, the high-temperature automotive grade version has an operating temperature range of -40°C~150°C.

Figure 4: With up to 98% efficiency, the LT8708/-1 can be used in 48V/12V and 48V/24V dual-battery systems.

The LT8708/-1 operates from a 2.8V to 80V input voltage range with a single inductor and can generate output voltages from 1.3V to 80V, delivering up to kW of power, simplifying bidirectional power conversion in battery/capacitor backup systems that require regulating VOUT, VIN and/or IOUT, IIN in the forward or reverse direction.

ADI's official website points out that LT8708/-1 has 6 independent adjustment forms that make it suitable for many applications. For example, LT8708-1 and LT8708 are used in parallel to increase power and phase number. LT8708 is the master device, and LT8708-1 always operates as a slave device; multiple slave devices can be connected to a single master device to proportionally increase the power and current capacity of the system. Another application is to power the load with the input voltage while also powering the LT8708/-1 circuit responsible for charging the battery or supercapacitor bank. When the input voltage disappears, the bidirectional capability of LT8708 can be used to continue to power the load from the battery or supercapacitor without interruption. The forward and reverse currents on the input and output sides of the converter can be monitored and limited. All four current limits (forward input, reverse input, forward output, and reverse output) can be set independently using four resistors. In conjunction with the DIR (direction) pin, the device can be configured to process power from VIN to VOUT or from VOUT to VIN, making it ideal for automotive, solar, telecom and battery-powered systems.

Figure 5: LT8708/-1 application circuit diagram

ADI believes that the high-performance bidirectional DC/DC converter will help the 48V mild hybrid system achieve the goal of significantly improving energy efficiency and reducing carbon dioxide emissions at a lower cost, meeting the increasingly stringent energy-saving and emission reduction policy requirements of various countries. It is believed that with the heavy investment and strong promotion of many auto giants and first- and second-tier suppliers, more and more 48V mild hybrid system models will be launched on the market, which will surely ignite the purchasing enthusiasm of end consumers and become the darling of the market as the industry generally desires.

Thumbs up for the LT8708/-1 with up to 98% efficiency!