Automotive Dual-Battery Redundancy with a 12 V to 12 V Bidirectional DC/DC Controller

By Bruce Haug

background knowledge

There are signs that the revolution of self-driving cars is about to come. Automotive companies are working with tech giants such as Google and Uber, as well as well-known startups, to develop a new generation of self-driving cars that will transform our urban roads and highways and lay the foundation for the smart cities of the future. They will use technologies such as machine learning, the Internet of Things (IoT), and the cloud to accelerate this development.

More importantly, self-driving cars will continue to drive the industry revolution that has already been initiated by ride-sharing service providers such as Uber and Lyft. All these technologies come together to create a world where the future of transportation is made up of intelligent, driverless vehicles.

Ultimately, all self-driving cars will achieve some degree of autonomy by integrating sensors, cameras, radar, high-performance GPS, light detection and ranging (lidar), artificial intelligence (AI), and machine learning. Connectivity to secure and scalable IoT, data management, and cloud solutions is also important, as they provide a resilient and high-performance foundation for collecting, managing, and analyzing sensor data.

From environmental benefits to improved safety, the rise of connected vehicles has far-reaching societal impacts. Fewer cars on the road also means less greenhouse gas emissions, which in turn reduces energy consumption and improves air quality.

Endpoint telemetry, intelligent software, and cloud technology are indispensable for autonomous vehicles and smart road systems. The onboard cameras and various sensors in autonomous vehicles collect a large amount of data, and this data must be processed in real time to keep the vehicle in the correct lane and drive safely to the destination.

Cloud-based networking and connectivity are also an important part of this system. Self-driving cars will be equipped with on-board systems that support vehicle-to-vehicle communication, allowing them to learn from other vehicles on the road to adjust to changes in weather and road conditions (such as bends and debris on the road). Advanced algorithms and deep learning systems are key to ensuring that self-driving cars can quickly and automatically adapt to various scene changes.

In addition to specific components, such as scalability and intelligent data management for cloud computing infrastructure, redundancy is also required for mission-critical systems, including power supplies. Previously released redundant battery redundancy solutions, such as the LTC3871, can work between two battery systems with different rated voltages, such as a 48 V lithium-ion battery and a 12 V lead-acid battery. However, most existing solutions do not provide redundancy for batteries of the same voltage, such as two 12 V, 24 V, or 48 V batteries, at least so far.

Clearly, a bidirectional buck-boost DC/DC converter that can operate between two 12 V batteries is needed. Such a DC/DC converter can be used to charge either battery, or both batteries can be used to power the same load simultaneously. In addition, if either battery fails, it is necessary to be able to detect the failure and isolate it from the other battery so that the load can continue to be powered without interruption. The recently released LT8708 bidirectional DC/DC controller from Analog Devices solves this critical problem by connecting two batteries with the same voltage.

Single channel bidirectional control IC solution

The LT8708 is a bidirectional buck-boost switching power supply controller with an efficiency of up to 98%. It can work between two batteries of the same voltage, making it ideal for battery redundancy in autonomous vehicles. It can also work with an input voltage higher, lower, or equal to the output voltage, making it ideal for two 12 V, 24 V, or 48 V battery systems commonly found in electric and hybrid vehicles. The LT8708 works between two battery systems, preventing the system from shutting down even if one of the batteries fails. The LT8708 can also be used in 48 V/12 V and 48 V/24 V dual battery systems.

The LT8708 uses a single inductor and can provide up to several kilowatts of conversion power when the input voltage range is 2.8 V to 80 V and the output voltage range is 1.3 V to 80 V, depending on the selected peripheral devices and the number of main circuit phases. It simplifies the bidirectional power conversion of battery/capacitor backup systems when forward or reverse regulating VOUT, VIN and/or IOUT, IIN. The LT8708 has six independent regulation modes that can be used in a variety of different applications.

LT8708-1 can be used in parallel with LT8708 to increase the maximum conversion power and phase number. LT8708-1 always works as a slave of the master LT8708, can set the clock out of phase, and can provide conversion power equivalent to the master. A master can connect up to 12 slaves at the same time, thereby increasing the power and current conversion capability of the system.

The forward and reverse currents at the converter input and output can be monitored and limited, and all four current limits (forward input, reverse input, forward output, and reverse output) can be independently set using four resistors. Combined with the direction setting (DIR) pin, the LT8708 can be configured to process power from VIN to VOUT or VOUT to VIN, making it ideal for automotive, solar, telecom, and battery-powered systems.

The LT8708 is available in a 5 mm × 8 mm, 40-lead QFN package and is available in three temperature grades, including extended and industrial grades for a –40°C to +125°C operating range and an automotive grade for a high temperature operating range of –40°C to +150°C. Figure 1 shows a simplified LT8708 block diagram.

Complete Solution

The block diagram in Figure 2 shows the additional parts needed to complete a dual battery redundancy circuit design in an automotive application. As shown, the LT8708, along with two LT8708-1s, forms a three-phase solution that can deliver up to 60 A of conversion current in either direction. By adding additional LT8708-1 devices, high power applications with 12 phases and more can be achieved. The AD8417 is a bidirectional current sense amplifier that senses the current flowing into and out of the battery. When this sense current exceeds a preset value, the LTC7001 high-side NMOS static switch driver turns on the back-to-back MOSFETs to isolate the battery from the circuit.

The LTC6810-2 monitors and controls lithium-ion batteries. It accurately measures battery cells and guarantees a total measurement error of less than 1.8 mV. Connecting multiple LTC6810-2s in parallel to the host processor will create additional redundancy for monitoring other voltages in the circuit. The LTC6810-2 has an isoSPI™ interface for high-speed, RF-immune, long-distance communications and supports bidirectional operation. The LTC6810-2 also supports passive balancing of each cell and battery measurement redundancy through PWM duty cycle control.

Control Overview

The LT8708 supports power conversion when the output voltage can be higher, lower or equal to the input voltage, and has bidirectional current monitoring and regulation functions at the input and output ends. ADI's patented control architecture allows the use of inductor internal resistance as a current sensing resistor in the buck, boost or buck-boost operating areas. The inductor current is controlled by the voltage on the VC pin, which is the combined output of six internal error amplifiers EA1 to EA6. These amplifiers can be used to limit or regulate the corresponding voltage or current, as shown in Table 1.

Table 1. Error Amplifiers (EA1 to EA6)

The minimum-maximum range of the VC voltage is typically 1.2 V. The maximum VC voltage controls the maximum forward inductor current, thereby controlling the maximum power flow from VIN to VOUT. The minimum VC voltage controls the maximum negative reverse inductor current, thereby controlling the maximum power flow from VOUT to VIN.

In the simple example VOUT regulation process, the FBOUT pin receives the VOUT voltage feedback signal and compares it with the internal reference voltage through EA4. A lower VOUT voltage increases VC, so more current flows into VOUT. Conversely, a higher VOUT voltage decreases VC, thereby reducing the current flowing into VOUT, or even absorbing current and power from VOUT.

Figure 1. LT8708 simplified bidirectional dual 12 V battery system application schematic.

Figure 2. Complete dual-battery redundancy solution system block diagram.

As mentioned previously, the LT8708 also features bidirectional current regulation at both the input and output. The VOUT current can be regulated or limited in both the forward and reverse directions (EA6 and EA2, respectively), and the VIN current can also be regulated or limited in both the forward and reverse directions (EA5 and EA1, respectively).

In some common applications, VOUT may be regulated via EA4, with the remaining error amplifiers used to monitor input or output overcurrent or input undervoltage conditions. In other applications, such as battery backup systems, the battery connected to VOUT may be charged at a constant current (EA6) to a maximum voltage (EA4) or reversed, with the remaining error amplifiers feeding energy back to VIN to regulate VIN and limit the maximum current. For more information on this topic, see the LT8708 data sheet.

in conclusion

The LT8708-1 brings a new level of performance, control, and design simplification to automotive DC/DC systems with dual batteries of the same voltage. Whether it is used for energy transfer between two power sources for redundancy or for backup power in critical situations, the LT8708 can work between two batteries or supercapacitors with the same voltage. This feature paves the way for automotive system engineers in the development of automotive electronic technology, making cars safer and more efficient.