How to use optocoupler to isolate high voltage to improve the safety of electric vehicle lithium-ion battery pack

Currently, in all-electric or hybrid vehicle applications, the management of high-voltage lithium-ion battery packs faces many challenges. In addition to having to monitor the charge and discharge cycles, it is also necessary to isolate the battery pack that provides hundreds of volts for safety reasons. This article specifically addresses the needs of lithium-ion battery monitoring and discusses the architecture and components used in battery monitoring systems, digital communication systems, and isolated interfaces.

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In the management system, the battery monitoring board uses two key subsystems to reliably monitor the battery status and provide digital results to the master processor that controls the operation of the control system. In order to separate these subsystems, in high-voltage battery sensing circuits and boards An optically isolated signal interface is used between the communication devices to ensure that high voltages do not affect the digital subsystem.

Lithium-ion battery characteristics

The complex electronic system that must meet the performance, safety, and reliability requirements of the electric vehicle is directly affected by the characteristics of the lithium ion battery. When the lithium ion battery is discharged, the lithium material is usually ionized at the graphite anode, and then the lithium ions are used. The movement of the electrolyte through the separator to the cathode causes charge flow, and the charging process reverses the entire process, bringing lithium ions back from the cathode through the separator back to the anode.

The performance and reliability of this chemical inversion program is controlled by the temperature and voltage of the cell. At lower temperatures, the chemical reaction is slower, so that the voltage of the cell is lower. As the temperature increases, the reaction rate increases until the lithium The ionic unit begins to collapse. When the temperature exceeds 100 ° C, the electrolyte begins to decompose, releasing a gas that may cause the pressure of the battery unit without designing a pressure relief mechanism. At a sufficiently high temperature, the lithium ion battery unit may be oxide Decomposition faces thermal runaway and releases oxygen, further accelerating the temperature rise.

Therefore, maintaining optimal operating conditions for lithium-ion batteries is a key requirement of battery management systems. The main challenge in designing control and management systems is to ensure reliable data acquisition and analysis for monitoring the status of lithium-ion batteries in automobiles. This is the characteristic problem of the lithium ion battery itself.

In the Chevy Volt electric vehicle, the battery pack contains 288 prismatic lithium-ion batteries, which are divided into 96 battery groups and provide 386.6V DC system voltage through connection. These battery groups combine with temperature sensor and cooling unit to form four mains. a battery module, a voltage sensing line connected to each battery group is terminally processed when connected to each battery module and connected to a battery interface module above each battery module through a voltage sensing band combination connector, 4 The color-coded battery interface module operates in different positions of the battery pack, corresponding to the low, medium and high voltage ranges of the DC voltage offset of the four modules.

The data provided by the battery interface module is sent to the battery energy control module, which then provides the fault condition, status and diagnostic information to the hybrid control module as the vehicle diagnostic master controller. At any time, the entire system will be More than 5,000 system diagnostics were run, with 85% of the diagnostics focusing on battery pack safety and others as target battery performance and lifetime control.

Multi-layer circuit board

Battery performance analysis begins with a battery interface control module such as that used in the Chevy Volt electric vehicle. Please refer to Figure 1. The design is specifically designed for high signal integrity. The four-layer design uses trace layout techniques, isolation techniques, and grounding. A combination of planes to help ensure signal integrity in such a challenging environment, with the top layer containing most of the components, including optical isolators, ground planes, and signal traces with multiple vias, providing access The lower connection path, the second layer is distributed below the high voltage area of ​​the board using the power and ground planes, and the third layer contains the signal traces below these areas, the other side of the printed circuit board, the fourth layer It acts as a ground plane and signal trace and contains some extra components.

Figure 1: Each of the four battery interface control module boards in the Chevy Volt electric vehicle contains multiple sensing circuits and CAN communication circuits that are isolated by optocouplers at the edge of the communication subsystem.

Signal isolation

In electric vehicle applications, communication and control are important cornerstones of vehicle operation. In the Chevy Volt, multiple network isolation and protection independent subsystems are used, and complex algorithms are used to manage independent lithium-ion battery groups and monitor special batteries. The interface controls the battery pack in each sensing subsystem, but the key data for overall battery management is included in the controller area network (CAN) signal signal interface and a high voltage fault signal, and the system is safe. Sex and reliability also depend on the safety isolation between the CAN bus network and the high voltage sensing circuit. Although isolation can be achieved using a variety of methods and components, the harsh environment and multiple safety regulations make optocouplers suitable for this type of application. Preferred solution.

Optocouplers provide high common-mode noise rejection and are fundamentally immune to high electrical noise environments such as EMC and EMI in automobiles. In addition, the high isolation provided by this type of device requires long-term exposure to battery pack DC voltage. And the rapid high-voltage transients that can occur during testing, charging connections and removal, and DC-DC conversion are critical.

When selecting this critical component, the main requirements for automotive applications include proper package and operating voltage specifications. Although performance specifications including speed, data rate, and power consumption are still important, EMI considerations for fast switching and high current variations are essential. This will limit the need for ultra-high-speed devices, which will shift to higher flexibility requirements for adjusting slew rate and limiting EMI performance.

In order to meet the stringent specifications of automotive applications, Avago offers a wide range of optocoupler products that can be used in the voltage sensing of battery packs to provide safe isolation of data communication interfaces and other applications. Table 1 provides a variety of Optocoupler products for automotive applications.

Table 1: Appropriate automotive applications for various optocouplers

Automotive optocoupler

For example, Avago's ACPL-M43T optocoupler provides the isolation required for the battery interface control module board. As part of the AvagoR2Coupler family, the ACPL-M43T is available in a compact 5-pin SO-5 JEDEC package. Automotive grade single channel digital optocoupler for surface mount applications. In addition to enhanced insulation, Avago's R2Coupler products use two wires to enhance critical functional connections, as shown in Figure 2. In addition, the sealed optocoupler exhibits greater reliability and a wider operating temperature range, significantly surpassing optocoupler products using consumer grade LEDs. Designed specifically for automotive applications, Avago's products use automotive-grade LEDs that are manufactured to ISO/TS16949-compliant quality systems and meet AEC-Q100 specifications.

Figure 2: For automotive grade R2Coupler products such as the ACPL-M43T optocoupler, Avago uses a two-wire enhancement key function connection. (highlighted in the picture)

This device is very suitable for electric vehicle battery pack requirements, including 567V continuous working voltage, 6000V maximum transient overvoltage, 5mm creepage distance and 5mm clearance, etc., at 10mA forward input current, regardless of logic high level Or low-level outputs have a common-mode transient immunity of 30kV/μs, which reduces the chances of other automotive subsystems changing into the CAN transmission line network.