Optocouplers safeguard battery systems from transients, shorts, and other faults

The transition from gas-powered vehicles to gas-electric hybrids or all-electric vehicles (EVs) presents many design challenges for the circuits and subsystems employed in the vehicle due to transients, extreme temperatures, and many other factors.  The high-voltage battery array and its connection to various subsystems such as the drive train and other high-power electrical systems requires isolation so the battery system “floats”—thus preventing leakage currents or high voltages from reaching low-voltage systems and the vehicle chassis.

In addition, the onboard charger of plug-in electric vehicles runs off a high voltage line (240 V) and draws high currents for overnight charging. The need for high-voltage protection against transients is critical. Car manufacturers are looking to standardize battery management systems to handle battery arrays that deliver up to 1,000V.

To deliver the necessary levels of isolation, optical isolators are the defacto standard way to provide high electrical isolation and high noise immunity while consuming little power, compared with systems that use transformer coupling for isolation. Battery subsystems are especially challenging due the large number of cells typically used, the high levels of electrical noise, and substantial transients that occur from the loads presented by the vehicle to the charging currents that recharge the cells. Additionally, in the design of the battery array itself and the charging subsystem, monitoring each cell’s voltage within the array is a key concern such that a cell failure will not cause the entire array to stop functioning or cause the charging system to overload.

A typical battery array in an electric vehicle consists of multiple battery modules, with each module typically consisting of many individual cells and specialized circuits that monitor these cells. The total array usually delivers an output of several hundred volts (i.e. about 400V). The monitoring circuits capture the battery voltage along with other parameters. The collected data are digitized and sent over a serial peripheral interface (SPI) bus to the microcontroller (MCU) that manages the battery subsystem (Figure 1). The MCU, in turn sends control signals over the controller area network (CAN) bus to various subsystems in the vehicle.

Figure 1: A typical battery management subsystem requires multiple optocouplers to provide isolation between the SPI bus and the microcontroller, and between the microcontroller and the CAN bus.

To isolate the battery subsystem from the MCU, optical isolators take the serial data from the SPI output of the cell monitoring circuits and provide a physical barrier thanks to the separation of the LED emitter and the light-sensing receiver. Isolation levels of several thousand volts prevent transients, electrical noise, or other factors from breaching the system—allowing the battery system to “float” with no direct connection to the body of the vehicle. Additionally, current leakage is also minimized since there is no connection to the vehicle body.

In the battery subsystem different types of optocouplers provide different levels of voltage isolation and performance to match the performance needs of each portion of the system (Figure 2).  For instance, the SPI interface would typically operate at data rates of over 1 MHz and would have to deliver its performance over a -40 to +125C operating temperature range. Those requirements would lead to the use of optocouplers, such as the ACPL-K49T or a similar optocoupler from various vendors, to isolate the low-speed chip-select signal, and higher-speed optocouplers, such as the ACPL-M72T or equivalent, for the three higher-speed SPI signal lines (serial clock, serial data in, and serial data out) from each battery monitoring circuit.

Figure 2: Four signals from the SPI port on the battery measurement circuit use optical isolators to ensure no high-voltage spikes cross from the battery array to the low voltage microcontroller