Protecting vehicle battery systems from transients, shorts, and other faults

March 08, 2012 // By Junhua He, Avago Technologies
The transition from gas-powered vehicles to gas-electric hybrids or all-electric vehicles presents many design challenges for the circuits and subsystems employed in the vehicle due to transients, extreme temperatures, and many other factors. Particular care has to be taken to the separation of high-voltage and low-voltage segments in the automotive power supply.

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. The onboard charger of plug-in electric vehicles takes in high voltage line voltage (240 V) and draws high currents for overnight charging. The need for high-voltage protection against transients is extremely critical. Car manufacturers are looking to standardize battery management systems to handle battery arrays that deliver up to 1000 V.

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 to provide the 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 the cells in the module. The total array typically delivers an output of several hundred volts (typically about 400 V). The monitoring circuits capture the battery voltage and other parameters and digitize the collected data and send the data 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 control-area-network (CAN) bus to various subsystems in the vehicle.

To isolate the

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