OEM&Lieferant Ausgabe 1/2020

82 eMobility BMS Transformers for High-Energy Storage How to Select the Right Battery Monitoring IC Features for Reliable Isolated BMS Communications By Cathal Sheehan, Market Manager Power Conversion, Bourns, Inc. It is no surprise that analysts have predicted continued growth in the usage of Lithium Ion (Li-Ion) battery cells for energy storage and automotive applications through 2025 with growth rates of up to 30 percent forecasted to sup- port China’s transportation market alone. In various forms of mobility applications, Battery Management Systems (BMS) are used to connect to high-energy battery packs and manage the charging, discharging of the pack. The BMS also monitors vital operational factors such as temperature, state of the charge along with the battery pack’s overall health. If needed, the BMS can connect and disconnect the battery from the load or charging source for added protection. This article highlights the specifications that high voltage battery systems and BMS transformers need to have in order to meet industry standards for insulation. A typical IC used as a monitoring device in a BMS functions to measure cell voltage, pack temperature and to perform cell balancing shown in Figure 1. High-voltage battery pack IC designs do not require a current sense function. That is because these packs need only one current sensing chip and several hundred monitoring ICs to monitor the in- dividual cells in the pack. A simple way to determine the number of monitoring ICs re- quired is if each monitoring IC can check 10 cells, then at least 130 monitoring ICs will be needed. Another consideration in high-volt- age battery packs is that the BMS IC module or board needs to be located on top of the shunt resistor, which may pose a mechanical design challenge. Designing for Isolated Communications Isolated communications in a BMS is typically handled with two ports that allow battery monitoring modules to be daisy-chained throughout the battery pack. The source and sink currents of the serial port drivers are bal- anced enabling the IC to drive a transformer without saturating it. A transformer with a rated working voltage of several hundred volts is required to provide the necessary protection of the communications line from any hazardous voltage coming from the bat- tery pack. Furthermore, the drivers on the IC encode a four-line serial peripheral protocol into the differential signal to enable isolated communication from board to board. The Serial Peripheral Interface (SPI) is an inter- face bus commonly used to send data where one device or “master” transmits a clock pulse and control bit to a series of slaves. On each clock pulse, the slave either reads a command from the master or if the control bit is inverse, transmits its data on the data line. This enables a central battery controller IC (master) to inter- rogate each monitoring IC (slave) in turn and, hence, retrieve necessary voltage and tempera- ture information from the whole pack. In addi- tion, the transformer and integrated common mode choke are employed to filter out common mode noise from the daisy-chained network. Although BMS ICs have balanced currents on their I/Opins, most manufacturers recommend a centre-tapped transformer shown in Figure 2. These have been found to improve CMNR if a filter capacitor and termination resistor is used. BMS Transformer Safety Testing The safety standard which system engineers refer to for battery is UL1973. The most com- mon standards referred to for insulation are IEC 60664 (insulation coordination for low voltage systems) and IEC 62368 (Audio, Video and Communication Technology Equipment). IEC60664 refers to tests which are necessary for systems with working voltages higher than 700 V which are partial discharge and impulse over voltage. Both tests check the long term reliability of the insulation. Partial discharge tests require specialist equipment designed to measure miniscule levels of elec- trical charge generated if micro voids develop inside the insulation material over time. Recommended Electrical Characteristics The recommended primary inductance values provided by various IC manufacturers will depend on the voltage of the communication signals, the pulse widths and the frequency. For example, Bourns designed its Model Image/Graphics: © Bourns This block diagram of a typical Battery Management System shows the functions for monitoring essential battery pack health. S H A R E Battery Cell Chemistries: BMS IC and Transformer Functionality Figure 1