Application of Ferroelectric Random Access Memory F-RAM in Power Battery Management

With the development of electric vehicle technology and the encouragement and support of government policies, electric vehicles (hybrid + pure electric) are growing at a high rate of more than 50% per year. As the core components of electric vehicles, the market demand for batteries and battery management systems has also grown rapidly. This article will analyze the demand for memory in battery management systems .

The Battery Management System ( BMS) mainly implements three core functions: prediction and calculation of battery charge and discharge status (i.e. SOC ), equalization management of single cells, and battery health status logging and diagnosis. The functional block diagram is as follows:

In the entire battery management system, the prediction and calculation of the battery state of charge (SOC) is its most important function, because only with the accurate prediction / calculation of the battery charge/discharge state can effective balance management be carried out. Therefore, the higher the SOC accuracy requirement, the better.

In order to improve the accuracy of SOC , in addition to collecting the voltage and current parameters of the battery, it is also necessary to provide multiple parameters such as impedance, temperature, ambient temperature, charge and discharge time, etc. The inherent parameters of the battery will be mathematically modeled to establish a software model, while the dynamic parameters will be collected in real time through the data acquisition card and transmitted to the MCU unit for storage in real time. The MCU will then perform algorithm calculations on the extracted data to obtain an accurate battery state of charge.

Therefore, the SOC function will store the models of different batteries into the memory, which needs to have low power consumption, fast reading and writing, simple interface and data retention time of 20 years. The SOC function requires the acquisition card to continuously store the collected battery voltage/current data into the memory in real time. If an MCU unit is connected to the collection data of 10 single cells, the data acquisition card will generally use a 1MB isoSPI bus for communication, that is, for the memory of the MCU unit, the interface rate is required to be high and data write operations must be performed almost every second. The battery life is required to be at least 10 years. If a car runs for 8 hours, the data write operation of the MCU unit's memory will be written 150 million times during the life cycle of the battery pack .

From the above analysis , it can be seen that the SOC function in the BMS is very critical, so the performance and reliability of the memory are also very high: it must be a non-volatile memory, the number of erase and write times must be at least more than 110 million times, the interface rate must be greater than 8MHz , low power consumption and data can be reliably stored for 20 years. It needs to comply with AECQ-100 and pass functional safety certification in the future , with at least ASILB level.

The mainstream non-volatile memories are EEPROM ,  Flash  and F-RAM . The interface of EEPROM is  SPI interface, the speed can reach 10Mhz , but there is a 5ms write waiting time for each write, the number of erase and write is 1 million times, the power consumption is medium, there are automotive grade devices, but there is no functional safety certification, and the data retention capacity can also reach 20 years.

Flash has a slow read and write speed, and each write operation must be erased, so it takes at least several hundred milliseconds to complete a write operation. The number of erases and writes can only support 100,000 times , far below the requirement of 110 million times. The data retention capacity is between 10 and 20 years.

F-RAM uses ferroelectric materials as storage media. It has high reliability, data retention time of 100 years, high read and write efficiency with completely random data without write waiting, SPI interface rate can support up to 50Mhz or 108MHz QSPI , and has very low power consumption. Due to its special ferroelectric material, the number of erase and write times of this type of memory can be as high as 10 billion times. As shown in the figure below:

As shown in the figure above, F-RAM , as a unique non-volatile memory, is currently the best memory choice for realizing high-reliability BMS systems in terms of write speed, durability, power consumption and reliability .

As the world's leading F-RAM core supplier, Cypress Semiconductor Inc. provides a very complete range of ferroelectric random access memory F-RAM products with capacities ranging from 4Kb to 8Mb . The interface is I2C/SPI  interface, with almost unlimited read and write times ( 10 billion read and write cycles). The QSPI interface rate is as high as 108Mhz , no write waiting time is required, and the operating current is as low as 0.6mA . It is an automotive-grade chip solution that can withstand high temperatures of 125 degrees and complies with ASIL-B.

About Cypress Automotive Electronics Systems

Cypress works with the world's top automotive companies to develop industry-leading automotive electronic systems, including advanced driver assistance systems ( ADAS ), 3D graphics display, wireless connectivity, full-featured touch screens, and superior body electronics. Cypress's automotive product portfolio includes: Traveo and PSoC® MCUs , CapSense® capacitive sensing solutions, TrueTouch® touch screen solutions, Wi-Fi® , Bluetooth® and USB connectivity solutions, power management ICs ( PMICs ), NOR flash, F-RAM™ and SRAM memories. Cypress is committed to providing zero-defect products and excellent services, and complies with the most stringent automotive industry standards.