Smart Battery Backup for Uninterruptible Energy Part II: Functionality and Operation of BBU Microcontrollers

summary

The Open Compute Project (OCP) is a nonprofit organization focused on promoting communication among companies on data center product design and best practices. Recently, the organization released the Open Rack Version 3 (ORV3) specification. The more notable change in the specification is the migration of the design architecture from 12 V to 48 V. This is the second in a five-part series focusing on Analog Devices’ battery backup unit (BBU) reference design. “Smart Backup Batteries for Uninterruptible Energy Part 1: Electrical and Mechanical Design” discusses the electrical and mechanical design considerations for the BBU. Part 2 will take a deep dive into the microcontroller’s software, which is primarily responsible for ensuring that processes run smoothly, thereby ensuring high efficiency and capacity for the BBU. Hardware and software must work together smoothly to achieve a system-level solution that meets the specification requirements.

Introduction

Implementing good housekeeping in BBU modules is critical and can provide significant benefits. The main benefit of keeping the circuits organized is that safety is enhanced and improved, thereby reducing the risk of electrical fires and other hazards in the module. In addition, good housekeeping practices can prevent electrical faults that can interfere with normal operation, thereby improving circuit performance and extending service life. It is essential to regularly monitor and organize the circuits to identify and resolve possible problems or faults. Prioritizing good housekeeping practices can ensure excellent circuit safety and performance, and this method is simple but effective and should not be underestimated. Each BBU module uses a microcontroller and a battery management system (BMS) microcontroller. In the Analog Devices reference design, the ultra-low power Arm® microcontroller MAX32690 acts as the microcontroller and is responsible for executing six important processes (see Figure 1). The MAX32625 microcontroller acts as the BMS microcontroller of the reference design. The BMS microcontroller is responsible for communicating with the ADBMS6948 chip and passing all measurement data to the microcontroller.

The six processes of the microcontroller are as follows:

►Performs housekeeping tasks and communicates with various peripherals via I2C protocol.

►Processing of discharge sequence provided by backplane voltage data.

►Choose constant current mode or constant voltage mode to handle battery charging.

►Change the charge and discharge status of the BBU module.

►Fault handling and response.

►Respond to Modbus commands as a Modbus slave.

Figure 1. Management operation cycle of the main controller 1

Microcontroller Process:

Performs housekeeping tasks and communicates with various peripherals via I2C

The microcontroller acts as the I2C master device when many auxiliary devices are connected to the module circuit. The core microcontroller collects and saves data from the auxiliary devices while acting as the I2C master device. In order to facilitate the smooth progress of various tasks, the microcontroller is also responsible for controlling various I2C auxiliary devices. Examples of several I2C auxiliary devices are as follows:

►The BMS microcontroller uses MAX32625.

►LTC2971, dual-channel power system manager.

►MAX31760, precision fan speed controller.

►24AA512T/EEPROM, used as data memory to retrieve and store important data.

►LTC2991, temperature sensor.

BMS Microcontroller (MAX32625)

The microcontroller communicates with the BMS microcontroller (MAX32625) at regular intervals to receive updates about the cell voltage, state of charge (SOC), state of health (SOH), cell temperature of each battery pack, and any faults that may have occurred in the battery pack. Updates are made every four minutes because the cell voltage, SOC, SOH, and temperature are not expected to change rapidly. If any fault occurs, the shared pin between the two microcontrollers is asserted high and triggers an interrupt on the microcontroller, which immediately reads the BMS microcontroller to obtain information about the fault. There is a dedicated I2C port in the microcontroller that is used only to communicate with the BMS microcontroller to support fast communication between the two microcontrollers.

LTC2971 (Power Management IC)

The microcontroller constantly communicates with the LTC2971 via the Power Management Bus (PMBUS) protocol to check voltage, current, and temperature measurements as well as warnings and faults. When measuring the backplane output voltage parameters, the LTC2971 provides fast feedback so that the microcontroller can adjust its routines. In addition, the device adjusts the feedback voltage of the power converter and allows it to reduce the output voltage by 1%, ensuring that the output voltage is within regulation during the discharge mode of operation.

MAX31760 (Fan Controller)

The MAX31760 is responsible for regulating the fan speed of the BBU module. The duty cycle of the pulse width modulation is configured by the microcontroller to adjust the fan speed through I2C. The microcontroller calculates and adjusts the required fan speed based on the temperature and the backplane load current or battery pack load current.

24AA512TT (EEPROM/data storage)

The onboard EEPROM acts as an external storage device for the entire BBU module. The microcontroller writes to the flash pages via I2C to periodically save important information such as battery voltage level, SOC, SOH, cell type and model year, and board temperature to the EEPROM. This data is updated every hour and can be accessed by the user during maintenance and troubleshooting.

LTC2991 (Digital Onboard Temperature Monitoring)

The LTC2991 is an octal voltage, current and temperature sensor. The device monitors the temperature of the battery module with the help of various digital sensors placed at strategic locations inside the battery module. Based on the temperature readings, the microcontroller can adjust the fan speed to ensure that the operating temperature of the power board and battery stack remains at an appropriate level and is always below 40°C.