Smart Battery Backup for Uninterruptible Energy Part 1: Electrical and Mechanical Design

summary

This article outlines the system requirements for the Open Compute Project Open Rack Version 3 (OCP ORV3) Battery Backup Unit (BBU). The article highlights the importance of an efficient, intelligent BBU that can provide power during power outages. In addition, the article shows the analog and digital design solutions, electrical and mechanical solutions, and the architecture developed to meet the written specifications.

introduction

Data centers power the internet, connecting communities around the world. Social media companies such as Facebook, Instagram, and X (formerly Twitter) rely on data centers to disseminate and store information, while search engines such as Yahoo and Google use data centers to support their primary search engine and storage functions. Almost all large companies and government agencies around the world require reliable data center capabilities to operate and maintain their primary business functions through intelligent computing, storage, and search. As the number of users increases year by year, data center capacity continues to grow at an astonishing rate to keep up with demand and technological advancements. In order to keep up with the growing demand, the system architecture of data centers must also be continuously updated and upgraded.

OCP is an organization that shares data center designs, and its system architecture definition is based on the Open Compute Project Open Rack Version 2 (OCP ORV2), where the backplane voltage is nominally 12 V and the system power is 3 kW. On the other hand, increased usage has led to increased power requirements, which makes the power requirements of 12 V systems too high, which is detrimental to overall system performance. To solve this problem, the backplane voltage was increased to 48 V while the system power remained the same, thereby minimizing the required current and copper traces, and reducing the heat dissipated by the backplane. This change improves overall system efficiency and reduces the need for complex cooling systems. This is the basis for the new Open Rack Version 3 standard (OCP ORV3).

Figure 1. OCP ORV3 power architecture.

Data center reliability is a basic condition for ensuring normal operation. Adding BBU to the system can provide system redundancy. If a power outage or brownout occurs, the system needs time to detect the situation, save important data, and switch operations to another data center server (most likely located in a different data center facility and location). These response operations must be completed in a seamless manner. Each rack uses a backup power system to regulate the delayed power supply of the system. This requirement is clearly defined in the latest standard ORV3 BBU: Based on the power stored and regulated by the lithium-ion battery, each BBU unit must provide 15 kW power output to maintain system operation for 4 minutes.

Under the guidance of this specification, ADI has worked with the OCP organization to complete and produce a reference design solution, which includes: a bidirectional power converter for dedicated charging and discharging operations through a single circuit, a battery management system (BMS) device, an on-board design system host microcontroller with firmware and GUI support, and hardware amplification.

Design requirements and hardware implementation

The OCP organization provides a specification (version 1.3) that outlines the concept and design requirements needed to meet the BBU module standard. The BBU module reference design is based on the ORV3 48 V proposal and consists of a battery pack with a BMS, charge/discharge circuits, and other functional blocks, as shown in Figure 2.

Figure 2. OCP ORV3 BBU block diagram.

In addition to the circuit requirements, the BBU module needs to have several main operating modes during its service life, as follows:

►Sleep Mode: The BBU module is in shipping or inventory status, or is not connected to an active bus. The battery discharge current is minimal at this time to extend storage time. BBU monitoring or reporting functions are not available in sleep mode. When the bus voltage is detected to be above 46 V and the duration is greater than 100 ms and less than 200 ms, and the PSKILL signal is low, the BBU will wake up and exit sleep mode.

►Standby Mode: The BBU module is fully charged and operating normally, and continuously monitors the bus voltage to prepare for a discharge event. The BBU module operates in this mode for most of its service life. The status and parameters of the BBU module are displayed on the upstream rack monitor via the communication bus.

►Discharge mode: When the bus voltage drops below 48.5 V and lasts longer than 2 ms, the BBU module discharge mode is activated. The BBU module is expected to take over the bus voltage within 2 ms, with a backup time of 4 minutes.

►Charge Mode: When all conditions are met, the BBU module enables its internal charger circuit to charge its battery pack. The charge current can be anywhere between 0 A and 5.5 A, depending on the last depth of discharge of the battery capacity. It also allows the upstream system to override the charge current through the communication bus. There should be a charger timeout control scheme based on the calculated charge current.

►State of Health Check (SOH) mode: The BBU module routinely tests the battery pack capacity by forcing the battery pack to discharge. The BBU module should perform an SOH test every 90 days to determine the EOL status of the battery.

►System control mode: The BBU should allow the upstream system to control the charging/discharging operation through the communication bus.

In addition to the BBU module operation requirements, OCP also specifies standards for battery pack capacity, cell type, and battery pack configuration. The specific instructions are as follows:

►Battery pack capacity: The BBU module can provide 3 kW backup power for no more than 4 minutes over a 4-year period.

►Battery cell type: The BBU module should be lithium-ion 18650 type, with a cell voltage of 3.5 V to 4.2 V, a battery capacity of at least 1.5 AH, and a continuous rated discharge current of 30 A.

►Battery pack configuration: The battery pack configuration of the BBU module is 11S6P (6 series combinations are connected in parallel, and each series combination consists of 11 cells connected in series)

In addition, the BBU module requires a BMS to provide battery charge/discharge algorithms, protection, control signals, and communication interfaces. The BMS is also responsible for establishing a cell balancing circuit to keep the cell voltage in the battery pack within a ±1% (0.1 V) tolerance.

The reference design block diagram (see Figure 3) shows the selected devices and various components integrated to accomplish certain tasks, forming a circuit capable of providing uninterruptible power, determining module health and faults, and performing module communications. The LT8228 is a bidirectional synchronous controller that resides within the BBU module. The device provides power conversion in the event of a line power interruption and provides battery charging during non-fault operation. The LT8551 is a 4-phase synchronous step-up DC-DC phase expander that works in conjunction with the LT8228 to increase the discharge power delivery capability to 3 kW per BBU module. In addition to the power conversion IC, the BBU module also includes the MAX32690, an ultra-low power Arm® microcontroller that is responsible for overall system operation. The LTC2971 is a 2-channel power system manager that implements precision sensing and fault detection of the power path, as well as critical voltage drop functions. The MAX31760 is a precision fan speed controller that performs system cooling functions during charging and discharging operations. The EEPROM is used as a data storage device to allow the user to recover any useful data during the time the BBU module is available. In addition to the power converter and the microcontroller for general housekeeping tasks, the design also includes a BMS IC. The ADBMS6948 is a 16-channel multi-cell battery monitor that monitors the battery voltage level, while its inherent coulomb counter is used to determine the state of charge (SOC) and SOH levels for cell balancing and battery life expectancy calculations. The battery health monitoring routine is completed by the ultra-low-power Arm microcontroller MAX32625. Both microcontrollers have been carefully selected to reduce the overall power consumption, thereby extending the battery life during the BBU sleep mode of operation.

In addition to the supplied components, the reference module also provides and builds a BBU module (see Figure 4a) and a BBU laminate (see Figure 5) to house and demonstrate a reference design that complies with the OCP ORV3 BBU module and laminate mechanical specifications. The BBU laminate includes six BBU module slots so that a single BBU laminate can provide up to 18 kW of backup power as needed.

Figure 3. ADI OCP ORV3 BBU block diagram.

Figure 4. (a) 3D rendered mechanical overview of the ADI BBU module and (b) airflow simulation.

Mechanical rendering and airflow simulation are two architectural advantages of the BBU module reference design. First, it supports visualization, which provides an accurate and attractive representation. Mechanical structure analysis can identify design issues and potential changes as early as possible, which helps the entire design process. Last but not least, it can reduce the need for time-consuming and expensive actual prototypes. In addition, airflow simulation can provide performance analysis, help identify potential problems and improve design efficiency. It is also responsible for thermal management, which can assist in identifying hot spots, optimizing heat losses, and enhancing overall system reliability. In addition, it can also plan the battery pack space according to safety and compliance requirements, thereby reducing risks. See Figure 4b for more information.