Power supply units (PSUs) and base unit bundles (BBUs) are the lifeblood of data centers, networks, servers, and storage devices in the Open Compute Project's (OCP) cutting-edge Open Rack Version 3 (ORV3) architecture. The central power converter supplies most of the necessary electrical power. However, the unsung hero is the auxiliary power component, which plays a vital role in maintaining the overall robustness, reliability, and security of the entire power ecosystem, including both the PSU and BBU.

In the following detailed analysis, we will examine the role of the auxiliary power supply in the BBU module's reference design, exploring its functions and internal mechanisms. By delving into the complexities of the auxiliary power supply, our aim is to highlight its important contribution to maintaining a continuous power supply and protecting our valuable technical resources from potential damage.

Standby Power:
Auxiliary power is used to provide secondary power within a BBU (Battery Module Unit). Even if the motherboard power is interrupted or unavailable, the supplemental power continues to power the devices inside the module, enabling the smooth operation of key functions stored in the BBU. This standby power capability allows the unit to maintain and sustain critical operations, such as a safe transition process, vigilant monitoring, management of complex control circuits, and continuous support of low-power devices. Auxiliary power ensures the modules' ability to distribute power when needed, providing a continuous and uninterrupted supply during power outages. This, in turn, acts as a buffer against potential disturbances and prevents data loss.

Voltage Regulation:
Ensuring a constant state of equilibrium and a constant voltage output is vital for achieving optimal performance of the devices within the BBU. The importance of auxiliary power in this scenario cannot be overstated, as it acts as a crucial protective measure, monitoring the sensitive area of ​​voltage control within the sophisticated architecture of a BBU module. Auxiliary power constantly monitors the output voltage, coordinating adjustments and stabilization within the precise limits of a meticulously defined tolerance spectrum.
This regulation acts as a shield, improving the interaction between the BBU and the connected devices. Auxiliary power guarantees a stable and reliable power source, protecting against voltage fluctuations that could otherwise lead to malfunctions, data corruption, or physical damage.
Auxiliary power becomes essential thanks to its precise calibration. It not only keeps the module operating efficiently but also protects the connected devices. This combination of precision and reliability paves the way for the PSU, BBU, and a host of associated devices in the data center to have a longer lifespan, greater efficiency, and lasting operational vitality.

Cooling and Fan Control:
Effective cooling management is critical to preventing electrical equipment from overheating. Auxiliary power supplies and coordinates the fans within the BBU. This cooling process helps preserve the BBU and compatible devices. This configuration uses auxiliary power to manage fan speeds and create a smooth and efficient cooling environment. The result is a well-balanced system that maintains optimal operating temperatures and protects against the damaging effects of overheating.
Heat dissipation involves complex interactions within thermal dynamics. This system manages heat to prevent overheating failures. Careful temperature regulation and auxiliary power increase performance and reliability.

Protection and Safety Features:
BBUs have a number of vital safety and protection features that safeguard both connected devices and the power converter. The integration of auxiliary power is essential for enabling the deployment and monitoring of these functions. Auxiliary power allows for a range of proactive measures, including overvoltage, overcurrent, and short-circuit protection, as well as temperature monitoring. This real-time monitoring via auxiliary power ensures a rapid response to anomalies or problems through continuous parameter checks. The rapid activation of protection mechanisms effectively prevents damage to the power supply and connected devices, mitigating electrical risks and significantly improving overall system safety.

Diagnostic Evaluation:
The BBU performs periodic self-diagnostic tests to validate its functionality before supplying power to connected devices. In this process, the auxiliary power supply provides the necessary voltage and control signals to initiate and complete the diagnostic routine. This self-assessment helps quickly detect potential faults within the BBU, including component problems or voltage irregularities. The involvement of the auxiliary power supply contributes to improved performance and extended PSU lifespan by enabling early identification and precise localization of faults. This proactive approach increases PSU reliability and readiness, ensuring uninterrupted power delivery and significantly reducing the risk of system failures.

ADI's LTpowerCAD® power supply design tool provides customized technical information and performance data for components specifically designed for BBU auxiliary power. This powerful combination successfully streamlines the complex electrical evaluation process, accelerates the prototyping phase, and significantly speeds up the overall development timeline for auxiliary power circuits. As a result, review time and circuit design complexity are noticeably reduced.
Figure 1 illustrates an auxiliary circuit designed to optimize power flow during BBU operation in charge or discharge mode, reflecting the intensive engineering efforts involved. In contrast, Figure 2 shows a low-power auxiliary circuit for BBU standby mode, featuring low-dropout regulators (LDOs) and a single buck converter.

Figure 1. Design of an auxiliary circuit for a BBU module during operation in charge and discharge mode.

Figure 2. Design of an auxiliary circuit for a BBU module during sleep mode.
To power these power converters, microcontrollers, and other peripherals, the BBU module's auxiliary circuit comprises six voltage rails, as shown in Table 1.

Table 1. BBU Voltage Rails

During operation in charge or discharge mode
During operation in sleep mode
► 12V bias rail
► 5V, 3.3V, 1.8V and 1.2V bias rails
► -3.0V bias rail

► 3.3V, 1.8V and 1.2V bias rail

Auxiliary power supply during BBU charging or discharging operation.
12V bias rail.
The LT8645S is a high-voltage synchronous buck driver with a remarkable load capacity of up to 8A. Its primary function is to efficiently convert the 48V mainboard voltage supply into a 12V auxiliary voltage supply, which it does with expert precision. The integration of bypass capacitors in the device sets it apart from other high-voltage buck drivers—a strategic choice that not only eliminates the need for a larger printed circuit board but also skillfully addresses challenges such as rapid current loops and electromagnetic interference (EMI) emissions. This combination results in a significant increase in overall efficiency, enhancing the driver's ability to optimize power consumption.

The 12V bias rail serves as the backbone for critical components, including power converters, fan power supplies, and current-sharing bus circuitry. By acting as the primary power conduit for these essential components, the 12V bias rail enables uninterrupted operation and performance. These components are seamlessly integrated under a single power supply, and the LT8645 plays a crucial role in enhancing efficiency and functionality.

5V, 3.3V, 1.8V, and 1.2V Bias Rails.
The use of the LT8692S, a four-channel synchronous buck driver, was a deliberate choice to provide a wide range of outputs: 5V, 3.3V, 1.8V, and 1.2V. This setting was especially important to accommodate the auxiliary voltage reduction, in line with the lower bus voltages. Guided by a unique oscillator operating at 2 MHz, the driver coordinates its outputs with precision and synchronization.
What truly sets the device apart is the integrated capacitors, a critical feature for meeting stringent EMI performance requirements, as it minimizes the design's susceptibility to noise. This feature significantly improves its compatibility with noise-sensitive environments and applications. The four-channel architecture, characterized by its high integration, not only saves space but also fosters a more concise and efficient design approach.
To explain how it's used, the 5V rails provide power for the amplifier's power rail, the Modbus UART controller, the digital temperature sensors, and the power management device. The 3.3V, 1.8V, and 1.2V outputs power the EEPROM device, the main microcontroller (MCU), and the battery management system (BMS) MCU, respectively.

The LTC1983 is a good choice as a charge pump inverter, providing a critical 3V negative feed rail for the operational amplifiers driving the BBU. Its ability to create a powerful 100mA output load requires only a couple of additional capacitors, demonstrating its inherent efficiency. Notably, this efficiency is further enhanced by its small size, a design feature that offers specific advantages, as it fits within a mere 1mm of the BBU's power board.

Auxiliary power supply during BBU sleep mode operation.
3.3V, 1.8V, and 1.2V bias rails.
The MAX17551 stands out as a synchronous buck converter, transforming the 48V battery bank voltage into a stable 3.3V output. This voltage acts as a strategic conduit, channeling power to the general-purpose digital pins of both the main MCU and the BMS MCU while they are in sleep mode. This buck converter was chosen for its strengths: its versatility across input voltages ranging from 4V to 60V, its small size, and its exceptional operating efficiency, with a minimum power consumption of less than 10mW. This convergence of features makes it the best solution for improving the overall durability and resilience of the battery bank.
The ADP165 achieves more sophisticated voltage reduction, precisely lowering the supply voltage from 3.3 V to 1.2 V. This specific reduction serves as the primary power source for the basic operations of both the main MCU and the BMS MCU. The strategic integration of an LDO regulator within the microcontroller circuitry enables better voltage regulation, significant noise reduction, an optimized design architecture, superior operating efficiency, and a high level of reliability. Furthermore, the ADP165 consumes less than 15 µW of power, significantly contributing to energy savings and, consequently, extended battery life.
Finally, the MAX38911with its precise reduction of the supply voltage from 3.3 V to 1.8 V. This LDO regulator can supply up to 500 mA of load current, sufficient to provide the necessary load current for the microcontrollers. The power converter consumes approximately 19.2 μA of supply current in low-power mode, making it suitable for sleep mode operation.
The importance of effective thermal management for maintaining consistent optimal performance cannot be overstated, given the remarkable efficiency of the auxiliary power supply, which reaches 94% during charging or discharging and 62% in standby mode. Selecting the best device for the auxiliary circuitry is crucial beyond ordinary functionality, as it forms the basis for design integrity across the entire operating spectrum of the BBU. This strategic choice allows for predicting total power consumption and calculating the estimated battery life. Furthermore, it offers cost-effective opportunities to improve the auxiliary's performance, ultimately leading to an overall improvement in unit efficiency.¹

Summary:
In this final article of the series, auxiliary power is presented as a crucial component of battery unit units (BBUs) that coordinates several important functions. Its role entails a number of key responsibilities, all of which are fundamental to the smooth, reliable, and safe operation of these units. As a shield for backup power, auxiliary power adjusts voltage levels, manages fan power for thermal cooling, implements protection measures, and performs power-on self-tests. This sophisticated interplay of operations converges to provide an uninterrupted power supply, protect our devices, and extend the overall vitality of the system.
Auxiliary power is crucial for providing reliable and supplemental power to our analog and digital devices. Advancements in auxiliary power technology are constantly improving efficiency, reducing power losses, and increasing the safety of data center power supply units (PSUs) and battery unit units (BBUs), driving innovation to new heights. This continuous progress is an unwavering response to the growing power demands generated by our ever-changing technological world, which influences both our present and our electrified future.
Each installment in this series of articles serves as a comprehensive resource, providing precise guidance and equipping design and application engineers with the knowledge needed to build a smarter, more reliable, and more cost-effective solution based on the sophisticated specifications provided by the BBU OCP ORV3. This is achieved by providing a meticulous parts selection process, prepared guidelines, engineering methodologies, and procedural recommendations.
References
1 David Sun. “Open Compute Project Open Rack V3 48 BBU Rev: 1.3.” Open Compute Project, November 2022.

About the author

Christian Cruz is an Application Development Engineer at Analog Devices, Inc., Philippines. He holds a Bachelor of Science in Electronic Engineering from Eastern Manila University, Philippines. He has over 12 years of engineering experience in analog and digital design, firmware design, and power electronics, including the development of power management integrated circuits, as well as AC-to-DC and DC-to-DC power conversion. He joined ADI in 2020 and is currently responsible for power management requirements for cloud computing and systems communications applications.

About the author

Ralph Clarenz Matociños graduated with a Bachelor of Science in Electronic Engineering from Pamantasan ng Lungsod ng Maynila (PLM) in Manila, Philippines. He has over a year of engineering experience and specialized knowledge in analog and digital design, as well as power electronics, including the development of integrated circuits for battery management systems and DC-to-DC power conversion.