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Why are 54V BLDC motors used in server applications today?

You may have heard of “cloud computing”, but most people’s understanding of “cloud computing” is limited to the fact that we can access important data through computers and smartphones, regardless of where the data is actually stored. So what exactly is cloud computing? Cloud computing refers to a network of many remote servers responsible for storing and moving data around the world, which we can access via Wi-Fi, LAN, or cellular networks. These remote servers act like large storage facilities, consisting of clusters of multiple servers and housed in warehouses often called server farms. These server farms need to be at a constant ambient temperature (the ideal temperature range is 68 to 71 degrees Fahrenheit) to perform at their best and minimize failures. These server farms are usually cooled by a central air-conditioning system, or heated by a central heating system, depending on where they are located (for example, a common office area).

Server racks actually use a series of fans to cool the electronics inside. As most of us should have experienced, Electronic devices can get hotter and hotter during use, ultimately causing them to not perform at their best. To minimize the cost and size of the heat sink, we can switch to brushless direct current (BLDC) fans, which use airflow from the environment to cool the electronics, which means that the heating, ventilation, and air conditioning (HVAC) system maintains a constant ambient temperature, and then blow air through BLDC fans to cool the electronics in the server racks.

In traditional server applications, 12V BLDC fans are often used to cool the electronics in the cabinet. But just like automotive applications, server applications tend to switch to 54V BLDC motors for some reason. This article will discuss the two main reasons server manufacturers are adopting 54V BLDC motors to replace traditional 12V BLDC motors, and discuss typical components required for 54V motor drive applications, as well as some common motor control algorithms.

Server manufacturers are using 54V BLDC motors instead of traditional 12V BLDC motors, first because they can use a quarter of the current, which in turn can use thinner copper wire. In addition, since the same amount of work can be done with less raw material, the motor manufacturer can also reduce the size of the motor and therefore reduce the overall cost of the motor. The second point is because server manufacturers can save on expensive cabling costs, a 54V BLDC motor can power four times the number of motors with the same power bus gauge cable compared to a 12V BLDC motor. High voltage motors can achieve the same power using thinner cables or narrower PCB traces.

For example, in a 450W server, a 12V BLDC fan consumes 32W. The current required to power it is easily calculated by the power formula (P= V x I, I = P/V, 32W/12V=2.67A). With the same power requirements, the current required with a 54V BLDC fan is reduced to about 0.67A, allowing server engineers to use 26-gauge American Wire Gauge (AWG) wire, compared to 12V Powering the BLDC fan requires 20 gauge AWG wire. In terms of PCB trace width, a 12V BLDC fan needs 0.1 inches, while a 54V BLDC fan only needs 0.012 inches, which allows server engineers to save a lot of board space when routing all power traces to the server system.

Server manufacturers have an added advantage of using 54V BLDC motors, which is the ability to run the motors at higher RPMs in the same form factor as traditional 12V BLDC motors, thereby increasing air density. However, increasing the torque power of the motor requires higher power, which will require additional current to support it. For example, server manufacturers can use 50W BLDC motors instead of traditional 32W motors to achieve greater airflow. A 54V BLDC motor only needs 0.93A of current, which is much less current than a 12V BLDC motor needs to drive a 50W motor. If you use a 12V BLDC motor to do the same amount of work, you need at least 4.17A. This means wider PCB traces and thicker cables, which are expensive. Using a 54V bus voltage allows server manufacturers to run fans at higher speeds, increasing airflow density while reducing cabling costs.

Challenges of using a 54V power bus

There is a challenge for the electronics driving the 54V BLDC fan motor. That is server engineers can’t use old 12V hardware to drive 54V motors. They need to use higher operating voltage electronics to have enough headroom for a 54V supply. Figure 1 shows a simplified block diagram of a 54V BLDC control circuit, which identifies typical components used to drive a 54V BLDC motor without affecting the mature motor control algorithm.

Why are 54V BLDC motors used in server applications today?

Why are 54V BLDC motors used in server applications today?

Figure 1: Simplified block diagram of a 54V BLDC control circuit (which identifies typical components used to drive a 54V BLDC motor without affecting mature motor control algorithms)

However, there are several hardware solutions on the market that can help simplify this transition. For example, Microchip’s MIC28514 75V synchronous buck regulator is an excellent solution for first-stage power conversion (Figure 2). With a 5A output current capability, the device is capable of powering multiple BLDC systems from a single 54V rail. The MIC28514 converts a 54V power bus rail to a traditional 12V power rail with over 90% power efficiency, allowing server engineers to continue using the original motor control algorithms and proven active components.

Why are 54V BLDC motors used in server applications today?

Why are 54V BLDC motors used in server applications today?

Figure 2: Typical Power Efficiency-Output Current Curve Using Microchip’s MIC28514 75V 5A Synchronous Buck Regulator

MOSFET drivers and MOSFET inverter circuits must also meet the requirements of high voltage MOSFETs, and 54V BLDC applications typically require 80V power MOSFETs. However, compared to 12V systems, the current requirements have been reduced by a quarter, and the MOSFET on-resistance has become less critical.

Figure 3: Microchip’s MIC4607 is an 85V three-phase MOSFET driver with adaptive dead-time and shoot-through and over-current protection

High Voltage Electronics Enables Choice of 54V BLDC Motors in Server Applications

Chipmakers such as Microchip Technology have developed high-voltage integrated circuits such as the MIC28514 75V synchronous buck regulator, enabling customers to take advantage of 54V BLDC motor technology while still using proven motor control algorithms and other active components. With these high-voltage devices, server manufacturers can adopt 54V power bus technology and thus be able to use smaller motors, thinner copper wires and narrower wiring on the PCB, thereby reducing the overall system cost. In addition, due to the increased voltage, these devices can also increase the air density in the same form factor. With the gradual popularization of cloud computing and the continuous improvement of cloud computing functions, server manufacturers must adopt solutions with the most reasonable price and the best performance in order to remain competitive in the price war and performance war.

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