WBG Devices Drive Efficiency in Data Centres

Author : Rolf Horn, Digikey

06 July 2023

Figure 1: WBG semiconductors provide better performance than other substrates across numerous different parameters (Source: ResearchGate)
Figure 1: WBG semiconductors provide better performance than other substrates across numerous different parameters (Source: ResearchGate)

Data centres play a crucial role in this increasingly digital, connected and virtualised world. Since these facilities have huge energy requirements, power solutions that can reduce losses, boost efficiency and enhance thermal control are required.

Traffic on the Internet has risen considerably in recent times - due to a more significant number of users, the widespread utilisation of mobile devices and social networks, plus the remote storage of information in the cloud. According to analysts, the growth of this traffic still needs to reach full saturation.

Forecasted increases raise questions regarding equipment efficiency and electricity consumption. This, in turn, spurs the development of new energy-efficient power conversion technologies - like those offered by wide bandgap (WBG) devices.

Efficiency is paramount
In addition to the associated physical infrastructure, a data centre houses large numbers of networked computer servers - for the processing, storage and distribution of data. These will be key for powering the Internet, cloud computing resources and corporate intranets.

Energy demand is moving upwards - because of the growing volume of digital data being created and subsequently worked with. In addition to powering racks, data storage and network units, data centres also need auxiliary cooling and ventilation equipment, in order to remove the heat produced during data processing workloads and the conversion of electrical power.

The typical structure of power conversion systems used in data centres comprises several AC/DC, DC/AC and DC/DC voltage converters, on which the efficiency of the entire data centre strictly depends. There are two key benefits that lowering the losses in the converters that power the data processing and storing devices will bring. These are: 

•Firstly, there is no need to supply the energy not being converted to heat.
•Secondly, there is a reduction in the energy required to dispose of the waste heat.

Data centre efficiency is often measured with the power usage effectiveness (PUE) metric. Developed by the Green Grid as a standard way to compare data centre energy use, the PUE is defined as the overall data centre energy use ratio to information technology (IT) equipment energy use.

The PUE measure is a basic enough statistic to identify areas for potential development. Despite not being a perfect metric, it has become an industry standard. The PUE should ideally be close to unity, meaning that the data centre only requires electricity to support its IT demands. However, according to the National Renewable Energy Laboratory (NREL), the average PUE is around 1.8. Data centres' PUE values range widely, but efficiency-focused ones frequently attain PUE values of 1.2 or less.

A high PUE can have different causes, such as the following:

•‘Zombie’ (or ‘comatose’) servers and uninterruptable power supplies (UPSs), meaning equipment is turned on but not fully utilized. It comprises unintentionally idle devices that consume electricity without visibility or external communications.
•The implementation of inefficient backup and cooling strategies.
•Data centre operations are more focused on reliability than on efficiency.

Adding variable frequency drives (VFDs) to cooling fans and minimising the number of servers and UPSs are two common methods for lowering PUE. In the last few years, the transition from legacy 12V architectures to more efficient 48V ones (as shown in Figure 1) has reduced I2R power losses substantially, providing increasingly power demanding processing systems with more efficient solutions. Using 48V in power architectures results in 16x lower I2R losses. This helps meet the most stringent of energy efficiency requirements, considering that a 1% efficiency improvement can save several kW at the whole data centre level.

Benefits of WBG semiconductors in data centres
Although silicon (Si) is the most well-known technology, it has a smaller bandgap than WBG materials, like gallium nitride (GaN) and silicon carbide (SiC). Consequently, Si can only support lower operating temperatures and it has voltage restrictions too.  

Adopting WBG power devices, in place of Si ones, can be a more effective alternative. A WBG approach overcomes the limitations of Si technology - providing solutions with higher breakdown voltages, faster switching frequency, lower conduction and switching losses, better heat dissipation, plus smaller form factors (see Figure 1). This results in higher efficiency of the power supply and power conversion stages. As mentioned earlier, in a data centre, even a single percentage point increase in efficiency can translate into substantial energy savings.

GaN devices 
GaN is an emerging WBG material, with an electron bandgap that is 3x larger (3.4eV) than Si (1.1eV). Additionally, GaN has twice the electron mobility compared to Si. GaN's well-known and unparalleled efficiency at very high switching frequencies is made possible because of this exceptional electron mobility. Such properties allow GaN-based power devices to withstand stronger electric fields in a smaller die size. More compact transistors and shorter current paths result in ultra-low resistance and capacitance, allowing for up to 100x quicker switching rates to be attained.

Reduced resistance and capacitance also increase power conversion efficiency, making more power available to carry out data centre workloads. Instead of producing more heat, which would require a greater amount of cooling, an increased number of data centre operations may be done per W. High-speed switching also decreases the size and weight of energy-storing passive components, because each switching cycle stores substantially less energy. Another advantage of GaN is its ability to support different power converter and power supply topologies.

GaN’s key features that prove most relevant to data centre applications are:
•Support for hard and soft switching topologies.
•Fast turn-on and turn-off (with the GaN switching waveform being almost identical to the ideal square wave).
•Zero reverse recovery charge.
•Properties that outperform Si technology.
o10x higher breakdown field
o2x higher mobility
o10x lower output charge
o10x lower gate charge and linear output capacitance (Coss) characteristics

Figure 2: High-efficiency GaN SMPS for data centre servers (Source: Infineon)
Figure 2: High-efficiency GaN SMPS for data centre servers (Source: Infineon)

These features allow GaN power devices to make solutions that achieve:
•High efficiency, power density and switching frequencies.
•Reduced form factor and on-resistance.
•Lower weight.
•Nearly lossless switching operation.

A typical target application for GaN power devices is shown in Figure 2. These high-voltage bridgeless totem-pole PFC stages and high-voltage resonant LLC stages can meet the uncompromising requirements of server switched mode power supplies (SMPSs), achieving a flat efficiency above 99% over a wide load range and high power density.

SiC technology
One of the first applications of SiC power devices in data centres was in relation to UPS equipment. A UPS capability is essential for data centres to prevent the potentially disastrous effects of a mains-voltage failure or operational interruption. Power supply redundancy is crucial to ensure that a data centre maintains continuity and dependability. Optimising the data centre's PUE is a priority for operations management.

A reliable, constant power source is necessary for a data centre. Voltage and frequency-independent (VFI) UPS systems are regularly employed to meet this requirement. An AC/DC converter (rectifier), a DC/AC converter (inverter) and a DC link are the constituent elements of a VFI UPS device. A bypass switch, primarily used during maintenance, connects the UPS output directly to the AC power source at the input. In the event of a mains power breakdown, the battery (typically made up of many cells) connects to a buck or boost converter and feeds the power supply.

Because the alternating voltage at the input is converted to direct voltage and then again into a precisely sinusoidal output voltage, these devices are typically double-conversion circuits. In addition to isolating the system from the power source, the conversion process shields the load from voltage fluctuation issues.

Until recently, insulated-gate bipolar transistors (IGBTs) with three-level switching topologies had the best efficiency results. 96% efficiency levels were achieved thanks to this approach, which is a significant improvement over earlier transformer-based models.

SiC transistors have made it possible to significantly reduce (by >70%) power losses and increase efficiency in double-conversion UPS systems. The remarkable efficiency levels that result (over 98%) persist whether in low or heavy load scenarios. They are obtainable because of the intrinsic properties of SiC. Compared with traditional Si-based devices, such as MOSFETs and IGBTs, SiC equivalents can operate at higher temperatures, frequencies and voltages.

An additional benefit of SiC-based UPS implementations is a better heat loss value (or heat rejection), which enables operation at higher temperatures. This feature lets designers adopt more compact and economical cooling solutions. Overall, a SiC-based UPS is more efficient, lighter and smaller than an equivalent model with Si-based components.

SiC devices can operate at higher temperatures than traditional Si devices previously used, due to their inherent properties. The customer's cooling costs can thus be reduced because of the UPS's lower heat loss and ability to operate at higher temperatures.

When maximising the available floor space in a data centre, a SiC-based UPS reduces weight and size compared to the equivalent Si-based UPS. Moreover, a SiC-based UPS requires less floor space - increasing the available power capacity in a given area.

Conclusion
In summary, WBG semiconductors will establish a new trajectory for power electronics in demanding applications such as data centres. Their benefits include increased system efficiency, lower cooling system requirements, operation at heightened temperatures and higher power densities. With the integration of GaN and SiC power devices into voltage converters and power supplies, data centre operators’ goal of achieving greater efficiencies, maximising the floor space utilisation and reducing operating costs across facilities are being realised.


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