How SiC MOSFETs Improve Power Conversion Efficiency in AI Data Centers

Everything is stored in the cloud these days, but where exactly is the cloud?

The answer is in the data center. Our insatiable appetite for pictures, videos, and other content is driving the data center industry to boom.

The rapid development of the artificial intelligence (AI) industry is leading to a surge in power demand in data centers. Data center power consumption is expected to more than double in the three years between 2022 and 2025, according to the International Energy Agency (IEA). This not only increases operating costs, but also puts tremendous pressure on the aging power infrastructure that is already overwhelmed, and requires large-scale investment upgrades.

As data center power consumption increases dramatically, the industry is more eager to use power semiconductors that can efficiently convert power. This demand is driven by reducing operating costs and greenhouse gas emissions to achieve net zero emissions. In addition, the industry is constantly pursuing lower-cost and smaller power systems.

Heat dissipation is another major challenge facing data centers. It is estimated that most data centers today use more than 40% of their power for cooling systems. In fact, for power efficiency, the energy wasted is mainly lost as heat, which needs to be discharged through the data center's air conditioning system. Therefore, the more efficient the power conversion, the less heat is generated, and the lower the electricity bill for cooling.

AC-DC Conversion Requirements in Data Centers

Let’s take a closer look at the demands of data center power systems and how component vendors are addressing these challenges. Power density in data centers is accelerating, and power supply unit (PSU) vendors are working to increase the power capabilities of a standard 1U rack (Figure 1). About a decade ago, the average power density per rack was about 4 to 5 kW, but today’s hyperscale cloud computing companies (such as Amazon, Microsoft, or Facebook) typically require power densities of 20 to 30 kW per rack. Some specialized systems go even further, requiring power densities of more than 100kW per rack.

High power density requires power supplies that are compact and energy efficient, due to limited space for power supply storage and space for heat dissipation and management of power conversion heat losses.

However, the challenge is not only to improve overall energy efficiency, the power supply must also meet the specific needs of the data center industry. For example, all AI data center PSUs should meet the stringent Open Rack V3 (ORV3) base specification.

Recently, a server rack provider introduced a new AC-DC PSU with a nominal input range of 200 to 277 VAC and an output of 50 VDC, which meets the ORV3 standard. The standard requires a peak efficiency of more than 97.5% at 30% to 100% load conditions and a minimum efficiency of 94% at 10% to 30% load conditions.

Topology Selection for Server Rack Power Supplies

The power factor correction (PFC) stage is a key component of the AC-DC conversion in the PSU and is very important for achieving high energy efficiency. The PFC stage is responsible for shaping the input current to maximize the ratio of useful power to total input power. PFC design is also key to meeting electromagnetic compatibility (EMC) standards in regulations such as IEC 61000-3-2 and ensuring compliance with energy efficiency specifications such as ENERGY STAR?.

For many applications, such as data centers, the “totem pole” PFC topology is the best choice for designing the PFC stage. This topology is commonly used for the PFC block in 3 kW to 8 kW system power supplies in data centers (Figure 2). Based on MOSFETs, the totem pole PFC stage improves the efficiency and power density of the AC power supply by removing the bulky and lossy bridge rectifier.

However, to achieve the 97.5% or higher efficiency required by hyperscale data center companies, totem pole PFC requires the use of MOSFETs based on “wide bandgap” semiconductor materials such as silicon carbide (SiC). Today, all PFC stages use SiC MOSFETs for the fast switching leg and Si-based superjunction MOSFETs for the phase or slow leg.

Compared to Si MOSFETs, SiC MOSFETs offer better performance, higher efficiency, and are more robust and reliable, perform better at high temperatures, and can operate at higher switching frequencies.

Compared to Si-based superjunction MOSFETs, SiC MOSFETs have lower energy stored in the output capacitor (EOSS), which is critical for achieving low load targets because the switching losses in the PFC stage are mainly due to EOSS and devices with relatively high gate charge. Lower EOSS significantly reduces energy losses during switching, thereby improving the efficiency of the fast leg of the totem pole PFC. In addition, SiC MOSFETs have a better positive temperature coefficient RDS(ON) than Si-based superjunction MOSFETs due to their excellent thermal conductivity, which is three times higher than that of Si-based devices.

The figure below shows the on-resistance of a 650V SiC MOSFET versus junction temperature. (Figure 3) (The on-resistance at a junction temperature of 175°C is 1.5 times higher than the on-resistance at room temperature.)

Similarly, the figure below (Figure 4) shows the on-resistance vs. junction temperature of a 650 V super junction MOSFET. The on-resistance at a junction temperature of 175°C is more than 2.5 times higher than the on-resistance at room temperature.

Comparing a silicon-based 650 V super junction MOSFET with similar RDS(ON) ratings to a 650 V SiC MOSFET, the on-resistance (RDS(ON)) of the former increases to about 50 mohm at a junction temperature (Tj) of 175°C, while the RDS(ON) of the latter is about 30 mohm. 650 V SiC MOSFETs have lower conduction losses during high temperature operation.

In the Totem Pole PFC slow leg functional block and LLC functional block, conduction losses account for the majority of the total power loss. SiC MOSFETs have lower RDS(ON) at higher junction temperatures, which helps improve system efficiency.

Thanks to the smaller RDS(ON) increase and excellent EOSS at high temperatures, SiC MOSFETs excel in Totem Pole PFC topologies, which further helps improve efficiency and reduce energy losses.

New SiC MOSFET technology enables excellent system efficiency

ONSemi's 650V M3S EliteSiC MOSFETs, including NTBL032N065M3S and NTBL023N065M3S, provide superior switching performance and greatly improve system efficiency at the PFC and LLC levels. The M3S EliteSiC technology outperforms its predecessors by 50% lower gate charge, 44% lower EOSS, and 44% less charge stored in the output capacitor (QOSS). When used in hard-switching topologies in the PFC stage, the superior EOSS performance further improves system efficiency at light loads. In addition, lower QOSS simplifies the design of resonant energy storage inductors for soft-switching topologies in the LLC stage.

Thanks to their superior switching performance and efficiency, the M3S EliteSiC MOSFETs dissipate less heat. In addition to helping reduce cooling requirements in data centers, the devices also run cooler in high-frequency PFC and DC-DC functional blocks, such as wall-mounted DC chargers for electric vehicles (EVs).

In addition, at the same voltage level, the M3S EliteSiC MOSFETs have superior gate charge Qg and lower gate drive losses. At the same time, superior Qgs and Qgd also help reduce switching turn-on and turn-off losses. In the LLC functional block, when VDS transitions from the off state to the diode conduction state, the output capacitor needs to be charged. To achieve this quickly, a low transient output capacitor (COSS(TR)) is required. Transient COSS is important here because it minimizes the circulating losses of the resonant energy storage and reduces the LLC dead time, which reduces the circulating losses on the primary side. Low on-resistance minimizes conduction losses, while low EOFF helps to further reduce switching losses. Overall, improving system efficiency is a key performance criterion, making SiC MOSFETs a preferred option for PFC and LLC stages in data centers.

The new EliteSiC MOSFETs are also well suited for energy infrastructure applications such as photovoltaic (PV) generators, energy storage systems (ESS), uninterruptible power supplies (UPS), and electric vehicle charging stations. Design engineers can use M3S EliteSiC MOSFETs to reduce the overall system size, which in turn helps increase the operating frequency. From a system perspective, the M3S EliteSiC MOSFET can help design engineers reduce system costs compared to silicon-based 650 V superjunction MOSFETs.

In summary, the new EliteSiC MOSFETs are comparable to the superjunction MOSFETs on the market in terms of cost, EMI, high temperature operation and switching performance based on the same RDS(ON). Compared to superjunction MOSFETs, the 650V M3S EliteSiC MOSFET in the same package can achieve lower RDS(ON), which helps improve the system efficiency of LLC topologies. The outstanding advantage is the significant reduction in switching losses compared to other silicon-based alternatives.

Conclusion

This article briefly explored the higher standards for efficient power conversion set by the growing power demand of hyperscale data centers. Artificial intelligence is expected to lead the world to change, and in order for our existing power grid to meet the rapid development of AI-driven cloud computing, we urgently need to improve energy efficiency.

The use of SiC MOSFETs can significantly improve the efficiency of both PFC and LLC levels. ON Semiconductor's 650 V M3S EliteSiC MOSFETs can significantly improve the energy efficiency of the PFC and LLC stages of hyperscale data centers. With lower gate charge, EOSS and QOSS, the 650 V M3S EliteSiC MOSFETs can improve energy efficiency and simplify the hard switching topology design in the PFC and LLC stages, helping to reduce power consumption and lower operating costs.