Power Electronics Europe Issue 4 - November 2022

28 WBG DATACENTER www.wolfspeed.com Issue 4 2022 Power Electronics Europe www.power-mag.com conduction losses with power and consequently junction temperature increase. This requires GaN devices to be made oversized to compensate for higher conduction losses regardless of switching frequency. The GaN testing had to be stopped at 3 kW due to power limitations of the device. The study clearly demonstrated that SiC results in significantly lower total losses, especially at the high power levels at which WBG semiconductor use is most compelling, such in as datacenters. The various device-level performance specs of the three semiconductor technologies are compared in the radar chart in Figure 4. At first glance, we notice GaN’s benefits are the lowest reverse recovery charge Q rr for the lowest switching loss in continuous conduction mode (CCM) synchronized rectifier, the lowest time-related output capacitance C oss(tr) for low dead time, and high frequency and efficiency, and the lowest energy-related output capacitance C oss(er) for minimum switching loss in hard- switched topologies. Notice that SiC trails close behind GaN in these attributes, while Si lags significantly. Silicon wins include the lowest junction- to-case thermal resistance R thjc , which confers better thermal performance, and the highest threshold voltage V th , which offers better immunity to noise and makes Si devices easier to drive. Note that GaN has an extremely low V th . The maximum junction T jmax and the avalanche energy, single pulse E as indicate device robustness. SiC is the most robust as shown, while GaN has no E as capability. SiC also has the lowest R DS(ON) change over temperature, which results in low conduction loss at high temperature. This is where GaN lags considerably to undo all gains from low switching loss. Put together, SiC’s strengths help deliver the highest efficiency at higher power levels, as well as high power densities required for enterprise datacenters and similarly demanding applications. The package point of view Since Wolfspeed developed the SiC technology for a successful transition from Si, many of the common surface-mount and through-hole packages are available for SiC products. GaN, on the other hand, faces unique challenges toward package standardization. For instance, GaN through-hole packaging is uncommon because products need to have lower parasitics and allow very-high-frequency switching to best utilize the material’s strengths. GaN is often either offered in large QFN or custom packages. Large QFN suffers from board-level reliability concerns and custom packages lack multisource availability as well as tooling capability at subcontractors. GaN’s power device package challenges do not end here. Other common concerns include: Kelvin source pins, widely adopted in SiC for better switching control, are not feasible in cascode GaN since other internal parameters like the cascode FET and capacitances go unaccounted. The common source cannot be eliminated and the cascode GaN is limited to TO- 247-3 (three-lead) package in which the vulnerability to gate oscillation limits switching speeds. Some custom packages on the market are so thin, they constrain the space available for a heatsink. Another custom package on the market has a top-side cooled drain, which requires thermal interface materials (TIMs) with high thermal conductivity to extract heat away from the device. Yet another TO-Leadless (TOLL) package for GaN places the gate and the Kelvin source in a direction different from standard Si, which makes transition from the latter technology cumbersome. As the market moves towards high- power density design and tighter space constraints, the TO-Leadless (TOLL) package offers advantages of low height and smaller footprint, and its leadless form results in low lead inductances that would otherwise become a concern in high frequency operation. The package’s larger drain tab area addresses thermal performance concerns from small packages (Figure 5). TOLL is a relatively new package for the datacenter and server power supply market. Wolfspeed is, however, supporting that market with product development in this direction, such as with new TOLL package variants for datacenter and server power. A system-level comparison Compared with Si-based H-bridge, SiC- Figure 4: Silicon Carbide excels in high-voltage, high-power and high-temperature applications, such as datacenter power supplies Figure 5: The TOLL package is significantly smaller than the standard TO-263 and enables low-cost surface-mount assembly