Power Electronics Europe Issue 4 - November 2022

OPINION 5 www.power-mag.com Issue 4 2022 Power Electronics Europe According to market researchers the rapid evolution of the WBG compound semiconductor market has positioned both SiC and GaN as key materials within the power device market. There has been a remarkable shift of interest in SiC for automotive applications and in power supplies as well as GaN for mainstream consumer applications and more recently for automotive. Since the first commercialization of SiC diodes, the power SiC device market has been driven by power supply applications. Nevertheless, automotive is becoming the killer application, following SiC’s notable adoption for Tesla’s main inverters in 2018. And as such many leading semiconductor manufacturers are increasing their SiC investments, from raw materials to chips. Although Silicon is the most familiar technology, its smaller bandgap limits operating temperature, its low breakdown electric field restricts its use to lower voltages, and its low thermal conductivity limits power density compared to wide bandgap materials, like gallium nitride (GaN) and Silicon Carbide (SiC). GaN’s high electron mobility is the property that enables its well- known and unmatched efficiency at very high switching frequencies - GaN offers the lowest switching loss. In our feature “Finding the Right Technology to Solve Datacenter Power Challenges” Wolfspeed compared their 60-m Ω SiC device with a 50-m Ω GaN device in a totem pole PFC simulation to find that although GaN had slightly lower switching losses over the entire power range, any gains were offset by the increased 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. For the efficiencies needed in datacenter power supplies, it is important to compare switching and conduction losses. Conduction loss, which is the device’s I?R loss, is lower when the ON drain-to-source resistance (RDS(ON)) is low and changes less with temperature. It is interesting to note that both GaN and SJ devices boast a lower RDS(ON) below 25°C, which are temperatures not quite practical for datacenter power supplies. As datasheets for GaN and SJ devices often specify RDS(ON) at 25°C, it can mislead engineers into assuming that specification at the much higher operating temperatures for which systems are normally designed. 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. At first glance, GaN’s benefits are the lowest reverse recovery charge Qrr for the lowest switching loss in continuous conduction mode (CCM) synchronized rectifier, the lowest time-related output capacitance Coss(tr) for low dead time, and high frequency and efficiency, and the lowest energy-related output capacitance Coss(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 Rthjc, which confers better thermal performance, and the highest threshold voltage Vth, which offers better immunity to noise and makes Si devices easier to drive. GaN has an extremely low Vth. Compared with Si-based H-bridge, SiC-based CCM totem pole PFC can have not only higher efficiency but higher power density at similar or lower cost. A comparison of efficiency between technologies clearly shows that while both SiC- and GaN-based CCM totem pole PFCs can achieve >99 % efficiency, GaN has the efficiency advantage only at very light loads. SiC provides an efficiency similar to that of GaN at half load and better efficiency at full load. An automotive application describes our cover story from EPC. A 48 V mild hybrid is estimated to provide 70 percent of the benefit of a high-voltage hybrid at 30 percent of the cost while boosting electrical power available in the vehicle from 2.5 kW to 10 kW. These systems will require a 48 V – 12 V bidirectional converter, with power range between 1.5 kW and 6 kW. The design of a 2 kW, two-phase 48 V/12 V bi-directional converter using GaN FETs in QFN packages, achieving 96% efficiency is targeted for the 48 V mild hybrid system. The solution is scalable; two converters can be paralleled for 4 kW, three converters for 6 kW or only one phase can be used for 1 kW. GaN FETs suitable for 48 V applications typically have 4 times better figure of merit compared to equivalent MOSFETs. For the same gate voltage of 5 V, GaN FETs have at least 5 times lower gate charge than MOSFETs. Other important advantages of GaN FETs include lower COSS, faster voltage transition, zero reverse recovery and they are physically smaller. At full load, EPC eGaN FETs can operate with 96 % efficiency at 500 kHz switching frequency, enabling 1 kW/phase compared to silicon-based solutions, which are limited to 600 W/phase due to the limitation on the inductor current at 100 kHz maximum switching frequency. With that the potential users have a guideline on choosing the right technology. I have written this opinion in a hospital. I am looking forward to be back for electronica! Achim Scharf PEE Editor Good Times for WBG