Power Electronics Europe February/March 2022

30 SIC POWER MODULES www.wolfspeed.com/products/power/sic-power-modules Issue 1 2022 Power Electronics Europe www.power-mag.com load or grid-side power quality. In the off- board DC fast charger as well, the AFE connects the converter to the grid. It rectifies the grid voltage into a stable DC link voltage, which then can be used to charge the batteries. The off-board charger topology is simpler with the AFE interfacing directly with just the DC/DC converter to quickly charge the EV. In both applications, the AFE uses three half-bridge power modules – one for each phase. Defining the problem and design goals A key issue with IGBT-based AFEs is that they are large and inefficient. They have high switching losses and, because they are also significant heat sources, engineers have the option to either use bulky cooling systems or take a performance hit in order to lower the heat generated. But, although demands vary slightly, all customers want to pay for a high efficiency system, not a heater. The AFE design goals, therefore, can be defined as: • Regulate the DC link voltage under normal operation by controlling the input current magnitude • Minimize power quality issues by sourcing very low THD (<5%) current with very high-power factor • Minimize BOM component costs • Shrink system volume to enable more compact systems • Maximize efficiency With this in mind, IGBT and SiC variants of an AFE system were designed to output 200 kW of high-quality rectified power with a well-regulated DC bus. IGBT- versus SiC-based designs The IGBT- and SiC-based systems are presented broadly before delving deeper with a side-by-side comparison of the component sizes and losses. Si-based high-power designs, like the AFE example, typically use IGBTs. Shown in Figure 2 is the circuit diagram with the power module and its physical cooling requirements. To use a best-in-class component, a module was chosen from among the dominant IGBT modules today that come in EconoDUAL® package. The topology requires three such power modules — each red box shown in the figure includes a single power module, a heatsink, and two fans. The system could be optimized to switch at a frequency as high as 8 kHz while needing a 100 µH inductor. For a 40°C ambient temperature, the IGBT LEFT Figure 2: Each red box in the circuit comprises the EconoDUAL® power module and the associated cooling system shown above ABOVE Figure 3: Each red box in the SiC-based circuit design uses the smaller XM3, a smaller heat sink and a single cooling fan Table 1: The loss comparison demonstrates a per module loss reduction by SiC of 40 % over IGBT