Power Electronics Europe February/March 2022

www.wolfspeed.com/products/power/sic-power-modules SIC POWER MODULES 29 www.power-mag.com Issue 1 2022 Power Electronics Europe Maximizing Active Front End Efficiency Using Silicon Carbide Engineers designing UPS with great care to ensure smooth enterprise data center operation 24/7 are also aware that their power supplies are destined to be part of a setup that gulps for example 90 TWh of US electricity every year — enough to sustain thirty large and noxious coal-fired plants. Power engineers in another design camp, working to ensure their fast chargers can speedily top up EVs, are also aware of the cost of electricity and the environmental impact of its generation. This article addresses these concerns and, by doing a side-by-side comparison, demonstrates that Silicon Carbide (SiC) is by far the better choice over Silicon (Si)-based devices for high power applications. Daniel Martin, Sr. Manager applications Engineering, and Jonathan Hayes, Application & Systems Engineer, both Wolfspeed, USA competitiveness as well as the environment. Why Silicon Carbide? Silicon Carbide enables engineers to check items on the list above by virtue of material and resulting device properties. Compared with the traditional Si technology, SiC devices offer a 2-3X lower on-state voltage drop than Si, thus lowering conduction losses in SiC switches. Since SiC devices are majority carriers, they offer much higher edge rates (di/dt) than possible with Si. Their 10X higher breakdown field than Si allows SiC devices to withstand higher voltage in the same package. A higher thermal conductivity of 3.3-4.5 W/cmK versus Si’s 1.5 W/cmK enables SiC devices to conduct heat away much more quickly, helping to reduce cooling requirements in the system. Moreover, SiC chip temperatures can reach 250-300°C (versus Si’s 125°C) and junction temperatures in Wolfspeed devices can go up to 175°C before affecting reliability. This means that the devices can run hotter and with a smaller cooling rig. Wolfspeed’s SiC power modules offer the following advantages over Si versions: • They are application-targeted with module selections offered in a variety of voltage and current ratings, and form factors, as well as with switching and conduction optimization • They have lower R DS(ON) compared with IGBT modules • They offer faster switching speeds • They have lower switching losses Application advantages of the AFE topology The AFE is applicable to almost all grid-tied converters. Two prominent topologies in today’s emerging markets are shown in Figure 1. The double-conversion UPS architecture comprises an AFE or rectifier, a DC/DC converter and an inverter. In normal power flow, a small current goes into the DC/DC converter that maintains the battery charge. Most of the power is sent through the DC link into the inverter where it feeds the load. Under a power fault, the AFE stops switching and the DC/DC converter sends power from the battery into the inverter to feed the load. Some applications may use the battery also to compensate for poor Engineers targeting any application area are joined in their concerns over efficiency, power density, and cost. And, even if they have yet to design with it, they are aware that the solution may lie in SiC technology. The article content uses an essential part of UPS and charger systems, the active front end (AFE), to explore improvements in size and power density, power losses and efficiency, and bill of materials (BOM) costs. It aims to turn that general awareness of SiC benefits into a clearer understanding, clearing a path through an entrenched less-efficient technology toward greater SiC-based design experience. The challenge in AFE design can be broadly expressed as a wish list of changes an engineer would want: 1. Lower switching and conduction losses in the semiconductor devices 2. Smaller and lighter cooling system 3. Smaller and lighter passives — capacitors and inductors 4. All of the above with reduction in operational cost as well as BOM cost Any technology that resolves all of these challenges — simultaneously — can indeed have a significant impact on product Figure 1: The AFE ties both applications — the double conversion UPS (left) and the EV off-board fast charger (right) — to the grid, rectifying AC input to DC