By switching to SiC MOSFETs, power system designers can significantly boost power density, lower switching losses, and improve thermal management in OBCs for electric vehicles.
Power systems in electric vehicles (EVs), on-board and off-board battery chargers, and other energy storage systems are demanding greater energy efficiency to pack more power into the same form factors. Here, silicon carbide (SiC) devices optimize power system designs in a way that enables designers to maximize power density, reduce size and weight, and hit the new energy-efficiency standards all at the same time.
Today’s traditional power factor correction (PFC) circuits can’t get the required efficiency with silicon-based solutions without adding more components and raising design complexity. On the other hand, switching to SiC MOSFETs allows power system designers to meet and exceed the most stringent power-efficiency requirements.
The 650V SiC MOSFETs with low on-state resistance lead to lower conduction losses, and the properties of silicon carbide itself minimize the on-resistance temperature variations to maintain high efficiency at full load. They are also crucial in managing the light and peak loads at DC/DC converters, which significantly reduces the overall power consumption.
That translates to less thermal management, which, in turn, leads to smaller and more economical heat sinks as well as fewer costly cooling parts. Better thermal management also leads to fewer support components, which means that there are fewer parts at risk of breaking.
A system-level BOM comparison that shows 15% cost savings with SiC MOSFETs
Bidirectional power flow in OBCs for EVs
Electric vehicles are relying on on-board chargers (OBCs) more than fast chargers. More than half (51%) of all reported charging takes place at home, while another 16% of charging happens when EVs are plugged in at work. However, the silicon-based MOSFET designs for OBCs lead to more waste per kilowatt, which means the end user is paying more for less.
The OBC design for EV battery systems is another use case in which 650V SiC MOSFETs offer competitive differentiation by enabling designers to boost efficiency while adding the ability to support bidirectional power flow without compromise in weight, size, and design complexity. That significantly reduces the size and weight of OBCs that convert AC power from the grid into DC power for the battery all within the vehicle.
Unlike unidirectional OBCs, which sit idle in the vehicle while losing the charge, bidirectional OBCs not only pull charge from the grid, they also replenish it. This bidirectionality also gives the end user the ability to provide energy to other AC-powered devices or provide the EV equivalent of a “jump start” to another vehicle that has run out of charge.
The OBC designs based on 650V SiC MOSFETs pack totem-pole PFC and a DC/DC converter into a single box to save precious space in EVs
The 650V-rated MOSFETs also meet the voltage stress rating requirement mandated by EV designs.
Lower system cost with SiC
Wolfspeed’s 650V SiC MOSFETs are targeted at not only OBCs for EV charging but also power systems in quest of higher power density and energy efficiency, including server and telecom power supplies, uninterruptible power supplies (UPS), energy storage systems, and more. For example, the 15-mΩ and 60-mΩ MOSFETs, which again support both unidirectional and bidirectional OBCs, shrink the size of both the AC/DC PFC front end and the DC/DC converter connected to the battery. According to Goldman Sachs, SiC devices can reduce the EV manufacturing cost and the cost of ownership by up to $2,000 per vehicle. The 650V SiC MOSFETs used in a bidirectional 6.6-kW OBC reference design facilitate a peak efficiency of 96.5% for both charging and discharging. The reference design, built around Wolfspeed’s C3M0060065D MOSFETs, operates at 90 VAC to 265 VAC and works with battery voltages ranging from 250 V to 450 V.
The block diagram of the 6.6-kW OBC reference design based on the C3M0060065D SiC MOSFETs
Wolfspeed’s third-generation SiC MOSFETs — based on the company’s third-generation C3M technology — is what provides lowest on-resistance as well as the lowest conduction and switching losses in the industry. In the case of the 6.6-kW OBC design, at the system level, 650V SiC MOSFETs can provide up to 15% lower costs even if SiC components cost more than their silicon equivalents.
Furthermore, in this 6.6-kW OBC design, a SiC-based solution offers 3.3-kW/L power density and 97% efficiency, as compared to 2.1-kW/L power density and 94% efficiency provided by a silicon-based solution. The system not only achieves lower total cost but also enables a smaller size and weight due to this superior power density.
Optimization of entire power designs
The efficiency enabled by 650V SiC MOSFETs ripples through the entire power system design. A faster switching speed and higher power conversion at the front end of OBC design eases design requirements for the rest of the power system.
Click here to learn more about the new C3M 650V SiC MOSFETs and reference designs.
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