Dr. Thomas Barbieri, Dr. Jeffrey Casady, Edgar Ayerbe, Dr. Ty McNutt, & Lauren Kegley, Wolfspeed
As the market demand for electric vehicles continues to increase — driven in part by government regulations on fuel efficiency and CO2 emissions, social awareness, and the overall trend toward greener transportation options — a growing number of automotive manufacturers are incorporating the latest power electronics technology in their designs to improve overall performance, increase efficiency, and reduce cost, weight, and complexity.
Hybrid electric vehicles (HEVs), plug-in hybrid electrical vehicles (PHEVs), and battery electrical vehicles (BEVs) all contain several critical systems that can benefit from using silicon carbide (SiC) power devices. These devices enhance both the energy efficiency and performance of electric vehicles (x-EVs), and have enabled early adopters to achieve a significant market advantage over their competitors.
As a mature and industry-accepted wide bandgap semiconductor material for power applications, SiC offers several proven performance advantages over conventional silicon (Si) technology, including: higher voltage blocking capability, faster switching speed, lower on-state and switching losses, higher thermal conductivity, and a more compact footprint. These characteristics provide the platform for developing advanced, efficiency-critical power electronics subsystems that are fundamental to any electric vehicle, such as drivetrains, power converters, and on- and off-board charging systems.
In a typical electric drivetrain vehicle, sophisticated power electronics are employed in the drivetrain, motor drive, battery charger, and power management systems used to control the flow of energy between these systems and energy storage devices (i.e., batteries). Maximizing the efficiency of each of these power electronics systems is critical for improving overall electric vehicle efficiency and reliability.
SiC’s impact to the EV drivetrain
EV drivetrain (or motor drive) systems range from 80kW to 300kW, depending on the vehicle. It has been well documented that motor drive losses comprise from 3% to 7% of the overall total vehicle loss, and are higher in city driving conditions than in typical highway driving conditions.
Optimized SiC MOSFETs and power modules being developed by companies like Wolfspeed for x-EV drivetrain applications outperform all other power devices currently available, and can significantly reduce these motor drive losses. An example of the enabling technologies behind this reduction is Wolfspeed’s high-performance 900V SiC 62mm half-bridge power module, with a low inductance (5.5 nH) and low volume, which enables record low on-resistance and conduction losses. This module, shown in Figure 1, utilizes the newly developed, industry-leading, 900V 10mΩ SiC MOSFET, which is based on Wolfspeed’s third-generation planar MOSFET technology.
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Figure 1: Wolfspeed’s high performance 900V SiC 62mm-mounting, half-bridge power module is based on the company’s 900V 10mΩ SiC MOSFETs.
While retaining the reliable, industry-leading DMOS structure, the specific RDSON of the new 900V module, which features eight MOSFETs in parallel per switch position, is driven down to 1.25mΩ at 25°C and 2.1mΩ at 175°C under light-load conditions, which is significantly lower than competing Si and SiC MOSFET products in this voltage range. As a direct result, the dramatically improved conduction losses for this module can enable up to 70% lower inverter losses than those for similarly rated Si-IGBT-based power inverters.
The majority of drive cycle operation is under light-load conditions, in which the SiC-MOSFET-based inverter offers unparalleled reduction in conduction losses. These reduced conduction losses translate into an extended maximum driving range, as well as reductions in size, weight, regeneration losses, battery, and overall system costs. Initial testing of the high performance 900V half-bridge power module have shown it to be capable of >800A on-state capability, at Tj = 175˚C.
Unlike comparable Si IGBTs, the switching losses present in these SiC MOSFETs are mostly temperature-independent, and they lack the knee voltage of the Si IGBT. The on resistance of the 900V SiC 62mm is only 1.25mΩ at room temperature, with a significant portion of that resistance coming from the module lead frames. At 175˚C and 800A current, the on-resistance remains below 2.1mΩ (see Figure 2), which is also a record-low on-state loss for power modules in this class. Additionally, the 900V VDS rating of the switches not only satisfies the breakdown voltages required for most commercial EV batteries, it also allows system designers to move toward higher battery voltages of 700V+.
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Figure 2: RDSON temperature dependence of the 900V SiC 62mm half-bridge power module
Increasing efficiency and power density in chargers
EV battery charging systems contain several critical power electronic subsystems that can be designed to enhance owner experience. Plug-in vehicle owners want rapid charging from readily accessible electrical outlets, while hybrid owners desire reliable and long-lasting battery charging systems. The key to achieving both of these performance enhancements is a power electronics system design that features high efficiency power conversion, high operating temperature capabilities, and high power density.
Increasing the power density of power electronics systems can be achieved by operating the power system at higher switching frequencies to reduce passive filter component size. However, existing silicon MOSFETs and IGBTs have limitations in high-frequency switching circuits due to inherent high switching losses and poor internal body diodes. The on-resistance of silicon MOSFETs further limits their use, as it typically increases 3x over temperature, causing thermal issues and significant de-rating of device capabilities. For example, Wolfspeed’s recently-released family of 900V SiC MOSFETs with optimized low-induction packaging addresses many of the traditional design limitations imposed by silicon devices.
With a Rds(on)•Qg figure of merit that is magnitudes lower than any comparable silicon device, on-resistance that only increases by a factor of 1.3× over the full operating temperature range, and the increased voltage margin and associated reliability of the higher breakdown voltage, Wolfspeed’s discrete 900V SiC MOSFETs enable smaller, faster, and more efficient EV charging designs.
Further, the newly-improved discrete SiC MOSFET packages include Kelvin connections to help minimize gate ringing and allow system designers to achieve higher switching frequencies that were not possible with previous packaging technology. Figure 3 illustrates the reduction in total switching loss achievable for the 900V, 65mΩ MOSFET in a conventional TO-247 package versus the new Wolfspeed D2PAK-7L package.
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Figure 3: A switching comparison between a D2PAK with Kelvin connection and a TO-247-3 package. Both packages were assembled with Wolfspeed’s 900V 65mΩ SiC MOSFET die.
Another significant performance enhancement can be achieved by implementing SiC Schottky diodes in a high voltage DC/DC boost converter circuit in the on-board battery charging system or in the secondary rectifier section on an off-board DC fast-charger. Silicon PiN diodes are typically used in these systems since they require high voltage devices (>300V), and high voltage silicon Schottky diodes are not available.
Bipolar silicon PiN diodes have poor reverse recovery characteristics, which reduce achievable switching frequency and efficiency. In comparison, the zero-reverse-recovery characteristic of unipolar SiC Schottky devices virtually eliminates diode switching losses, which enables increased switching frequencies and makes the overall power management system much more efficient.
Figure 4 illustrates that the reverse recovery current of a silicon diode increases with temperature, while it remains constant for the SiC Schottky diode, which is yet another advantage of SiC Schottky diodes over their silicon counterparts. As the operating temperature of a charger or inverter in a silicon system increases, the switching efficiency of silicon diodes decreases.
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Figure 4: Wolfspeed™ SiC Schottky diodes show zero-reverse-recovery current (IRR), which is a result of silicon carbide’s ability to enable unipolar devices at high voltage.
Alternately, the switching efficiency of SiC diodes remains unchanged over temperature. Since vehicle charging systems are subject to such high operating and ambient temperatures, these characteristics make SiC devices a better choice for many automotive applications.
Application example: on-board 6kW charger
Although high-frequency SiC power devices are a technical breakthrough for developing novel, compact, and high-efficiency on-board chargers, the development of both a package and system capable of implementing the near-ideal nature of the SiC MOSFET is also critical for achieving the benefits of the wide bandgap material in an end-application.
Wolfspeed has demonstrated a high performance on-board battery charger, designed around the initial high performance 41mm H-Bridge module technology, that was successfully integrated into a Toyota Prius plug-in hybrid for testing purposes. The converter was designed to achieve high power density and high efficiency in a simple, minimized, high-temperature-capable footprint. A battery charger is a conventional two-stage converter comprised of a bridgeless boost AC-DC power factor correction (PFC) converter and a phase-shifted full-bridge (PSFB) isolated DC-DC converter.
The hard-switched PFC converter topology allowed for a significant improvement in the conversion efficiency, and demonstrated a maximum efficiency of 98.5% at a switching frequency of 250kHz. The PSFB converter achieved 96.5% efficiency at 200kHz operation, and the final system design achieved more than 95% efficiency an output power of 3.1kW with a switching frequency of 200kHz.
Reducing volume & weight of magnetics
Another challenge to taking maximum advantage of SiC technology benefits is reducing the volume and weight of the magnetics parts. To achieve maximum system density, it is critical to reduce the size and weight of all components within a conversion system. As such, the high performance modules and magnetic components designed into the on-board charger tested in the Prius were mounted directly to a heat sink.
The module used an isolated metal-matrix-composite baseplate to improve thermal conductivity and CTE mismatch in the stack-up, and the magnetic components were planar designs to increase the density of the solution. The resulting gravimetric power density of 3.8W/Kg and volumetric power density of 5.0kW/L represent a 10x improvement over the previous silicon-based charger design.
The high performance 41mm module at the heart of this SiC-based charger is designed to cater to high-power-density applications, such as the EV market. Based around the most powerful SiC MOSFET on the market, this package can be reconfigured as either a half- or full-bridge module with up to 100A continuous current, depending on the device configuration. Similar to the high-performance, high-current half-bridge discussed above, the 41mm footprint is capable of utilizing the entire Wolfspeed MOSFET product line, which spans 900V to 1700V with ambient operation up to 175°C.
Upgrading to SiC MOSFETs, SiC Schottky diodes, and SiC modules results in significant performance improvements in EV power electronic systems, such as drivetrain and on-board chargers. The space and weight reductions enabled by SiC material properties provide EV designers with the ability to achieve enhanced efficiency, eliminate auxiliary cooling systems, and deliver increased battery range in form factors that were previously unimaginable.