Reference Board Optimizes Gate Driver, SiC FET Module Design for EV Applications
Skyworks’ reference design pairs isolated gate drivers with Wolfspeed high-voltage SiC FET power modules.
Skyworks has announced the release of the Si828x gate driver board (GDB) that serves as an isolated (5 kV) gate driver reference design for Wolfspeed’s XM3 Silicon Carbide (SiC) half-bridge FET power modules. The new reference design is aimed to help accelerate the design and evaluation process of high-voltage power conversion and inverter circuits in applications like EV traction inverters, DC fast charging, and high-voltage power supplies.
Si828x gate driver board. Image used courtesy of Skyworks
Along with the basic drive circuitry, the reference design incorporates fault monitoring to communicate switching anomalies to the system MCU along with overload and short circuit protection to minimize damage to the XM3 module FETs under fault conditions.
As part of the GDB, Skyworks has published an extensive test report intended to save design engineers time when evaluating the platform for their own applications. Full schematic, layout, and BOM files are also provided to help jump-start the design process.
High and Low Side Gate Drivers
The GDB reference board features the Skyworks Si8281 and Si8284 for the high and low side gate driver blocks. The basic function of the gate drivers is to pass the system MCU control signals across the high voltage isolation boundary and generate the gate drive signals that control the FET switching sequences.
To meet isolation requirements, the gate drivers use Skyworks' proprietary silicon isolation technology, supporting up to 5.0 kVrms of withstand voltage per UL1577.
In addition to the gate drivers, the Si828x family also integrates a small DCDC controller that can be used to construct an isolated supply for generating the drive voltages (VDDB and VSSB). To generate the appropriate VGS for switching the FETs, the high drive (IOH) peak output current is about 2.7 A (VDDB = 15 V, VDDS = -4 V) with a low drive current (IOL) of a little over 5 A.
Input to the device is a complementary digital input that can be utilized in several configurations. On the output side, the Si828x devices provide separate pull-up and pull-down pins for the FET gate.
A dedicated DSAT (desaturation) pin helps protect against abnormal load conditions, or excessively large load currents, that could cause permanent damage to the FETs. Under these overload conditions, the driver is shut down in a controlled manner using soft shutdown. The drivers also integrate a Miller clamp to ensure a stable off state for the FET switch and avoid shoot-through conditions from Miller effect gate voltage transients.
Si828x GDB gate driver functional Bbocks. Image used courtesy of Skyworks
Si828x gate driver functional block diagram. Image used courtesy of Skyworks
Wolfspeed XM3 Power Modules
The XM3 is a high-voltage, half-bridge power module suitable for power conversion applications like EV traction inverters and DC fast charging. The modules have a VDS withstand voltage up to 1200 V and can handle 450 A of continuous DC current, or 900 A of pulsed current. With a low on resistance of 2.6 mΩ the FETs dissipate ~525 W of power at the rated 450 A, against a potential conducted power of up to several hundred kW or more. The part can deliver the full rated current of 450 A up to a case temperature of about 80 C.
The modules are in an 80 x 53 x 19 mm encapsulated package that incorporates silicon nitride insulation and a copper baseplate to help with thermal management. Bolt mounts allow the module to be securely fastened to the host system.
The modules are not cheap. Low-volume unit pricing through distribution is just under $700, but direct, high-volume OEM pricing will likely be a couple of hundred dollars or less.
Wolfspeed recommends a gate drive voltage (VGS) of -4 to +15 V, in this case, provided by the Skyworks drivers.
XM3 half-bridge power module. Image used courtesy of Wolfspeed
XM3 SiC FET module current derating. Image used courtesy of Wolfspeed
EV Market Drives SiC FET Development and Adoption
Much of the growth in the SiC FET market is being driven by the emerging EV market as e-OEMs battle to deliver cars and trucks with more range, reduced charging times, and lower costs. The key design challenge involves efficiently moving large amounts of current to and from batteries, charging stations, and vehicle powertrains at high voltage (traction inverter, OBC, fast charging).
EVs look to a higher voltage to increase power and performance. Image used courtesy of Adobe Stock
To get better EV performance, and higher power densities, engineers are increasingly working with higher-voltage powertrains. Tesla recently announced the first 1000 V power train that will be used in their Semi Class A commercial truck that just released into production. Indications are that this higher voltage powertrain technology will migrate to their other vehicle platforms as well. Other OEMs will follow and voltage levels will continue to march higher.
SiC FETs, along with IGBTs and GaN, is emerging as the go-to solutions to optimize EV efficiency and performance at these high voltages. The stakes could not be higher as automotive OEMs look to transition entire vehicle lineups and roadmaps from ICE to electric powertrains.
