ISSUE 012 Winter 2021 Sigma Powertrain EMAX transmission dossier l In conversation: David Hudson l 48 V systems focus l 2021 Battery Show North America and Cenex-LCV reports l Everrati Porsche 911 digest l Switching insight l Motor laminations focus

Because of the slow reverse recovery of the high-side diode (or body diode of the MOSFET), the voltage at the cathode of diode D 2 (in the diagram) cannot jump from ground to DC+ voltage instantly, and this causes a large current spike. Because of that, the designer cannot use a silicon MOSFET in a CCM totem pole PFC. This is where the SiC MOSFET and GaN transistors come in, as they have a low reverse recovery. The biggest advantage of the totem pole PFC is the reduced power losses in the conduction path. One 6.6 kW totem pole bridgeless power factor correction design for an OBC uses SIC MOSFETs driven by a C2000 microcontroller from Texas Instruments with SiC-isolated gate drivers. The design implements three- phase interleaving and operates in CCM to achieve a 98.6% efficiency at a 240 V input voltage and 6.6 kW full power. The C2000 enables phase shedding and adaptive deadtime control to improve the power factor at light load. The gate driver board implements reinforced isolation and can withstand more than 100 V/ns common-mode transient immunity. The gate driver board also contains a two-level turn- off circuit, which protects the MOSFET from voltage overshoot during a short- circuit. “If you are looking at just OBCs, the topology of the PFC is driven by the power level, and then you get all the other factors – faster charging, higher efficiency, smaller size and then cost considerations,” says Jay Nagle, senior product marketing engineer for the automotive market segment at onsemi. “We are seeing 3.3 kW OBCs with a single-phase boost PFC using one power switch and single diode, then interleaved multiphase boost to get to 6.6 and 7.2 kW. “The three-phase totem pole is used for power levels of 11 to 22 kW, and that raises the fundamental question of whether to use SiC and GaN,” he says. “GaN does not have the same power density as SiC and has limited avalanche capacity, so it struggles to handle voltage stresses beyond the rated voltage, so you have to be careful in fast-switching configurations at high frequencies with lots of overshoot. “Bidirectional OBCs need efficient use of the body diode, and SiC has zero reverse recovery so there are advantages there.” That said, Navitas and Xaomi have shown a 6.6 kW OBC design using GaN with a 240-420 V output in a package measuring only 222 x 168 x 60 mm, and with a power density of 3 kW/litre – three times higher than silicon-based OBCs. Navitas says its OBC roadmap continues to more than 5 kW/litre. VisIC, in Israel, has used a different type of GaN device in depletion mode to achieve a 100 kW inverter switching at 40 kHz. The depletion-mode GaN transistor is ‘normally on’ when the device is on with a gate-source voltage (V gs ) of 0 V and requires a negative V gs to turn off. D-mode transistors have been turned into ‘normally off’ devices by using a proprietary normally off circuit and driving scheme. This D3GaN technology is used in a 100 kW inverter reference design that can be adapted to work on an 800 V or 900 V power bus. The inverter measures 26.9 x 21.4 x 3.5 cm and has a power density of 50 kW/litre. VisIC has also developed a 6.7 kW OBC that provides a power density close to 3 kW/litre. Vienna recti iers Although many topologies exist for active three-phase power factor conversion, a Vienna rectifier is popular owing to its operation in CCM and the fact that it supports multi-level switching with less voltage stress on the power devices. The Vienna rectifier is a unidirectional three-phase, three-switch, three- level PWM rectifier – essentially a three-phase diode bridge with A 6.6 kW totem pole onboard charger design (Courtesy of Texas Instruments) 58 Winter 2021 | E-Mobility Engineering Deep insight | Switching