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

“With bidirectional chargers, the power must flow in both directions, so the hardware is two full bridges and there are a lot of PWM strategies for this. The choice of strategy depends on the power balance, whether it is the same power in both directions. “For chargers using resonant switching at 3-5 kW that don’t need the same power rating in both directions, superjunction MOSFETs are still preferred. For an OBC rated to 22 kW where the power is almost the same in both directions and works at full load most of the time then SiC is preferred.” Totem poles All plug-in EVs require an OBC between the power grid and the HV battery pack, and the OBC needs a power factor correction (PFC) converter to connect directly to the grid to maximise the real power that flows. Conventional PFC converters implement a passive diode bridge for rectification, which is now known as a passive PFC technique. This is a simple, reliable design with a slow control loop, but the passive components are heavy and generate significant power losses, resulting in bulky heat sinks and a lot of heat dissipation. Owing to the various interoperability requirements, the industry uses many different PFC topologies. The vast majority of PFC stages in OBCs are operated in continuous current mode (CCM), which needs transistors that are robust against hard commutation on their body diode. Superjunction devices, such as the CoolMOS CFD7A series from Infineon, can be used in CCM PFC stages with an SiC diode as the commutation partner. An alternative is to use either IGBT or CoolSiC wide-bandgap MOSFETs, as these technologies are inherently robust against hard commutation. In this case, two switches can be used in a half-bridge configuration, whereas the superjunction MOSFET works in combination with an SiC diode. The input bridge consumes about 2% of the input power at the low end of a wide input voltage range. If the designer can suppress one of the series diodes they can save 1% of the input power, which allows the efficiency to rise from 94% to 95%, but this can limit the power capability of the OBC. As a result, the trend is to eliminate the traditional diode bridge by using a SiC MOSFET switch. A totem pole bridgeless PFC boost rectifier consists of a boost inductor, two high-frequency boost SiC switches (SiC 1 and SiC 2 in the diagram left) and two components for conducting current at the line frequency. The line frequency components can be two slow diodes, as the diagram shows. The left side shows two silicon MOSFETs (Si 1 and Si 2 ). The right side shows that the use of Si 1 and Si 2 further increases the efficiency. The key issue in a totem pole PFC is the operation mode transition at the point the AC line crosses the zero-voltage line. When the AC input changes from the positive half line to the negative half line at the zero crossing, the duty ratio of the low-side high-frequency switch SiC 2 changes from 100% to 0%, while the duty cycle of SiC 1 changes from 0% to 100%. the full primary side of an OBC with the rectifier. Switches with power levels of 2.2 to 3.3 kW could be IGBTs or SiC.” The more complex topologies such as a totem pole design are switching below 70 kHz, typically at 30-40 kHz, where MOSFETs and IGBTs can be used. For example, STMicroelectronics has a 3.6 kW power factor correction reference design that uses a silicon carbide MOSFET switching at 72 kHz with an isolated FET driver controlled by a 32-bit microcontroller, with a peak efficiency of 97.5% and a total harmonic distortion of 3.7%. “For the OBC, the problem is that there is both a single- and a three- phase input,” Di Turi says. “That means the topology has to move from single to three phase, and the Vienna rectifier is not suitable for that. “There is still development with superjunction MOSFETs, and the devices are very efficient in DC- DC converter designs. Most of our customers prefer to use a variable frequency range of 120-200 kHz. “With a GaN inverter the competition with SiC is complicated, as SiC is the acknowledged solution with a full- bridge topology with multiple devices per phase, while GaN requires a multi- level topology. The future of GaN is in the DC-DC or AC-DC portion of the charger in the kW range. ;Oe Iasic structure of a toteT pole IriKNeless po^er factor correction Ioost rectifier (Courtesy of Texas Instruments) Winter 2021 | E-Mobility Engineering 57 Deep insight | Switching