61 Isolation technologies | Deep insight consumption increases with LED forward current, which can range from 1 mA to over 15 mA. In some cases, the LED may require an external driver, further decreasing system efficiency, while increasing BOM complexity and cost. The optocoupler output impedance can be low or high, depending on its architecture. High-speed optocouplers have an active photocoupler and output driver that requires an external bias voltage. Such devices have low output impedance, but at the expense of increased total operating current, which can range from 15 mA to over 40 mA. Compared with optocouplers, CMOS digital isolators offer significantly higher operating efficiency, consuming about 1.7 mA per channel at 10 Mbps at VDD = 5.0 V with a 15 PF (power factor) load. Its high-impedance input buffer consumes only microamps of leakage current, while its 50 W CMOS output driver can source or sink 4 mA. The bulk of the CMOS digital isolator’s power savings results from the use of an RF carrier instead of light, eliminating the power-hungry LED. Losses in the isolation path are minimised by the isolation capacitor structures, which are optimised for robust data transfer and minimum power loss. The CMOS digital isolator’s power dissipation remains relatively flat and substantially less than that of the optocoupler. Integration challenges New power-conversion architectures can create issues for isolation. The integration of the onboard battery charger with the traction drive reduces the overall size and complexity of the system, but can present safety issues due to the absence of an isolation stage. Most carmakers require an isolation stage galvanically separating the grid from the battery, but this is not provided by most integrated chargers. Additionally, some designs produce torque at the shaft during charging, which may cause vibration and the need for rotor locking. Some designs even require free-shaft continuous rotation of the rotor during charging at grid electrical frequency, complicating the mechanical arrangement, and introducing relevant friction and ventilation losses during charging. They are also unidirectional, so do not allow vehicle-to-grid (V2G) operation. To address these issues, researchers at the University of Torina, Italy, have integrated the onboard battery charger with the traction drive of road EVs equipped with a six-phase, traction motor drive. They developed a charger that is deeply integrated within the e-drive powertrain to reduce the cost and volume of the e-axle with respect to non-integrated solutions, but still providing galvanic insulation. Dedicated control strategies are developed and tested for regulating the AC grid current at unitary power factor and low total harmonic distortion (THD), and to avoid torque production or rotor movement during charging independently of the rotor position. Extensive simulation results show the feasibility of the proposed solution, together with a proof-ofconcept validation on a commercial traction motor. Multi-phase designs are increasingly popular, permitting a reduced phasecurrent rating and higher system reliability. Torque-ripple mitigation, a higher number of degrees of freedom for the control and better thermal management are also additional benefits. In particular, the adoption of six-phase motors appears to be the best trade-off between the benefits of multi-phase machines and increased system complexity, permitting a relatively easy transition from the traditional three-phase drives. The Isolated Fully Integrated OBC (IFI-OBC) concept developed by the researchers adds a control strategy that requires a dedicated transformation matrix for defining the inverter reference voltage. This integrated OBC enables accurate control of grid current quality and the absence of motor torque during charging. Finally, differently from many other topologies, the proposed OBC is bi-directional, permitting V2G operation. The design has been validated by accurate simulation models and experimental tests on a proof-ofconcept test rig using a commercial, 96 kW traction motor. The isolated onboard battery charger has been developed and integrated with a traction motor drive, based on a sixphase permanent magnet synchronous machine (PMSM). The topology implements galvanic isolation between the grid and the battery by exploiting the traction motor as a transformer, without E-Mobility Engineering | January/February 2025 The InnoSwitch3 with FluxLink isolation (Image courtesy of Power Integrations)
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