High Frequency Magnetics and Soft-Switching for Differential Mode Electric Vehicles Superchargers
Creators: name: Mohamadi, Moien
Publisher: name: University of Illinois at Chicago
Year: 2023

description: In this dissertation, confluence of integrated magnetics and soft switching is presented for a differential mode EV charger aimed at improving power density and efficiency simultaneously. The trade-off between these two characteristics of power electronics systems (PES) is usually favoring only one, by using triangular conduction mode (TCM) the major loss producing mechanism (turn on switching loss of the ac-side device) is eliminated without additional components. This is done through extending the switch current ripple to a slightly negative value in order to discharge the Coss of the incoming device. Increasing current ripple in the switch results in elevated losses in the magnetics (increased flux density swing) as well as additional turn off loss and conduction loss in the device. To lower the required ripple for TCM each phase in the charger is divided into three interleaved sub-modules. In doing so, the ripple current in the high frequent ac- and dc-side capacitors is reduced significantly, which enables the designer to select much smaller capacitor size while the conducted electromagnetic emission cause by switching transitions is maintained. The hardware implementation of this architecture is critically important to gain the desired effect. An off-the-shelf six-pack 1.7 kV SiC MOSFET is used with similar footprint compared to the hard switch counterpart to enhance the power stage layout. That is further aided by multilayer high current PCB design for equal current sharing between interleaved sub-modules and minimizing power loop inductance. A custom gate driver motherboard is design to hist the charging/discharging circuitry alongside device protection in a small footprint (slightly larger than the device). The design is further optimized by several noise mitigation techniques in the signal feedback and sensing system, such as, isolated power and signal delivery, decoupled ground architecture, differential PWM signal transmission, etc. Integrated magnetic is developed first through scaled power prototype of the target PES. Its performance is compared with the discrete magnetic counterpart, showing 30 % current ripple reduction through flux cancellation. The trade-offs between coupling factor with the performance of the rectifier is analyzed. Through heuristic optimization k=0.5 is selected, efficiency is improved around ~2 % and the length of the footprint is reduced to by 70\ mm with similar width that translate into 51.5\ nH reduction in parasitic capacitance and 3.3\ m\Omega reduction in parasitic resistance in the power loop. The same conceptual magnetic charger is scaled up to high power (60\ kW) and high-power design challenges are studied both in the integrated magnetics and other aspects of the power stage design. Inspired by the improvements in the hard switching setup the integrated magnetics idea is extended to the soft- switching setup with all the inductors of a phase (module) integrated into the same magnetic device called integrated interleaved magnetics (IIM). Doing this further improves the performance of the soft-switching solution. The efficiency of a single module at various operation points changes between 97.5\ %\ to\ 98\ % while the power density improved by around 6712 W/m3.