DC/DC converter for EV charging from trolley grid

Abstract

Trolley bus systems provide modern low-emission public transport for urban areas. The trolley bus demands a peak power at acceleration that is at least 4 times the regular operational power demand of the bus. To provide the acceleration power for a fleet of buses, the substations are inherently oversized. With the excess capacity in the trolley grid, EV charging can be integrated into the DC trolley grid, making the trolley grid more multi-functional. This EV charger should be able to charge from a DC grid with fluctuating line
voltage, has high power, low volume, high efficiency, and conform to different charging protocols. In this thesis, a 25kW DC/DC converter for EV charging from the trolley grid is modeled, designed, and compared. The isolated full-bridge PWM ZVS topology is chosen and simulated on MATLAB Simulink. The working principle of the full-bridge converter is analyzed and modeled, obtaining the operating modes and circuit behavior. Then the influence of the circuit parameters can be investigated on the model, and the optimal design choices are made. The converter uses SiC MOSFET switches, and its modular two secondary phase design provides 2 connection modes, ensuring a wide output voltage and current range. The zero-voltage switching can be achieved over a wide load range, and the efficiency at rated load 25kW is 96.4%. Finally, another widely used high power DC/DC converter topology dual active bridge (DAB) is investigated and modeled in Simulink, and the waveforms and efficiency of DAB are presented in both power transfer directions. The performance of DAB is compared with that of the full-bridge converter.