Design and implementation of a universal onboard battery fast charger for transportation electrification applications

Abstract

Electric vehicles (EVs) have signaled a new era in the transportation sector. One key area that is still in development and has a future scope for advancement is the efficient charging systems of the EVs. Due to safety limitations and the lack of charging infrastructure, most of EVs are opted with a two-stage on-board charger (OBC) that allows charging of battery packs from standard utility sockets (85-265 V) as per SAE J1772. Existing EVs such as Neighborhood Electric Vehicles (NEVs), Plug-in-hybrid Electric Vehicles (PHEVs), forklifts, golf kart, and buses have different operating voltages, power levels and vehicle ranges. The output voltage of the OBC falls under one of the three possible ranges, 36-72 V, 72-150 V, and 200-450 V, depending on the type of electric vehicle mentioned above. This thesis presents the design of a universal on-board charger that can address the wide range of battery pack voltages. In a two-stage OBC, this requirement is achieved by varying the AC/DC converter output voltage using the concept of variable DC link rather than fixed DC link in conventional converters. An interleaved boost cascaded-by-buck (IBCBB) power factor correction (PFC) converter has been proposed as an AC/DC converter. Small signal analysis of the converter for the design of the closed loop control structure is also presented. The design consideration for component selection in hardware implementation is also presented. A 1.0 kW system of proposed converter has been designed for proof-of concept. The detailed current stress analysis and loss modelling of the proposed converter was done using empirical calculations and verified using PSIM. To enhance the efficiency of the converter, a simple zero current switching (ZCS) method is applied to most operated boost switch to reduce the turn-off losses. The peak efficiency of 96.4% was observed on proposed soft switched converter. A 1.0 kW system for proof-of concept was implemented. To reduce the turn-on and turn-off losses of the converter and enhance the efficiency of converter further, a simple auxiliary resonant circuit has been implemented to most operated boost switch. This auxiliary circuit provides the zero voltage transition (ZVT) and zero current transition (ZCT) of the boost switch and the remaining components are also soft switched. The detailed timing waveforms of converter operation is presented. The peak efficiency of 97.4% is observed on a 1.0 kW proof-of-concept prototype. Finally, an existing zero voltage switching (ZVS) phase shifted DC-DC converter with the designed high frequency transformer is integrated to the proposed AC/DC converter for wide range of output voltages. Simulations of the overall system is presented and a proof-of-concept is also implemented.