Abstract

Electric transportation worldwide has witnessed a tremendous increase in the use of electric vehicles (EV's) due to increased awareness of environmental issues. Road EV's compromise a broad spectrum of vehicles right from two-wheelers three-wheelers (rickshaws/Auto/Trio), cars and electric buses. E-Rickshaw has gained popularity in the Asian market post-2010 because of their symbolic resemblance with traditional auto-rickshaw. The fast growth of the market is principally pushed by the low ownership cost of electric three-wheelers, falling battery prices, and favorable government policies and support. These EVs run on low-cost 48 V, 120 Ah lead acid battery packs having low depth-of-discharge (DOD). Hence, frequent battery charging becomes essential for such EVs. Conventional battery chargers available in the market utilize flyback converter based topologies in order to charge such battery packs. On one hand such battery chargers are easy to implement, these topologies fail to achieve unity power factor (UPF) operation leading to high total harmonic distortion (THD) and poor input power quality at the input. Thus active power factor correction (PFC) becomes a vital constituent in AC-DC converters. By understanding the constraints posed by continuous current mode (CCM) based battery chargers, the proposed converters are designed to operate in discontinuous current mode (DCM) because of its evident benefits such as inherent PFC, zero current turn-on and zero diode reverse recovery losses. By omitting sensors at the input and utilizing only the output sensors, regulated voltage or current can be obtained which makes the system cost-effective and improves its reliability and robustness to high frequency noise.
This thesis presents both isolated and non-isolated battery charger for local e-transportation EVs utilizing 48 V lead acid battery pack. At first, a non-isolated single-stage interleaved buck-boost float charger is proposed by considering the advantages such as reduced current stresses, minimum number of semiconductor devices and absence of bulky high frequency transformer. DCM operation of the proposed converter ensure UPF operation for variable input voltage and utilizing just a single sensor makes this charger configuration economical and easy to implement. However, such a configuration had high current stress on the semiconductor devices leading to increased thermal requirement and reduced efficiency at light loads. Thus addressing these problems, a high efficiency two-stage battery charger is proposed. The battery charger uses an interleaved DCM buck-boost converter in order to achieve PFC at variable input voltage, whereas the second stage is an unregulated half-bridge LLC resonant converter which provides isolation as well as soft-switching for the primary switches. Synchronous rectification (SR) along with only capacitive filter is used on center tapped transformer secondary to improve converter efficiency. Due to DCM of the front-end AC-DC converter achieves zero current turn-on of the switches and DC-DC converter switches achieve zero voltage turn-on because of the LLC resonant. The proposed battery charger implements constant current (CC) and constant voltage (CV) method of charging using simple PI controllers, thus making it suitable for commercial use. Small signal models for both the battery charger configurations are developed using the current injected equivalent circuit approach and a detailed controller design is illustrated. Simulation results using PSIM11.1 software and experimental results from proof-of-concept laboratory hardware prototypes are provided in order to validate the reported analysis and design which demonstrates their performance.