Analysis and Design of High Efficiency Grid-to-Vehicle (G2V) Plug-in Chargers for Local e-Transportation

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

Single-Sensor DCM PFC Based Onboard Chargers for Low Voltage Electric Vehicles

Grid-connected plug-in electric vehicles (PEVs) are considered as one of the most sustainable solutions to substantially reduce both the oil consumption and greenhouse gas emissions. Electric vehicles (EVs) are broadly categorized into low power EVs (48/72 V battery) and high power EVs (450/650 V battery). Low power EVs comprise two-wheelers, three-wheelers (rickshaws), golf carts, intra-logistics equipment and short-range EVs whereas high power EVs consist of passenger cars, trucks and electric buses. Charger, which is a power electronic converter, is an important component of EV infrastructures. These chargers consist of power converters to convert AC voltage (grid) to constant DC voltage (battery). The existing chargers are bulky, have high components’ count, complex control system and poor input power quality. Henceforth, to overcome these drawbacks, this thesis focuses on the onboard charging solutions (two-stage isolated and single-stage non-isolated) for the low voltage battery EVs. Power factor correction (PFC) is the fundamental component in the EV charger. Considering the specific boundaries of the continuous conduction mode (CCM) operation for AC-DC power conversion and their complexity, the proposed chargers are designed to operate in discontinuous conduction mode (DCM) and benefiting from the characteristics like built-in PFC, single sensor, simple control, easy implementation, inherent zero-current turn-on of the switches, and inherent zero diode reverse recovery losses. Proposed converters can operate for the wide input voltage range and the output voltage is controlled by a single sensor-based single voltage control loop making the control simple and easy to implement, and improves the system reliability and robustness. This thesis studies and designs both single-stage non-isolated and two-stage isolated onboard battery chargers to charge a 48 V lead-acid battery pack. At first, a non-isolated single-stage single-cell buck-boost PFC AC-DC converter is studied and analyzed that offers reduced components’ count and is cost-effective, compact in size and illustrates high efficiency. While the DCM operation ensures unity power factor (UPF) operation at AC mains without the use of input voltage and current sensors. However, they employ high current rated semiconductor devices and the use of diode bridge rectifier suffers from higher conduction losses. To overcome these issues, a new front-end bridgeless AC-DC PFC topology is proposed and analyzed. With this new bridgeless front-end topology, the conduction losses are significantly reduced resulting in improved efficiency. The low voltage stress on the semiconductor devices are observed because of the voltage doubler configuration. Later, an isolated two-stage topology is proposed. The previously proposed bridgeless buck-boost derived PFC converter is employed followed by an isolated half-bridge LLC resonant converter. Loss analysis is done to determine optimal DC-link voltage for the efficient operation of the proposed conversion. The converters' steady-state operation, DCM condition, and design equations are reported in detail. The small-signal models for all the proposed topologies using the average current injected equivalent circuit approach are developed, and detailed closed-loop controller design is illustrated. The simulation results from PSIM 11.1 software and the experimental results from proof-of-concept laboratory hardware prototypes are provided in order to validate the reported analysis, design, and performance

LLC resonant charger with variable inductor control

The present work pretends to study the operation and behavior of the LLC resonant converter topology considering a battery charging application, using the traditional switching frequency control and a new control variable, the variable inductance, provided by a current controlled device, the Variable Inductor (VI). During this work, a brief state of the art regarding general types of power converters and resonant power converters is presented. The LLC resonant converter topology and its advantages and disadvantages are described. The VI principle of operation and structure is presented and discussed and, in the end some information about batteries and its behavior under charging and discharging conditions is presented. The considered batteries characteristics for the studied battery charger are shown and the adopted charging profile is presented. In the following chapters, a theoretical analysis of the LLC resonant converter operation and behavior under switching frequency or VI control is performed and presented. A design methodology is proposed for the converter considering both switching frequency and VI control, separately or simultaneously. Simulations of the converter operation under open-loop condition were made, and simulation results were obtained and discussed. A prototype was built and test results were obtained. The prototype uses a SiC MOSFET (Silicon Carbide Metal Oxide-Semiconductor Field Effect Transistor) based inverter working at 100 kHz controlled with fiber optic drivers. To build the prototype, Printed Circuit Boards (PCB) were designed, manufactured and built. An high-frequency transformer and a VI were also design and built. Finally, theoretical, simulation and experimental results are confronted in order to reach conclusions regarding to the proposed design methodology and the prototype operation. This final analysis allows validating the LLC-VI resonant converter as a good option for a battery charger

High-Efficiency Three-Phase Current Source Rectifier Using SiC Devices and Delta-Type Topology

In this dissertation, the benefits of the three-phase current source rectifier (CSR) in high power rectifier, data center power supply and dc fast charger for electric vehicles (EV) will be evaluated, and new techniques will be proposed to increase the power efficiency of CSRs. A new topology, referred as Delta-type Current Source Rectifier (DCSR), is proposed and implemented to reduce the conduction loss by up to 20%. By connecting the three legs in a delta type on ac input side, the dc-link current in DCSR can be shared by two legs at the same time. To increase the switching speed and power density, all-SiC power modules are built and implemented for CSRs. The switching waveforms in the commutation are measured and studied based on double pulse test. Four different modulation schemes are compared for high efficiency CSR considering the switching characteristics of different device combinations. The most advantageous modulation scheme is then identified for each of the device combinations investigated. A compensation method is proposed to reduce the input current distortion caused by overlap time and slow transition in CSRs. The proposed method first minimizes the overlap time and then compensates the charge gain/loss according to the sampled voltage and current. It is verified that the proposed method can reduce the input current distortion especially when the line-to-line voltage is close to zero. The dc-link current will become discontinuous under light load in CSRs, when the traditional control algorithm may not work consistently well. To operate CSR in discontinuous current mode (DCM), the CSR is modeled in DCM and a new control algorithm with feedforward compensation is proposed and verified through experiments. A protection scheme with fast response time is proposed, analyzed and verified to protect SiC devices from overvoltage caused by current interruption in CSRs. To deal with the harmonics and voltage sag in the input ac voltage, a new control algorithm is proposed. By adding ac current feedback control and proportional-resonant (PR) control, the proposed control algorithm can reduce the input current distortion and dc output voltage ripple under input voltage disturbance

Inverse sinusoidal pulse width modulation switched electric vehicles’ battery charger

This paper documents an efficient, cost-effective and sustainable grid-connected electric vehicles (EVs) battery charger based on a buck converter to reduce the harmonics injected into the mains power line. To utilize the switching converter as an effective power factor controller (PFC), inverse sinusoidal pulse width modulation (ISPWM) signals have been applied. A mathematical relationship between the sending-end power factor and the duty ratio of the switching converter has been presented. To ensure the sustenance of the proposed method, a simulation model of the proposed battery charging system has been tested on PSIM simulation platform. The simulation results yield to a lossless charging system with a sending-end power factor close to unity. An experimental testbed comprising a 60 V battery bank of 100 A-h capacity with a charging current of 7 A has been generated. The laboratory assessments present an 88.1% efficient charging prototype with a resultant sending-end power factor of 0.89. The laboratory framework concerns with the comparative analysis of the power efficiency, sending-end power factor and lines current total harmonic distortion (THD) values obtained for different charging methods and the evaluations corroborate the reliability of the proposed work

Analysis of a new family of DC-DC converters with input-parallel output-series structure

There is an increasing trend of development and installation of switching power supplies due to their highly efficient power conversion, fast power control and high quality power conditioning for applications such as renewable energy integration and energy storage management systems. In most of these applications, high voltage conversion ratio is required. However, basic switching converters have limited voltage conversion ratio. There has been much research into development of high gain power converters. While most of the reported topologies focus on high gain and high efficiency, in this thesis, the input and output ripple currents and reliability are also considered to derive a new converter structure suitable for high step-up voltage conversion applications. High ripple currents and voltages at the input and output of dc-dc converters are not desirable because they may affect the operation of the dc source or the load. A number of converters operating in an interleaved manner can reduce these ripples. This thesis proposes a dc/dc switching converter structure which is capable of reducing the ripple problem through interleaved action, in addition to high gain and high efficiency voltage conversion. The thesis analyses the proposed converter structure through a dual buck-boost converter topology. The structure allows different converter topologies and combinations of them for different applications to be configured. The study begins with a motivation and a literature review of dc/dc converters. The new family of high step-up converters is introduced with an interleaved buck-boost as an example, followed by small-signal analysis. Experimental verifications, conclusions and future work are discussed

Development and Evaluation of an Isolated Resonant Converter for Auxiliary Power Supply in DC Traction

This paper presents the implementation and evaluation of an isolated resonant converter and also compares the efficiencies of hard and soft switching isolated converter topologies using high-frequency transformer for auxiliary power supplies in DC traction. The half-bridge DC-DC converter with resonant network has been tested under zero voltage switching (ZVS), zero current switching (ZCS) operations, and also dead time variation of the power switches improving the overall system efficiency. This paper provides guidelines for a cost effective DC-DC converter design based on discrete 1200V/40A IGBTs driven with high switching frequency. That would allow optimization of passive elements by reducing their mass making the converter suitable for traction application. Simulations and test results of an experimental setup with output power up to 3kW are presented. The overall system efficiency of the ZVS and ZCS operations of half-bridge LLC DC-DC converter were compared with a classic hard switching topology

Analysis of a new family of DC-DC converters with input-parallel output-series structure

There is an increasing trend of development and installation of switching power supplies due to their highly efficient power conversion, fast power control and high quality power conditioning for applications such as renewable energy integration and energy storage management systems. In most of these applications, high voltage conversion ratio is required. However, basic switching converters have limited voltage conversion ratio. There has been much research into development of high gain power converters. While most of the reported topologies focus on high gain and high efficiency, in this thesis, the input and output ripple currents and reliability are also considered to derive a new converter structure suitable for high step-up voltage conversion applications. High ripple currents and voltages at the input and output of dc-dc converters are not desirable because they may affect the operation of the dc source or the load. A number of converters operating in an interleaved manner can reduce these ripples. This thesis proposes a dc/dc switching converter structure which is capable of reducing the ripple problem through interleaved action, in addition to high gain and high efficiency voltage conversion. The thesis analyses the proposed converter structure through a dual buck-boost converter topology. The structure allows different converter topologies and combinations of them for different applications to be configured. The study begins with a motivation and a literature review of dc/dc converters. The new family of high step-up converters is introduced with an interleaved buck-boost as an example, followed by small-signal analysis. Experimental verifications, conclusions and future work are discussed