Lixing Fu

Development of an 85-kW Bidirectional Quasi-Z-Source Inverter With DC-Link Feed-Forward Compensation for Electric Vehicle Applications

This paper presents a detailed operation analysis, controller design, and realization of a high-power, bidirectional quasi-Z-source inverter (BQ-ZSI) for electric vehicle applications. The circuit analysis shows that with a bidirectional switch in the quasi-Z-source network, the performance of the inverter under small inductance and low power factor can be improved. Based on the circuit analysis, a small signal model of the BQ-ZSI is derived, which indicates that the circuit is prone to oscillate when there is disturbance on the dc input voltage. Therefore, a dedicated voltage controller with feed-forward compensation is designed to reject the disturbance and stabilize the dc-link voltage during a non-shoot-through state. An 85-kW prototype has been built. Both simulation and experimental results are presented to prove the functionality of the circuit and the effectiveness of the proposed control strategy.

A Wide Bandgap Device-Based Isolated Quasi-Switched-Capacitor DC/DC Converter

This paper proposes an isolated quasi-switched-capacitor (QSC) dc/dc converter to serve as an auxiliary power supply in electric and hybrid electric vehicles, managing a bidirectional power flow between the high-voltage (HV) battery and the low-voltage dc bus. A QSC dc/ac circuit with a 3:1 voltage step-down ratio is proposed to serve as the front-stage circuit of the converter. Based on it, an isolated QSC dc/dc converter is proposed with a synchronous-rectifier, current-doubler post-stage circuit. Compared with existing full-bridge, half-bridge, and three-level converters, the features of the proposed converter include: 1) the voltage stresses on HV-side switches are reduced to two-third of the HV-dc-bus voltage; 2) the voltage stress on transformer is reduced to one-third of the HV-dc-bus voltage; 3) the transformer turns ratio is reduced; 4) it has soft-switching capability and high efficiency; and 5) bidirectional power-flow and simple control can be implemented. The operation principles, soft-switching analysis, and simulation results are presented. Wide Bandgap devices are selected for the proposed converter to shrink the size of passive components, provide high efficiency, and decrease the cooling requirement. Guidelines are given to estimate the key circuit parameters, including the capacitance of the switched capacitors, the transformer dc-bias flux density, and the average currents of the post-stage inductors. Experiment results provided from a 1-kW prototype built with SiC MOSFETs on the HV-side validate the feasibility and superior performance of the proposed converter.

Merits of gallium nitride based power conversion

Gallium nitride (GaN) based power electronics devices are actively being evaluated to determine if their theoretical advantages over silicon (Si) based switches can translate into improved performance of existing hardware as well as open the doors to new types of applications, such as high temperature implementations, or very high frequency power conversion. The following paper presents an overview of this activity. A brief summary about power electronics and the requirements of semiconductor devices used in this field is provided. Detailed analysis of the advantages and the challenges of using GaN devices is included along with a survey of demonstrations. This work also presents the test results from the evaluation of GaN devices from Efficient Power Conversion (EPC) and Transphorm. Included is a demonstration of EPCs devices in a high frequency, high efficiency, switched-capacitor voltage doubler. This circuit achieves an output of 480 W at a switching frequency of 893 kHz.

A Gate Drive With Power Over Fiber-Based Isolated Power Supply and Comprehensive Protection Functions for 15-kV SiC MOSFET

This paper presents a 15kV silicon carbide (SiC) MOSFET gate drive, which features high common-mode (CM) noise immunity, small size, light weight, and robust yet flexible protection functions. To enhance the gate-drive power reliability, a power over fiberbased isolated power supply is designed to replace the traditional design based on isolation transformer. It delivers the gate-drive power by laser light via optical fiber over a long distance (>1 m), so a high isolation voltage (>20 kV) is achieved, and the circuit size and weight are reduced. More importantly, it eliminates the parasitic CM capacitance coupling the power stage and control stage, and thus eradicates the control signal distortion caused by high dv/dt in switching transients of the high-voltage SiC devices. In addition, the gate-drive circuit design integrates comprehensive protection functions, including the overcurrent protection, undervoltage/overvoltage lockout, active miller clamping, soft turn off, and fault report. The overcurrent protection responds within 400 ns. The experimental results from a 15kV double-pulse tester are presented to validate the design.

A phase shift controlled current-fed Quasi-Switched-Capacitor isolated dc/dc converter with GaN HEMTs for photovoltaic applications

This paper proposes a phase shift controlled isolated dc/dc converter for photovoltaic applications, which is composed of a current-fed half bridge and a Quasi-Switched-Capacitor (QSC) cell. The proposed converter employs phase shift control to realize a wide voltage regulation range, and features zero-voltage-switching on all the switches. Compared to traditional H-bridge and half bridge, the secondary side QSC circuit exhibits a higher boost ratio and reduced voltage stress on devices and transformer. The circuit structure, operation principles and soft-switching analysis of the converter are presented. A 1-kW gallium nitride device based circuit prototype is used to verify the circuit analysis. Both simulation and experimental results are presented.

Small signal modeling and controller design of a bidirectional Quasi-Z-Source inverter for electric vehicle applications

This paper presents the small signal modeling and controller design of a bidirectional Quasi-Z-Source inverter (BQ-ZSI) for electric vehicle (EV) applications. The derived small signal model shows that the Quasi-Z-Source network (QZSN) is prone to oscillate when there is a disturbance from the dc input voltage. Since the battery pack voltage in an EV is easy to fluctuate because of the batterys internal impedance and rapid change of the load current, a dedicated voltage controller with feed forward compensation is designed to reject the disturbance and stabilize the dc-link voltage during non-shoot-through state. Simulation and experimental results are both presented to prove the effectiveness of the proposed controller.

An isolated DC/DC converter with reduced number of switches and voltage stresses for electric and hybrid electric vehicles

This paper proposes an isolated DC/DC converter to serve as an auxiliary power supply in electric and hybrid electric vehicles (EVs/HEVs), delivering power from high voltage (HV) dc bus to 12 V loads. A DC/AC circuit with a 3:1 voltage step-down ratio is proposed to serve as the front-stage circuit of the converter. Based on the proposed DC/AC circuit, an isolated DC/DC converter is proposed with a post-stage synchronous rectifying current doubler circuit. Compared with full-bridge, half-bridge, push-pull and flyback converters, the advantages of the proposed converter include: 1) the voltage stresses on HV-side switches are reduced to 2/3 of the input voltage; 2) the voltage stress on transformer is reduced to 1/3 of the input voltage; 3) reduced transformer turns ratio; 4) soft-switching capability, high efficiency and simple control. The operation principles, soft-switching analysis, simulation verifications, and experiment results of a 1 kW hard-switching prototype based on silicon carbide (SiC) MOSFETs on the HV side are provided.

A 15 kV SiC MOSFET gate drive with power over fiber based isolated power supply and comprehensive protection functions

This paper presents a 15 kV SiC MOSFET gate drive circuit, which features high common-mode (CM) noise immunity, small size, light weight, and robust yet flexible protection functions. To enhance the gate-drive power reliability, a power over fiber (PoF) based isolated power supply is designed to replace the traditional design based on isolation transformer. It delivers the gate-drive power by laser light via optical fiber over a long distance (>1 m), so a high isolation voltage (>20 kV) is achieved, and the circuit size and weight are reduced. More importantly, it eliminates the parasitic CM capacitance coupling the power stage and control stage, and thus eradicates the control signal distortion caused by high dv/dt in switching transients of the high-voltage SiC devices. In addition, the gate drive circuit design integrates comprehensive protection functions, including the over-current protection, under/over-voltage lockout, gate-clamping, soft turn-off, and fault report. The over-current protection responds within 400 ns. Experimental results from a 15 kV double pulse tester are presented to validate the design.

A Family of Quasi-Switched-Capacitor Circuit-Based Dual-Input DC/DC Converters for Photovoltaic Systems Integrated With Battery Energy Storage

In this paper, a family of bidirectional dual-input dc/dc converters is proposed to combine a photovoltaic system and battery energy storage system. This family of converters utilizes a full-bridge, or half-bridge current-source circuit, as the primary side, and a quasi-switched-capacitor circuit as the secondary side. Depending on the power level of the primary side and voltage level of the battery, different topologies can be selected. Compared to other bidirectional multiple-input dc/dc converters with galvanic isolation, this family of converters requires less switches and passive components, reduces voltage stress on switches, and realizes soft switching for all switches. The operation principle of the converter is presented, and a power sharing strategy between two input sources is proposed. A 2-kW full-bridge circuit prototype is built in the lab, based on Gallium Nitride (GaN) switching devices. Simulation and experimental results are also presented to verify the theoretical analysis.

The evaluation and application of wide bandgap power devices

This paper presents an overview of wide bandgap (WBG) power devices. The development and challenges of silicon carbide (SiC) and gallium nitride (GaN) power devices are summerized. A comprehensive evaluation of the performance of different devices is conducted, including static characterization and dynamic switching related tests. The paper also demonstrates the application of WBG devices in power electronic circuits. The testing results are provided to show the performance of WBG devices in different aspects.