Chengcheng Yao

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 GaN transistor based 90W AC/DC adapter with a buck-PFC stage and an isolated Quasi-switched-capacitor DC/DC stage

This paper presents a GaN Transistor based 90W ac/dc adapter with a Buck-PFC stage and an isolated Quasi-Switched-Capacitor (QSC) dc/dc stage. In the dc/dc stage, two different QSC converters are proposed. Compared to Flyback and LLC resonant converters, the QSC converters feature: 1) reduced voltage stress on the primary-side switches to 2/3 of the input voltage; 2) reduced voltage stress on the transformer to 1/3 of the input voltage and a lower transformer turns ratio; 3) a wide range for soft-switching operation and high efficiency; 4) a simple control strategy. The operation principles and simulation results are presented. A 90 W, 85 V/19 V, 1 MHz QSC resonant converter is built, using 100 V EPC eGaN FETs for all switches. This prototype achieves: 1) a high power density of 10.5 W/cm3; 2) wide-range soft switching and a peak efficiency of 92.8% at 900 kHz in preliminary test results. A Buck-PFC evaluation module from TI is tested with a GaN HEMT and a SiC Schottky diode. The peak efficiency reached 97.1%, and the experimental results are compared with those from the Si based version.

Small-signal modeling and controller design of an isolated Quasi-Switched-Capacitor DC/DC converter

This paper presents the small-signal modeling and controller design of an isolated Quasi-Switched-Capacitor (QSC) dc/dc converter. The converter serves as an auxiliary power supply in automotive applications, connecting the highvoltage battery and the 14 V power net. It employs a front-stage QSC dc/ac circuit with a 3:1 voltage step-down ratio, and a poststage synchronous-rectifier, current-doubler circuit. The output power of the converter is regulated by the switching frequency control. A small-signal model of the converter is derived by the method of state-space averaging. Comparison of the open-loop simulation results from the derived small-signal model and a detailed circuit model showcases the effectiveness of the small-signal model. A closed-loop voltage controller with feed-forward compensation is designed to regulate the output voltage. Closed-loop simulation results from both the small-signal model and the detail circuit model, and experiment results from a 1-kW prototype are provided to verify the closed-loop control.

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.

Evaluation of high voltage cascode GaN HEMTs in parallel operation

Paralleling devices is an effective way to achieve higher power applications while still having the convenience brought by discrete devices. However, very few papers investigate the challenges of paralleling Gallium Nitride high electron mobility transistors (GaN HEMTs) in cascode configuration, especially the potential failure modes and its related mechanisms. In this paper, a comprehensive study on paralleled high voltage cascode GaN HEMTs is presented. The influence of paralleling cascode GaN HEMTs on the circuits stray inductance is studied. Potential operation failure modes and the mechanisms of the cascode GaN HEMTs parallel operation were analyzed in detail. The Ansoft Q3D FEA tool and SPICE-based simulation model were used together to quantify the impacts of the circuit and device mismatch on the paralleled GaN HEMTs operation. The SPICE model is validated by the experimental results.

A Quasi-Switched-Capacitor Resonant Converter

A quasi-switched-capacitor (QSC) resonant converter is proposed for isolated dc/dc conversion in offline power supply application. Similar to the Φ2 resonant converter, the proposed converter is operated most efficiently at fixed switching frequency and duty ratio, and it features trapezoidal voltage waveforms on all switches. Full soft switching combining ZCS, ZVS on and off is achieved within a wide load range, and, therefore, the switching loss is minimized. In addition, compared to half/full-bridge converters (e.g., the LLC resonant converter and the dual-active-bridge converter), the proposed converter reduces the voltage stress on primary-side switches to 2/3 of the input voltage, and, thus, enables more choices of low-voltage devices, which are more efficient because of better figure of merit. Furthermore, the proposed converter reduces the transformer turns ratio by 2/3, and, thus, enables less number of turns of the winding, lower winding loss, and lower transformer leakage inductance, making it suitable for high-frequency operation. A 90-W 88-V/19-V 700-kHz prototype is built with 100-V eGaN FETs. The transformer design and PCB layout are presented to minimize the transformer leakage inductance and stray inductance. The prototype achieved a power density of 172 W/in3, and a flat efficiency curve with a peak value of 96%.

Design of a 10 kW GaN-based high power density three-phase inverter

The medium power rating two-level three phase voltage source inverter is among the most popular power conversion systems. The typical switching frequency of the commercial medium power rating inverter, however, is limited to tens of kHz. By increasing the switching frequency and using emerging gallium-nitride devices, the size of the overall system can be greatly reduced. This paper begins by reviewing all commercially available GaN power transistors and their package technologies. The GS66516T device from GaN Systems is selected due to its suitable ratings and superior package performance. Then, a half-bridge structure is designed for this device to achieve low parasitic inductance and strong cooling capability at the same time. The dynamic characterization results of this 650V/60A Enhancement-mode GaN transistor are extracted with the proposed half-bridge structure. A gate drive circuit with comprehensive protection function is integrated. Based on the proposed phase-leg structure, a 10 kW three phase inverter prototype is built and the experimental waveform is shown at the end.

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.