Suxuan Guo

Optimal design methodology of bidirectional LLC resonant DC/DC converter for solid state transformer application

The isolated DC/DC stage in solid state transformer (SST) is a very critical stage in the whole system. It converts the high DC voltage to low DC voltage with the galvanic isolation after the high voltage AC/DC rectifier and before the low voltage DC/AC inverter. In previously developed solid state transformers, dual active bridge (DAB) and dual half bridge (DHB) are the most common topology for this part. In this paper, a bidirectional LLC resonant converter is presented to replace the traditional DAB or DHB in SST application. Compared with DAB, the bidirectional LLC converter can achieve higher efficiency with wider range of zero voltage switching (ZVS), zero current switching (ZCS) and simpler control strategy, which makes it highly attractive for solid state transformer applications. The converter operation modes are presented and analyzed in this paper. For achieving higher efficiency, a methodology of hardware design optimization is introduced to minimize the power losses. Analysis, simulation and experimental results based on a scaled down prototype are shown and discussed.

Design and application of a 1200V ultra-fast integrated Silicon Carbide MOSFET module

With the commercial introduction of wide bandgap power devices such as Silicon Carbide (SiC) and Gallium Nitride (GaN) in the last few years, the high power and high frequency power electronics applications have gained more attention. The fast switching speed and high temperature features of SiC MOSFET break the limit of the traditional silicon MOSFET. However, the EMI problem under high dI/dt and dV/dt is an unneglectable problem. The overshoot and oscillation on drain-source voltage and gating signal could cause breakdown of the switches. This paper proposes a 1200V integrated SiC MOSFET module. With the ultra-fast gate driver integrated with the SiC MOSFET, the parasitic inductance and capacitance could be reduced dramatically, which accordingly suppress the EMI problem caused by the parasitic parameters. Thus zero gate resistance could be adopted in the module to further increase the switching speed. The switching performance of the integrated SiC module is shown better than the discrete package device. The switching loss of the SiC MOSFET module is measured by the inverter level measurement and composition method. Zero switching loss could be achieved when the drain current is lower than a critical value. The module has been tested at 1.5MHz and 3.38MHz switching frequency to prove its high speed capability. For isolated topology applications, the impact of high frequency on the power density and efficiency is discussed in this paper.

A SiC Power MOSFET Loss Model Suitable for High-Frequency Applications

The reduced chip size and unipolar current conduction mechanism make silicon carbide (SiC) metal–oxide–semiconductor field-effect transistors (MOSFETs) suitable for high-frequency power electronics applications. Modeling the switching process of the SiC power MOSFET with parasitic components is important for achieving higher efficiency and power density system design. Therefore, this paper proposes a new concise yet accurate switching loss model for SiC power MOSFETs. Addressing the limitations in experimental measurements, numerical simulations are conducted to validate the proposed model taking the output capacitance Coss discharge and charge into consideration. The role of the parasitic components in the second-order model is discussed in depth for switching losses. Furthermore, this paper also provides guidelines in designing the gate driver for ultrafast SiC power MOSFETs.

An Isolated Bidirectional Single-Stage DC–AC Converter Using Wide-Band-Gap Devices With a Novel Carrier-Based Unipolar Modulation Technique Under Synchronous Rectification

A novel carrier-based unipolar-sinusoidal pulse width modulation (SPWM)-oriented modulation technique with synchronous rectification for isolated bidirectional single-stage high-frequency-ac link dc-ac converters using SiC MOSFET is presented in this paper. The dc-ac converter is composed of a full-bridge (FB) inverter cascaded with a cycloconverter through a high-frequency transformer. A carrier-based unipolar-SPWM-oriented modulation technique with synchronous rectification is proposed to realize zero-voltage-switching (ZVS) for the FB inverter and zero-current or zero-voltage-switching (ZVS/ZCS) for the cycloconverter in all load ranges, and to suppress the voltage spikes introduced by the transformer leakage inductance as well. In order to increase the switching frequency, efficiency, and power density, this paper proposes to utilize SiC MOSFETs for the converter. Synchronous rectification is implemented to further increase the converter efficiency. With the novel modulation technique, there are two switches in the cycloconverter that are continuously on at each interval, which eliminates on-fourth of the switching loss. A simulation model and a 400 VDC-240 VAC, 1.2=kW prototype have been developed to validate the effectiveness and performance of the proposed unipolar soft-switching modulation technique and SiC converter.

Control and analysis of the high efficiency split phase PWM inverter

The traditional power switch phase leg is widely used in the power electronic devices. However, the short through of the phase leg is always a problem for reliability, efficiency, higher switching frequency. Besides, both of power device turn-on/turn-off time and the reverse recovery time of poor performance body diodes will limit the switching frequency and power conversion efficiency. This paper proposes a new split phase PWM inverter which could split the MOSFET based phase legs by coupled inductor to prevent the short through and disable poor performance body diode. The extended Schottky diodes could be adopted to eliminate reverse recovery loss. By using the traditional bipolar or unipolar PWM control schemes, the circulating current which is caused by the feature of inductors will exist in the circuit and will increase power loss. An advanced unipolar PWM control strategy is proposed in this paper to minimize circulation current and to improve power conversion efficiency. Simulation is done to verify the better performance of split phase PWM inverter and the proposed advanced unipolar PWM control strategy.

Soft-Switched Modulation Techniques for an Isolated Bidirectional DC–AC

Two carrier-based unipolar-sinusoidal pulse width modulation (SPWM)-oriented modulation techniques for an isolated bidirectional dc–ac converter are proposed, compared, and validated in this paper. The dc–ac converter is composed of a full-bridge (FB) inverter cascaded with a cycloconverter through a high-frequency transformer. Both modulation techniques proposed in this paper can realize zero-voltage switching (ZVS) for the FB inverter and zero-current switching or ZVS for the cycloconverter in all load range, and are able to suppress the voltage spikes introduced by the transformer leakage inductance as well. In order to increase the converter efficiency and power density, we propose to utilize SiC MOSFETs for the converter. The first modulation technique enables the utilization of Si-SiC hybrid switches with no synchronous rectification (SR), for the purpose of lowering the converter cost. The second modulation technique requires all switches to be SiC MOSFETs, but with SR, which increases the converter efficiency. A 400-V dc to 240-V ac 1.2-kW prototype has been developed to validate the effectiveness and performance of the proposed carrier-based unipolar-SPWM-oriented modulation techniques.

Design and optimization of the high frequency transformer for a 800V/1.2MHz SiC LLC resonant converter

The LLC resonant converter is very suitable for high frequency and high power density applications due to its excellent characteristics such as load independent ZVS turn-on, low turn-off current. The high frequency high voltage transformer is a key component in the LLC that provides galvanic isolation and power conversion. In order to improve the power density, the frequency can be pushed to several hundred kHz or megahertz range especially if advanced power devices such as the SiC MOSFET is used. However, optimal design of the transformer is a grand challenge which must consider magnetic core material, core and winding loss, and transformer thermal performance. In this paper, a megahertz high voltage transformer is designed for a high voltage SiC LLC resonant converter. The loss model and thermal model of the transformer are analyzed in details. The high frequency magnetic core materials are reviewed and compared. A comprehensive transformer optimization design method is proposed to maximize the utilization of the transformer, achieving the maximum power density with minimum transformer loss. A 1.2 MHz 4.5 kW transformer prototype is built and tested in the 800V SiC LLC resonant converter.