Accurate Measurement of Dynamic on-State Resistances of GaN Devices under Reverse and Forward Conduction in High Frequency Power Converter

Because of trapped charges in GaN transistor structure, device dynamic ON-state resistance RDSon is increased when it is operated in high frequency switched power converters, in which device is possibly operated by zero voltage switching (ZVS) to reduce its turn-ON switching losses. When GaN transistor finishes ZVS during one switching period, device has been operated under both reverse and forward conduction. Therefore its dynamic RDSon under both conduction modes needs to be carefully measured to understand device power losses. For this reason, a measurement circuit with simple structure and fast dynamic response is proposed to characterise device reverse and forward RDSon. In order to improve measurement sensitivity when device switches at high frequency, a trapezoidal current mode is proposed to measure device RDSon under almost constant current, which resolves measurement sensitivity issues caused by unavoidable measurement circuit parasitic inductance and measurement probes deskew in conventional device characterisation method by triangle current mode. Proposed measurement circuit and measurement method is then validated by first characterising a SiC-MOSFET with constant RDSon. Then, the comparison on GaN-HEMT dynamic RDSon measurement results demonstrates the improved accuracy of proposed trapezoidal current mode over conventional triangle current mode when device switches at 1MHz

Design and measurement considerations for WBG switching circuits

Wide Band Gap (WBG) transistors using materials such as Gallium Nitride (GaN) and Silicon Carbide (SiC) offer superior electrical and thermal properties, as well as fast switching capability. However, the high dv/dt and high di/dt may cause ringing with the parasitic inductances and capacitances in the switching loop, increasing overshoot voltages and reducing confidence in the design. Also, making accurate measurements of the switching behaviour without unduly loading the circuit under test is challenging and further impedes the development of WBG applications. This paper presents a prototype WBG Development Platform, built around a half-bridge switched inductive load test circuit. Additional circuits are integrated on to the main PCB for test and measurement purposes: These include a high bandwidth linear current gate-drive circuit and a high bandwidth on-board measurement system. These sub-circuits are described in detail in this paper, together with the switching waveforms that have been achieved during tests with SiC MOSFETs. The authors demonstrate a practical implementation for high frequency WBG power circuits

Trade-off between Losses and EMI Issues in Three-Phase SiC Inverters for More Electrical Aircrafts

Power converters will only be effectively used in future aircrafts if they are compact, efficient and reliable. All these aspects can be improved by the use of disruptive technology such as the so-called Wide Bandgap (WBG) semiconductors made of Silicon Carbide (SiC) or Gallium Nitride (GaN). These components can switch much faster than their silicon counterpart, which can reduce converter losses and also decrease differential mode filter given the increase of switching frequency. However, such a fast commutation increases Electromagnetic Interference (EMI) issues in the converter and load connected to it. This paper shows the approach developed at the French Institute of Technology (IRT) Saint-Exupery, in order to evaluate the trade-offs between losses and EMI issues of three-phase inverters used in future aircraft applications. Given the high voltage DC bus of 540V, SiC MOSFETs are investigated and experimental results show the impact of these components on losses and EMI for different parameters

Impacts of the use of SIC semiconductors in actuations systems

Driven by customers’ demands to improve aircraft performance on one hand, while ensuring compliance to ACARE (Advisory Council for Aeronautics Research in Europe) environmental requirements for 2020 on the other, the aircraft industry has been pushing toward the concept of More Electric Aircraft (MEA) for the last ten years or so. One of the main challenges associated to the More Electric Aircraft is thus to increase drastically the power density of electrical power systems, such as electromechanical chains applied to actuation systems, without compromising on reliability. This paper explains the advantages of using Wide Bandgap (WBG) semiconductors made of Silicon Carbide (SiC) in the power converters that are used in an electromechanical chain as well as the associated drawbacks when it comes to EMI and partial discharge, which are mainly related to high dv/dt and overvoltage during commutation. It also shows the development of a generic electromechanical chain platform at the Institut de Recherche Technologique (IRT) Saint-Exupéry and all related research. This platform is being designed in order to test different technologies composing an electromechanical chain (SiC transistors, passive filters, cables, innovative motor) and to evaluate the impact of the use of such technologies

Characterizing threshold voltage shifts and recovery in Schottky gate and Ohmic gate GaN HEMTs

Threshold voltage shift in normally-OFF GaN High Electron Mobility Transistors (HEMTs) is an important reliability concern in GaN devices. Differences in device architecture between Schottky gate and Ohmic gate normally-OFF GaN HEMTs means that there are important differences in the physical mechanism behind threshold voltage shift due to gate stress. In this paper, a non-intrusive technique for the characterization of threshold voltage shift is applied to both technologies. The technique relies on using a sensing current to measure the third quadrant voltage before and after gate-voltage stress. The results show that in Schottky Gate GaN HEMTs, a positive threshold voltage shift occurs at low gate stress voltages due to electron trapping in the GaN/AlGaN interface while at higher gate stress voltages, the threshold voltage shift becomes negative due to hole trapping and accumulation. The stress time has a fundamental role on the measured threshold voltage shift at medium gate voltage levels and pulsed gate stresses are able to capture this phenomenon. For the Ohmic Gate GaN HEMTs, only a negative threshold voltage shift is observed for all stress currents with no apparent shift as the junction temperature is increased

On-line Condition Monitoring, Fault Detection and Diagnosis in Electrical Machines and Power Electronic Converters

The objective of this PhD research is to develop robust, and non-intrusive condition monitoring methods for induction motors fed by closed-loop inverters. The flexible energy forms synthesized by these connected power electronic converters greatly enhance the performance and expand the operating region of induction motors. They also significantly alter the fault behavior of these electric machines and complicate the fault detection and protection. The current state of the art in condition monitoring of power-converter-fed electric machines is underdeveloped as compared to the maturing condition monitoring techniques for grid-connected electric machines. This dissertation first investigates the stator turn-to-turn fault modelling for induction motors (IM) fed by a grid directly. A novel and more meaningful model of the motor itself was developed and a comprehensive study of the closed-loop inverter drives was conducted. A direct torque control (DTC) method was selected for controlling IM’s electromagnetic torque and stator flux-linkage amplitude in industrial applications. Additionally, a new driver based on DTC rules, predictive control theory and fuzzy logic inference system for the IM was developed. This novel controller improves the performance of the torque control on the IM as it reduces most of the disadvantages of the classical and predictive DTC drivers. An analytical investigation of the impacts of the stator inter-turn short-circuit of the machine in the controller and its reaction was performed. This research sets a based knowledge and clear foundations of the events happening inside the IM and internally in the DTC when the machine is damaged by a turn fault in the stator. This dissertation also develops a technique for the health monitoring of the induction machine under stator turn failure. The developed technique was based on the monitoring of the off-diagonal term of the sequence component impedance matrix. Its advantages are that it is independent of the IM parameters, it is immune to the sensors’ errors, it requires a small learning stage, compared with NN, and it is not intrusive, robust and online. The research developed in this dissertation represents a significant advance that can be utilized in fault detection and condition monitoring in industrial applications, transportation electrification as well as the utilization of renewable energy microgrids. To conclude, this PhD research focuses on the development of condition monitoring techniques, modelling, and insightful analyses of a specific type of electric machine system. The fundamental ideas behind the proposed condition monitoring technique, model and analysis are quite universal and appeals to a much wider variety of electric machines connected to power electronic converters or drivers. To sum up, this PhD research has a broad beneficial impact on a wide spectrum of power-converter-fed electric machines and is thus of practical importance

A High Voltage High Frequency Resonant Inverter for Supplying DBD Devices with Short Discharge Current Pulses

In this paper, the merits of a high-frequency resonant converter for supplying dielectric barrier discharges (DBD) devices are established. It is shown that, thanks to its high-frequency operating condition, such a converter allows to supply DBD devices with short discharge current pulses, a high repetition rate, and to control the injected power. In addition, such a topology eliminates the matter of connecting a high-voltage transformer directly across the DBD device and avoids the issues related to the parasitic capacitances of the latter which disturbs the control the power transfer to the plasma. The design issues of the converter, including the inverter and its switches, the resonant inductor, and the parameter drift compensation are studied. An experimental validation is performed: a mega Hertz resonant converter using GaN FET switches has been manufactured and tested with an excimer lamp

CMOS Active Gate Driver for Closed-Loop dv/dt Control of Wide Bandgap Power Transistors

Wide bandgap (WBG) power transistors such as SiC MOSFETs and GaN HEMTs are a real breakthrough in power electronics. These power semiconductor devices have lower conduction and switching losses than their Silicon competitors. However, the fast switching transients can be an issue in terms of Electromagnetic Interferences (EMI). Consequently, one must slow down the switching speeds of WBG transistors to comply with EMI limitations, which reduces their advantages in terms of higher switching frequencies and lower total losses. In this work, an active gate driver is proposed to control the switching speed of wide bandgap semiconductor power transistors. An innovative closed-loop control circuit makes it possible to adjust separately the dv/dt and di/dt during the switching sequences. Overall, the dv/dt values can be reduced to comply with system-level limits of EMI, with less switching losses than existing methods. The proposed method is thoroughly investigated, with analytic and numerical models to assess the key performances: feedback loop bandwidth, optimal circuit design, area consumption. Selected and optimal designs are implemented in two integrated circuits in CMOS technology which demonstrate delay times below the nanosecond. With such performances, it has been shown experimentally that it is possible to actively control switching speeds higher than 100 V/ns under voltages of 400 V

A GaN-Based Synchronous Rectifier with Reduced Voltage Distortion for 6.78 MHz Wireless Power Applications

The call for a larger degree of engineering innovation grows as wireless power transfer increases in popularity. In this thesis, 6.78 MHz resonant wireless power transfer is explained. Challenges in WPT such as dynamic load variation and electromagnetic interference due to harmonic distortion are discussed, and a literature review is conducted to convey how the current state of the art is addressing these challenges.A GaN-based synchronous rectifier is proposed as a viable solution, and a model of the circuit is constructed. The precisely derived model is compared to a linearized model to illustrate the importance of exactness within the model derivation. The model is then used to quantify the design space of circuit parameters Lr and Cr with regard to harmonic distortion, input phase control, and efficiency. Practical design decisions concerning the 6.78 MHz system are explained. These include gate driver choice and mitigation of PCB parasitics. The model is verified with open loop experimentation using a linear power amplifier, FPGA, electronic load, and two function generators. Current zero-crossing sensing is then introduced in order to achieve self-regulation of both the switching frequency and input phase. The details of the FPGA code and sensing scheme used to obtain this closed loop functionality are described in detail. Finally, conclusions are drawn, and future work is identified