Andrew Lemmon

Instability in Half-Bridge Circuits Switched With Wide Band-Gap Transistors

Wide band-gap (WBG) field-effect devices are known to provide a system-level performance benefit compared to silicon devices when integrated into power electronics applications. However, the near-ideal features of these switching devices can also introduce unexpected behavior in practical systems due to the presence of parasitic elements. The occurrence of self-sustained oscillation is one such behavior that has not received adequate study in the literature. This paper provides an analytical treatment of this phenomenon by casting the switching circuit as an unintentional negative resistance oscillator. This treatment utilizes an established procedure from the oscillator design literature and applies it to the problem of power circuit oscillation. A simulation study is provided to identify the sensitivity of the model to various parameters, and the predictive value of the model is confirmed by experiment involving two exemplary WBG devices: a SiC vertical-channel JFET and a SiC lateral-channel MOSFET. The results of this study suggest that susceptibility to self-sustained oscillation is correlated to the available power density of the device relative to the parasitic elements in the circuit, for which wide band-gap devices, to include SiC and GaN transistors, are in a class approaching that of the radio frequency domain.

Stability Considerations for Silicon Carbide Field-Effect Transistors

Owing to their very low intrinsic capacitance and on-resistance, silicon carbide FETs have been shown to produce poor dynamics in certain power electronics applications, particularly those based on the half-bridge configuration. This letter catalogs three separate phenomena that are observed in the context of such applications and provides a detailed treatment of the most troublesome of these behaviors: the occurrence of sustained oscillation at switch turn-off. This behavior is analyzed in the context of established oscillator design theory; both simulation and experimental results are shown to verify this analysis; and practical suggestions are made to application designers to manage this behavior.

Comparative analysis of commercially available silicon carbide transistors

Since the release of power SiC JFETs in 2008 and power SiC MOSFETs in 2011, there are now more choices of SiC power transistors than ever before available to industrial power electronics markets. To inform prospective users, this paper surveys critical factors influencing the adoption of silicon carbide transistors for a wide range of power electronics applications. Citing publicly available documents, the analysis uses five key factors to compare and contrast the industrial viability of existing SiC transistor technologies: performance, availability, reliability, adoptability, and affordability. Special attention is devoted to the SiC transistors currently available to the commercial market, and the aspects of these devices related to the viability criteria are discussed.

Evaluation of 1.2 kV, 100A SiC modules for high-frequency, high-temperature applications

Cumulative advances in substrate quality and device manufacturing yields over the past few years have paved the way for the commercial introduction of Silicon Carbide (SiC) power modules capable of supporting applications in the 10-20 kW load class and beyond. This paper investigates the suitability of one such module for high-frequency operation at elevated temperatures by leveraging a high-peak-current gate-drive circuit and careful management of parasitic-induced oscillations. Clamped-inductive load experiments have been carried out at elevated temperatures, and the results compared to published results for similar-scale prototype modules. This work demonstrates achievement of very fast slew rates and switching times; the resulting switching losses are 50-70% lower than figures reported in the literature for modules of this scale.

Control and characterization of electromagnetic emissions in wide band gap based converter modules for ungrounded grid-forming applications

Electromagnetic emissions of a 1.2kV, 120A SiC-based half-bridge switching at 100kHz that includes grounding paths and that can be extended to a 100kVA inverter and, eventually, systems of paralleled and cascaded inverters suitable for shipboard and solar farm applications is studied. This switching pole forms the basis of a test platform specifically designed to discover sensitivities to resonant paths so that design guidelines for peripheral structures and EMI mitigating components can be developed. Ungrounded grid-forming inverters are considered in this work because such systems present a worst case scenario when it comes to the effects of resonances through grounding paths being excited by “near-RF” frequencies associated with Wide Band Gap implementations.

Comprehensive Characterization of 10-kV Silicon Carbide Half-Bridge Modules

This paper provides a comprehensive characterization of an example medium-voltage dc-based (MVDC) silicon carbide (SiC) MOSFET module of the type recently developed under Department of Defense (DoD) programs. As shown in this paper, these modules are capable of switching inductive loads very rapidly (<;100 ns), with minimal overshoot and manageable parasitic-induced ringing. The transient analysis contained herein demonstrates a total per-cycle switching losses of 2.5 mJ at a bus voltage of 2 kV and a load current of 20 A. This figure is approximately two orders of magnitude lower than values recorded in the literature for commercially available silicon Insulated-Gate Bipolar Transistor modules with multikilovolt ratings at similar operating conditions (but which have lower blocking capability than the MVDC SiC module under study). These results corroborate the excellent switching performance of these MVDC SiC MOSFET modules, which has been only sparsely documented in the literature to date. In addition, this paper provides a comprehensive analysis of the static characteristics of these modules, including forward conduction, transfer characteristics, capacitance-voltage measurements, and quantization of parasitic impedances within the module housing. Technical challenges associated with making accurate and repeatable measurements with SiC modules of this scale are identified and explained, along with the details of the identified solutions. Finally, this paper includes a detailed description of the transient characterization test stand designed in support of this paper, which offers support for safely demonstrating the high-performance transient characteristics of MVDC SiC MOSFET modules at reasonable cost.

Fast, configurable over-current protection for high-power modules

Protection from over-current situations and the resulting risk of device failure is a requirement when testing limited-run, experimental modules. This paper demonstrates a fast, configurable, and consistent means for protecting against over-current conditions during Clamped Inductive Load (CIL) testing of 10 kV SiC MOSFET modules. This protection circuit offers the user a convenient method of selecting a desired trip current. The protection circuit is able to sense current in the power loop of the CIL test stand, and will interrupt power to the device-under-test (DUT) upon detecting an over-current condition. A method of predicting and compensating the small delay introduced by the gate drive of the protection circuit is also demonstrated.

Active gate drive solutions for improving SiC JFET switching dynamics

Active and Non-Active Gate Drives (AGDs and NAGDs) are known for managing switching characteristics of silicon (Si) and silicon carbide (SiC) power semiconductors. As SiC adoption has grown, the need for intelligent gate drives which are capable of managing the dynamics associated with the fast-switching characteristics of these devices has become apparent. To propose a solution for managing driven and post-driven dynamic behavior, this paper first studies the most recent AGD solutions for silicon power semiconductors with focus on closed loop schemes. The study is continued by reviewing available AGD and NAGD solutions for silicon carbide power semiconductors concentrating on the SiC JFET. Oscillatory modes which can be observed in application circuits based on SiC devices are discussed, and an AGD design example is proposed for improving the final dynamic response of such circuits. The active gate drive design example is constructed, and both simulated and empirical results are shown to substantially reduce the occurrence of natural and forced oscillations at turn-off of the SiC JFET.

Characterization and Modeling of 10-kV Silicon Carbide Modules for Naval Applications

This paper presents a detailed characterization and modeling effort applied to a set of 10-kV SiC MOSFET modules, which have not been exhaustively described in the literature to date. This paper builds on a previous effort by the authors, in which an empirical performance evaluation was performed using a reduced-scale variant of the medium-voltage direct current (MVDC)-rated SiC MOSFET module. In this paper, full-scale module samples are used, which are capable of continuous operation at 120 A. Thus, the evaluation provided here offers improved relevance to the category of full-scale MVDC applications of which future naval shipboard power systems are expected to be a part. The evaluation effort described here considers both the static and dynamic performances of the considered 10-kV SiC MOSFET module, along with the identification of integration considerations that will be of use to designers of future applications based on this technology. Specific contributions of this paper that belong to this category include the presentation of a custom gate-drive circuit designed to operate the modules under consideration and the presentation of a detailed behavioral simulation model created to predict the performance of the same. The output of this model is compared with experimental waveforms captured during pulsed switching experiments at a bus voltage of 2 kV and a load current of 100 A. The simulation output is demonstrated to offer good agreement with the experimental waveforms during both turn-on and turn-off transitions. The availability of such a model is important because it makes possible the execution of a wide range of feasibility and trade studies for future applications by researchers without physical access to this technology.

Methodology for the volume minimization in non-isolated SiC based PV inverters

The opportunities for both power density and efficiency improvements of photovoltaic (PV) inverter have come with the development of commercially available wide bandgap (WBG) devices such as Gallium Nitride (GaN), and Silicon Carbide (SiC). The focus of this paper is to demonstrate a methodology for achieving a minimum volume of the differential and common mode parts of the output filtering while meeting power quality and electromagnetic interference (EMI) specification and maximizing efficiency. Temperatures are kept constant between both Si and SiC designs. Efficiency improvements are also characterized through an accurate calculation of device and magnetic component losses-the largest contributors to loss in the system. Frequency dependent behavior of filter components are taken into account as well as mitigations for high di/dt and dv/dt. MATLAB/Simulnk and PLECS are used to assist in the process and a 100 kVA PV inverter is considered in this work.