Turn-off Transition Research Articles

A Zero-Voltage-Transition H5-Type Transformerless Photovoltaic Grid-Connected Inverter

Efficiency and leakage current are two major issues for transformerless grid-connected inverters (TLI). Soft-switching technologies can reduce switching losses and be applied in TLIs. Herein, a zero-voltage-transition H5 type (ZVT-H5) inverter with soft turn-on and turn-off transitions of high-frequency main switches is derived from basic resonant tanks. Compared with the hard-switching H5 (HS-H5) inverter, the conversion efficiency of ZVT-H5 is significantly improved. Moreover, the reverse recovery problem of freewheeling diodes is alleviated under the operation of the auxiliary resonant network. Meanwhile, a diode clamping branch is employed to achieve constant common-mode characteristic for the ZVT-H5. The construction process of the proposed topology is described, and the operation principle of the auxiliary resonant network is analyzed. Moreover, the resonant parameter design of ZVT-H5 and its circuit performance are discussed in detail. Finally, the experimental results of a 3-kW prototype at 100 kHz switching frequency are provided to verify the effectiveness of the ZVT-H5.

Design of a Gate-Driving Cell for Enabling Extended SiC MOSFET Voltage Blocking

A series connection of SiC MOSFETs for kV blocking capability can enable more design flexibility in modular multi-level converters as well as other topologies. In this paper, a novel gate driver circuit capable of driving series-connected SiC MOSFETs for high voltage applications is proposed. The primary advantage of the proposed design is that a single gate driver was used to switch all the series devices. The circuit used switching capacitors to sequentially charge and discharge device gate capacitances during switching and enable a negative turn-off voltage to avoid device coupling from Miller-capacitive feedback effects. With the proposed gate driver design and appropriate component values selection, avalanche breakdown due to voltage divergence during switching transients could be avoided with only a minor imbalance in the top device. Simulations and experimental measurements showed that the zero-current turn-off transition of all switches was achieved, and this approved the validity of the design.

A New Gate Driver for Suppressing Crosstalk of SiC MOSFET

High switching-speed Silicon Carbide Metal-Oxide-Semiconductor Field-Effect Transistor (SiC MOSFET) has serious crosstalk issues. During the turn-ON transition and turn-OFF transition of the active switch in a phase-leg configuration, the voltage drops across the common-source inductor and the displacement current of the gate-drain capacitor of the OFF-state switch induce a spurious pulse on its gate-source voltage. This paper proposes a new gate driver using two Bipolar Junction Transistors (BJTs) and one diode to connect the gate terminal of SiC MOSFET and the negative driver voltage, which provides a low impedance path to bypass the displacement current of the gate-drain capacitor when crosstalk issues occur. The simulation results prove the proposed driver is valid on suppressing the crosstalk issue. The comparisons between the prior drivers and the proposed driver show the superiority of the proposed driver. Finally, the proposed gate driver is successfully implemented and experimentally verified on a 1.1 kW synchronous buck prototype.

Thin-drift-layer n-channel 4H-SiC IGBT with low switching loss and switching ringing reduction

An n-channel 4H-SiC insulated-gate bipolar transistor (IGBT) with an extremely low switching loss was demonstrated by making 70 μm thin drift layers designed for 6.5 kV blocking voltage, without substrates. A conductivity-modulated bipolar operation was successfully performed as a on-voltage of 4.96 V at a 100 A cm−2 collector current. The turn-off losses under 3.6 kV/32 A operation were 8.8 (at room temperature) and 22.7 mJ (at 150 °C), which were much lower than the estimated losses of SiC IGBTs with thick drift layers. A detailed investigation on turn-off transition of SiC IGBTs specifically focused on a voltage-slope increase was also conducted. This phenomenon can cause ringing in switching characteristics. A device structure has been proposed to eliminate it by controlling the expansion of a depletion layer. Voltage-slope control and ringing reduction without noticeable degradation in static characteristics and switching loss were demonstrated with the SiC IGBT with the proposed device structure.

Evaluation of the turn‐off transient controllability for high‐power IGBT modules

The efficient and flexible conversion of electrical energy is increasingly accomplished by megawatt insulated gate bipolar transistor (IGBT) modules. Their dynamic performance is influenced by the gate driver control, operating points and the switching loop parasitics, which are crucial to their optimized operational behaviours, efficiency and field reliability. This paper investigates the controllability of the transient voltage and current slopes of megawatt IGBT modules during their turn-off transitions and proposes an efficient assessment method accordingly. Firstly, the causal relationship of transient characteristic variations and controllability is investigated analytically. Then, major factors impacting the controllability are analysed and validated, including the charge carrier profile, the operating points (i.e. the load voltage, the load current, and the junction temperature), and the gate resistance. Finally, an efficient method using the MOS-channel turn-off point on calibrated gate voltage waveform is proposed for the controllability evaluation, which can guide the IGBT, gate driver, and converter design.

Active Auto-Suppression Current Unbalance Technique for Parallel-Connected Silicon Carbide MOSFETs

Nowadays, medium- and high-power applications make use of silicon carbide (SiC) MOSFETs, and many times their parallelization is necessary. Unfortunately, this requirement causes an inevitable current unbalance between power devices, affecting the performance of power switches. Over the last decade, numerous studies have been conducted, proposing various techniques with the capability of mitigating current unbalance for a number of discrete parallel SiC MOSFETs. However, the realization of most methods requires knowledge of the technical characteristics of power devices, adding extra cost to the system, since screening is a time-consuming and costly process. This necessity reduces the possibilities of such a technique being implemented in power electronics applications, preventing the exploitation of the exceptional features of SiC MOSFETs. In addition, most of these techniques can suppress only the current unbalance, which occurs during turn-on and turn-off transitions. In this paper, an active auto-suppression current unbalance technique is proposed, requiring no device screening. The active method is a closed-loop system capable of sensing and eliminating the entire current unbalance between parallel SiC MOSFETs automatically, actively, and independently of the cause. Simulation results are presented to demonstrate the feasibility and effectiveness of the proposed technique.

Impact of Carriers Injection Level on Transients of Discrete and Paralleled Silicon and 4H-SiC NPN BJTs

The 4H-SiC vertical NPN BJTs are attractive power devices with potentials to be used as high power switching devices with high voltage ratings in range of 1.7 kV and high operating temperatures. In this paper, the advantages of the 4H-SiC NPN BJTs in terms of switching transients and current gain over their silicon counterparts is illustrated by means of extensive experimental measurements and modelling, including investigation of high level injection, as a common phenomenon in bipolar devices that influences the switching rates and DC current gain. The two device types have been tested at 800 V with maximum temperature of 175 °C and maximum collector current of 8 A. The turn-ON and turn-OFF transition in Silicon BJT is seen to be much slower than that of the SiC BJT while the transient duration will increase with increasing temperature and decreases with larger collector currents. The common-emitter current gain of SiC BJT is also found to be much higher than silicon counterparts, increasing with temperature in low injection levels but decreasing in higher injection levels in both devices. The rate of increase of current gain slows down toward stability as the collector current increases, known as the high-level injection. Current sharing imbalance among parallel connected devices is also investigated, which are shown to be evidently dependant on temperature and base resistance in Silicon BJT, while the current collapse in also seen in SiC BJT at high injection levels with high base resistance. The turn-OFF delay is seen to be temperature dependant in single and paralleled Silicon BJTs while almost non-existent in SiC device.

A Novel Single-Switch High Step-Up DC–DC Converter with Three-Winding Coupled Inductor

This paper introduces a single-switch, high step-up DC–DC converter for photovoltaic applications such as power optimizers and microinverters. The proposed converter employs two voltage multipliers cells with switched capacitor and magnetic coupling techniques to achieve high voltage gain. This feature, along with a passive clamp circuit, reduces the voltage stress across the switch, allowing for the employment of low RDSon MOSFET. This leads to low conduction loss of the switch. The diodes operate with zero-current switching at their turn-off transition, eliminating the reverse recovery losses. Additionally, the switch turns on with zero-current switching, leading to insignificant switching loss associated with its turn-on transition. The operation principle and steady-state analysis are presented and validated through experimental results obtained from a 140 W prototype of the proposed converter.

A reliable voltage clamping submodule based on SiC MOSFET for solid state switch

The electron cyclotron resonance heating system is one of the most effective plasma heating systems for controlled nuclear fusion. The key part of the system called gyrotron is driven by a high voltage power supply with a rated output power of hundreds of kilowatts. When the system is in operation, breakdowns frequently occur in the gyrotron. During breakdowns, the gyrotron will endure a large volume of energy and may be damaged. A solid-state switch is required to protect it by blocking high voltage (∼40 kV) within 10 microseconds and limiting energy within a few joules. Compared with Si IGBT/MOSFET, SiC MOSFET with higher switching speed is more suitable for the switch. However, rapid switching speed exacerbates the voltage imbalance. To solve the problems, a reliable module named Advanced Chopper Sub-Model based on SiC MOSFETs for a solid-state switch is proposed. The module adopts a voltage-clamped circuit to achieve the capabilities of rapid switching-off speed, as well as low overvoltage and good voltage balancing. In addition, modules connected in series can tolerate large driver time delay. The SPICE simulation and the double-pulse test are used to validate the effectiveness of the proposed module. The protection performance test was also conducted by using a spark gap to simulate the breakdown fault. Finally, the switch that consists of 64 series-connected modules has been tested at 30 kV/6A. The turn-off time is ∼5 µs, and the energy during the turn-off transition is 0.283J. The results show that the switch has good performance.

A Soft-Switched Modulation for a Single-Phase Quasi-Z-Source-Integrated Charger in Electric Vehicle Application

A modulation for a single-phase quasi-Z-source-integrated electric vehicle charger (qZSC) system is proposed, which can achieve multiple zero-voltage-switching and zero-current-switching (ZCS) transitions without any auxiliary circuit. Though the modulation has a couple of hard-switching turn-off transitions, they occur at low current values. The switch of a quasi-Z-source network also can be turned on with ZCS. The inductor currents of the quasi-Z-source network operate in boundary conduction mode or discontinuous conduction mode to achieve all freewheeling diodes turned off naturally. Besides, there is no additional voltage stress and current stress on all switches in comparison with hard switching. Therefore, it is observed that the efficiency of the qZSC system using the novel modulation increases significantly. The details of operation principles for this soft-switching qZSC are analyzed. A simulation model is built and a 1.3-kW prototype is established to verify the proposed theory.

Active Optical Modulation for Series-Connected Emitter Turn-<sc>Off</sc> Thyristors

Series connection of high-power thyristors is proposed in the literature for high-voltage applications. Due to the high dv/dt across series-connected thyristors during turn-off, charge equalization techniques have been pursued that increase the fall time to avoid overshoot voltage and device breakdown. Longer fall time for thyristors yields reduced switching frequency and increased losses. As such, a new method to control turn-off transition for the series connection of recently developed optical emitter turn-off (o-ETO) thyristor is outlined in this letter. The turn-off speed of series-connected o-ETOs and their voltage balance are controlled by modulating the optical intensity of the triggering (instead of the high-power) device of the o-ETO.

Leakage current reduction for a double‐leg boost converter by switching transition synchronisation

With wide-bandgap transistors, the leakage current through the parasitic transistor to heat sink capacitance is increased with the operation at higher switching transition voltage slopes. This study presents a modulation synchronisation method for a double-leg boost converter to reduce these leakage currents. Having a similar leakage current path, the transistors turn-on transition of one leg is synchronised on the turn-off transition of the second leg.

Investigation on the Activation Energy of Device Degradation and Switching Time in AlGaN/GaN HEMTs for High-Frequency Application

In this paper, the influence of traps on the dynamic on-resistance ( $\text{R}_{\mathrm{ dson}}$ ) and switching time of AlGaN/GaN high-electron-mobility transistors is validated by means of a switching power converter with floating buck-boost topology. A new scheme based on the voltage-dependent dynamic $\text{R}_{\mathrm{ dson}}$ is proposed to extract the average activation energy of device degradation. The average activation energy obtained is 2.25 eV at 50–200 V and 2.60 eV at 200–600 V. In addition, the dynamic $\text{R}_{\mathrm{ dson}}$ is accurately extracted by a unique extraction circuit with high switching frequency up to 1 MHz and high off-state voltage up to 600 V. Meanwhile, the switching times at turn-on and turn-off transitions are captured in a floating buck-boost converter, showing that the transconductance decreases with increasing drain voltage. Furthermore, the effect of parasitic output capacitor on the dynamic $\text{R}_{\mathrm{ dson}}$ is investigated by an experimental method. Finally, a proper hard operating mode is proposed to alleviate the influence of the trapping effect on the performance of switching power converters.

Analytical Switching Model of a 1200V SiC MOSFET in a High-Frequency Series Resonant Pulsed Power Converter for Plasma Generation

The circuit parasitic components have a significant effect on switching performance in the high-voltage and high-frequency pulsed power converter. The SiC MOSFET has better electrical characteristics than Si MOSFET and it is preferable in this application. In this paper, the characteristics of comparative study between Si and SiC MOSFET is provided first. Then, a converter-level analytical switching model for a 1200 V SiC MOSFET is proposed in a high-frequency series resonant pulsed power converter for the application of plasma generation. To increase the accuracy of predicting the behavior of the SiC MOSFET, the proposed analytical model involves all parasitic components in the converter, including the nonlinearity of the junction capacitances and transconductance, stray inductances from the package, printed circuit board (PCB) and transformer, parasitic capacitances from the transformer, and the capacitive load of plasma. The turn-on and turn-off transitions are analyzed in detail and the modeling mathematical equations are resolved by the MATLAB ode-45 command. The accuracy of this model is validated by comparing the analytical, LTspice simulated and experimental waveforms of a 400 VDC input, −8.2kV output prototype. The proposed model enables us to evaluate and optimize the switching solution of a SiC MOSFET for the pulsed power converter.

Junction temperature estimation of IGBT module via a bond wires lift‐off independent parameter V gE‐np

An electrical method for junction temperature estimation of insulated-gate bipolar transistors (IGBTs) is presented in this study. Owing to the parasitic inductance between bond wire and main emitter terminal L E . The temperature-dependent falling collector current during turn-off transition would cause a negative voltage drop in the gate-main emitter voltage waveform v gE . Therefore, this negative voltage drop V gE-np is proportional to the junction temperature. A double-pulse test circuit is developed to verify the accuracy and feasibility of the proposed method. The impacts of collector-emitter voltage V ce , collector current I c and bond-wires cut-off are also be discussed theoretically and experimentally. The experimental results show that the proposed V gE-np has a linear relationship with junction temperature as theoretical analysis and it is a bond-wires cut-off independent parameter in some special test point, which offers an effective way to estimate junction temperature without package destruction. The advantages of the proposed method include good linearity, bond-wires failure immunity and adequate sensitivity with junction temperature.

Effective measures for improving switching performance of SiC JFET bi‐directional switches

Silicon carbide (SiC)-based junction field-effect transistor (JFET) bi-directional switches (BDSs) have great potential in the construction of many power electronic circuits, including matrix converters, multi-level converters, solid-state breakers, and cycloconverters. However, the oscillations of BDSs during turn-on and turn-off transition have great impacts on the stability and reliability of power electronic systems. Two methods are proposed to improve the switching performance, namely using a snubber capacitor and using a ferrite ring. It is found that the use of a snubber capacitor mainly improves the turn-off transition, and the use of a ferrite ring not only damps the oscillation of current waveform during turn-on transition but also makes the waveforms cleaner and less noisy. Besides, both methods can cut down settling time significantly. The effects have been demonstrated by theoretical analysis and verified by experimental results.

A Novel Active Gate Driver for Improving SiC MOSFET Switching Trajectory

The trend in power electronic applications is to reach higher power density and higher efficiency. Currently, the wide band-gap devices such as silicon carbide MOSFET (SiC MOSFET) are of great interest because they can work at higher switching frequency with low losses. The increase of the switching speed in power devices leads to high power density systems. However, this can generate problems such as overshoots, oscillations, additional losses, and electromagnetic interference (EMI). In this paper, a novel active gate driver (AGD) for improving the SiC MOSFET switching trajectory with high performance is presented. The AGD is an open-loop control system and its principle is based on gate energy decrease with a gate resistance increment during the Miller plateau effect on gate–source voltage. The proposed AGD has been designed and validated through experimental tests for high-frequency operation. Moreover, an EMI discussion and a performance analysis were realized for the AGD. The results show that the AGD can reduce the overshoots, oscillations, and losses without compromising the EMI. In addition, the AGD can control the turn-on and turn-off transitions separately, and it is suitable for working with asymmetrical supplies required by SiC MOSFETs.

An Improved Zero-Current-Switching Single-Phase Transformerless PV H6 Inverter With Switching Loss-Free

In this paper, a switching loss-free (SLF) concept for the first six-switches H-bridge inverter (H6-I) topology is proposed. SLF means that its switches are able to operate with soft turn-on and turn-off transitions. In order to implement the SLF goal, a new resonance-trajectory is proposed. Compared with the zero-current-transition H6-I (ZCT-H6-I) topology published in previous literature, the proposed resonance-trajectory can precisely compensate for losses of resonant tanks every switching period. With this intention, an implementing circuit is structured based on the H6-I topology, and its detailed operation principle and performance characteristics are analyzed. As a result, all active switches of the new circuit are switched under zero-current turn-on and zero-current turn-off conditions. Also, the reverse recovery problem of freewheeling diodes is alleviated owing to the zero-current turn-off property of diodes. The SLF target is realized in theory. Finally, experimental results from a 1 kW prototype at 50 kHz switching frequency are provided to verify the effectiveness of the proposed SLF concept in practice. Specifically, the conversion efficiency of the new circuit is over 95% in a wide load range, and there is roughly a 1.5% efficiency improvement compared with the hard-switching H6-I topology.

A 1 MHz Half-Bridge Resonant DC/DC Converter Based on GaN FETs and Planar Magnetics

A 1 MHz half-bridge resonant dc/dc converter based on GaN FETs and planar magnetics is proposed in this paper, which improves the system efficiency and power density. The resonant network can achieve satisfactory soft-switching characteristics based on a small impedance angle, which greatly reduces the losses of switches and diodes. The losses characteristics during the turn-on and turn-off transitions are analyzed in detail. The calculation results show that the GaN FETs with low output capacitance and on resistance can achieve fast switching speed and low losses in high-frequency conditions. To reduce the profile and increase the power density of the system, planar magnetics are used in this paper. The response surface methodology (RSM) and modular layer model (MLM) are adopted to help design the planar inductor and transformer, respectively. Both of the methods offer clearer and more effective ways to design the planar magnetics. A 25-W prototype is built to verify the feasibility of the proposed high-frequency converter.

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.