GaN Switching Research Articles

Mathematical Model-Based Analysis and Mitigation of GaN Switching Oscillations

Mathematical Model-Based Analysis and Mitigation of GaN Switching Oscillations

A fully integrated half-bridge driver circuit in All-GaN GAN-IC technology

To unlock the full potential of fast switching GaN technology, monolithic integration of power circuit is crucial. GaN-IC ensures a fast efficient switching operation by reducing the parasitic inductance of interconnections significantly. In this paper, we demonstrate a monolithically integrated half-bridge power switch with its corresponding driving stage and the protection circuits that is realized by using our GAN-on-SOI technology. A low gate threshold voltage along with a low gate overdrive makes the driving of a GaN HEMT difficult. An optimized gate driver design is important to fully utilize a GaN power switch. This is also crucial for overall system efficiency, robustness, and EMI performance. Along with the design description this paper discusses the challenges and the design considerations of the GaN power IC.

A Reconfigurable Setup for the On-Wafer Characterization of the Dynamic RON of 600 V GaN Switches at Variable Operating Regimes

Charge-trapping mechanisms observed in high-voltage GaN switches are responsible for the degradation of power converter efficiency due to modulation of the effective dynamic ON-resistance (RON) with respect to its static value. Dynamic RON degradation is typically dependent on the blocking voltage and the commutation frequency and is particularly significant in new technologies under development. The possibility to characterize this phenomenon on GaN switch samples directly on-wafer, under controlled operating conditions that resemble real operations of the DUT in a switching mode power converter is extremely valuable in the development phase of new technologies or for quality verification of production wafers. In this paper, we describe a setup that allows this characterization: dynamic RON degradation of on-wafer 600 V GaN switches is characterized as a function of the VDS blocking voltage, the VGS driving voltage, and at different temperatures. The dependency on the switching frequency is identified by measuring the current recovery of the switch after the application of blocking voltages of different durations.

Characterization of the Dynamic RON of 600 V GaN Switches under Operating Conditions

High-voltage GaN switches can offer tremendous advantages over silicon counterparts for the development of high-efficiency switching-mode power converters at high commutation frequency. Nonetheless, GaN devices are prone to charge-trapping effects that can be particularly relevant in the early-stage development of new technologies. Charge-trapping mechanisms are responsible for the degradation of the dynamic ON-resistance (RON) with respect to its static value: this degradation is typically dependent on the blocking voltage, the commutation frequency and temperature, and is responsible for the reduction of power converter efficiency. The characterization of this phenomenon is very valuable for the development of a new process to compare different technological solutions or for the final assessment of performance. This characterization cannot be made with traditional static or small signal measurements since RON degradation is triggered by application-like dynamic device excitations. In this paper, we propose a technique for the characterization of the dynamic RON of high-voltage GaN switches under real operating conditions: this technique is based on the design of a half bridge switching leg in which the DUT is operated under conditions that resemble its operation in a power converter. With this setup, the characterization of a 600 V GaN switch dynamic RON is performed as a function of variable blocking voltages and commutation frequency. Additionally, this technique allows the separation of thermal and trapping effects, enabling the characterization of the dynamic RON at different temperatures.

A Wide Bandwidth GaN Switching Power Amplifier of Active Magnetic Bearing for a Flywheel Energy Storage System

A switching power amplifier (SPA) provides the driving current for an active magnetic bearing (AMB) to achieve magnetic suspension (MS) control. The performances of the current bandwidth and current ripple should be significantly considered in the SPA of the AMB. The SPA can obtain a wide bandwidth by increasing the dc-link voltage. However, a high dc-link voltage will cause a high current ripple. To achieve a wide bandwidth and low current ripple, a new GaN-device-based SPA for the AMB of a flywheel energy storage system (FESS) is proposed in this article. The GaN SPA can operate at a high switching frequency and high dc-link voltage. Thus, the GaN SPA can improve the performance of the current bandwidth and current ripple compared to the conventional SPA. According to the characteristics of the GaN SPA, a low-loss PWM method and an optimal dead time strategy is proposed to increase the efficiency of the SPA. Furthermore, a high-response control approach of dead-beat model predictive control is proposed to further improve the current bandwidth. The proposed GaN SPA is applied in a 500-kW MS FESS prototype. The effectiveness of the SPA is verified experimentally.

Sliding Mode Input Current Control of the Synchronous DC-DC Buck Converter for Electro-Mechanical Actuator Emulation in More Electric Aircrafts

The main challenges of the input current control in synchronous DC-DC buck converters are the nonlinear model of the system, changes of the operating point in a wide range, and the need to use an input LC filter for current smoothing, which may result in the instability of the closed-loop system. In this paper, a step-by-step approach is developed for the design and improvement of a PI-feedforward closed-loop controller. It is shown that a linear PI controller cannot stabilize the closed-loop system properly during wide changes in model parameters, e.g., an equivalent series resistance of the input filter. To cope with the stability issues, a fixed-frequency sliding mode controller (SMC) has been developed in this paper for the implementation of an electro-mechanical actuator (EMA) emulator. Moreover, a systematic approach is proposed for controller tuning and the selection of the SMC’s gains. To achieve high power efficiency, high-frequency GaN switches are used for the practical implementation of the DC-DC converter. Despite large changes in the load current, the designed nonlinear controller can track the input current reference satisfactorily. Steady-state and dynamic responses of the proposed SMC are compared with conventional linear controllers. Considering the Lyapunov stability theorem, it is proved that the designed SMC can stabilize the closed-loop system in the entire utilizable domain. The proposed nonlinear SMC controller enjoys a very simple control law. Hence, despite having very high switching and sampling frequencies, it can be easily implemented. The experimental response of the designed synchronous DC-DC buck converter is evaluated experimentally by implementing the control strategy in a TMS320F28335PGFA DSP from Texas Instrument. Moreover, the comprehensive comparison of the proposed SMC controller and a PI-feedforward controller proved the superior performance of the developed closed-loop system, in terms of the transient time response, robustness, and stability of the EMA emulator.

Optimal Design of High Frequency Transformer for High Power Density Flyback Converter

This brief presents an optimal design of high power frequency transformer for flyback converter based on E-mode GaN device, which could be used in personal portable chargers. The new generation of wide band gap semiconductor GaN switch is used to improve the switching frequency of the circuit. The high-frequency magnetic core and the winding method of magnetic components are optimized. The leakage inductance of the transformer of magnetic components is greatly reduced. Consequently, the overall power density of the circuit is improved. The feasibility of the design scheme is verified by hardware implementation and experimental results. The test results show that compared with the traditional flyback magnetic element design, the volume of magnetic element is reduced by 30%. Meanwhile, the loss is reduced by 30%, and the peak efficiency of the converter can reach 94.2%. The ratio of leakage inductance of magnetic transformer to excitation inductance decreased from 2.2% to 1%. In addition, power density of the converter reached 20.26W/in3, which is much higher than the existing commercial ones.

Dynamic high temperature operating life test methodology for long-term switching reliability of GaN power devices

Dynamic High Temperature Operating Life (DHTOL) test is becoming a mandatory test for GaN power devices as it explores the device reliability under application-close conditions. However, the number of applications and circuitries where GaN power devices are intended to be used poses a serious challenge in terms of validity of the application-like test setup used to evaluate the device degradation. In this paper, an accurate model supported by real DHTOL tests was developed precisely including test board and device properties with parasitic elements providing an advanced environment to support the evaluation of the GaN switching behavior and its stability. In order to evaluate and validate the current and voltage peaks, which define the switching stress conditions, we present the methodology to extract device, package, and board parasitics of the complete test setup. Furthermore, an accurate junction temperature (TJ) estimation and prediction technique is also presented being essential to judge and discriminate the natural self-heating effects from the one induced by the RDSON degradation during long-term repetitive hard-switching events.

Self-Reverse-Blocking Control of Dual-Gate Monolithic Bidirectional GaN Switch With Quasi-Ohmic on-State Characteristic

Converter topologies such as current-source rectifiers and inverters require switching devices with bipolar voltage blocking and unidirectional current conduction capability. Recently available dual-gate GaN monolithic bidirectional switches (MBDSs) can mimic such self-reverse-blocking (SRB) behavior if the MBDS’ two gates are controlled accordingly, but thus need twice the number of gate signals and gate drive circuits. Therefore, we propose cascode-diode control (i.e., without additional sensing or gate drive circuitry) of one MBDS gate using a cascode configuration with a low-voltage silicon Schottky diode. The resulting SRB-MBDS features quasi-ohmic conduction characteristic and single-gate control. We provide static and dynamic measurements of a discrete proof-of-concept realization (600-V, 190-m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{\Omega }$</tex-math></inline-formula> GaN MBDS; 40-V, 10-A silicon Schottky diode) that demonstrate the proposed SRB-MBDS’ feasibility.

A Link-Independent Hybrid Inductive and Capacitive Wireless Power Transfer System for Autonomous Mobility

A link-independent hybrid wireless power transfer system (HPT) that uses both capacitive and inductive action for autonomous vehicle charging application is presented in this article. A three-leg inverter topology is utilized here. Two legs of the inverter, switched at 85 kHz, are used for inductive wireless power transfer (IPT) and the third leg, switched at 1 MHz, is used for capacitive wireless power transfer (CPT). The three-leg inverter module is realized using SiC and GaN switches. The dual switching frequency operation of the converter enables independent compensation techniques to be adopted for IPT and CPT, bringing the modularity feature in the HPT. The circular inductive-link outer diameter is 460 mm and the parallel plate capacitive-link dimension is 600 mm × 600 mm. The power transfer distance or air gap is 100 mm between both links. The output power ratio of IPT to CPT is 2:1. The effectiveness of the proposed topology is validated experimentally. The advantages of this HPT system are modular-link architecture, improved misalignment performance, and distinct frequency of operation for each link that allows the inductive link to meet the SAEJ2954 standard frequency limits compared with the existing HPT systems proposed for electric vehicle charging to support autonomous mobility.

Equivalent circuit modeling in GaN wireless power transmission including stray inductance and capacitance by measuring radiated emission

We establish the method for estimating the stray elements of the GaN-WPT circuit by measuring the radiated emission around the GaN switching device. By controlling the circuit supply voltage, the spectrum peak shift due to the output capacitance of the GaN-HEMT is observed. It is found that these peak shift characteristics include the influence of both the stray wire inductance and stray capacitance. By fitting using the series resonance model, the value of the stray inductance and stray capacitance can be estimated in the non-destructive measurement in the GaN-WPT circuit.

Fully Integrated GaN-on-Silicon Gate Driver and GaN Switch With Temperature-Compensated Fast Turn-on Technique for Achieving Switching Frequency of 50 MHz and Slew Rate of 118.3 V/Ns

In this article, a monolithically integrated driver fabricated by 12-V depletion mode gallium nitride (dGaN) and enhanced mode GaN (eGaN) driver is proposed. The proposed driver features an internal temperature-compensated (T-compensated) controller to drive an integrated 650-V eGaN power switch. Due to T-compensated characteristics, a slew-rate enhancement driver can be well-controlled by the fast turn-on (FTO) technique which is supplied by an on-chip regulator with reference voltage circuit. Therefore, the Miller plateau voltage can be tracked correctly by the proposed controller so that the switching frequency can be raised up to 50 MHz and the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit {dV}_{\mathrm {DS}}$ </tex-math></inline-formula> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dt</i> slew rate can reach 118.3 V/ns for high efficiency and high switching operation.

Comparison Investigations on Unclamped-Inductive-Switching Behaviors of Power GaN Switching Devices

This article makes the comparisons on the behaviors of three types of commercial GaN power switching devices, including Schottky gate p-GaN high electron mobility transistor (HEMT), ohmic gate p-GaN HEMT with hybrid drain, and Cascode GaN device, under single-pulse and repetitive unclamp-inductive-switching (UIS) conditions by experiments and simulations. It shows that all the three types of GaN devices withstand the UIS stress by storing energy in parasitic capacitances rather than by avalanche process, which is a different phenomenon compared with traditional Si/SiC devices. However, the failure phenomena under single-pulse UIS condition are different. For the two types of p-GaN gate devices, the inverse-piezoelectric induced punch-through makes the burnout under drain contact region. For the Cascode device, the breakdown of the inner Si device dominates the failure. As for the behaviors under repetitive UIS stresses, p-GaN gate device with hybrid drain performs the best, Schottky gate p-GaN HEMT shows the most serious electrical performance degradations due to the trapping effects and carrier storage phenomena, Cascode GaN device exhibits stable threshold voltage and acceptable degradations of <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -state resistance due to the existence of inner Si device.

Analytical Model for Two-Dimensional Electron Gas Charge Density in Recessed-Gate GaN High-Electron-Mobility Transistors

A physics-based analytical model for GaN high-electron-mobility transistors (HEMTs) with non-recessed- and recessed-gate structure is presented. Based on this model, the two-dimensional electron gas density (2DEG) and thereby the on-state resistance and breakdown voltage can be controlled by varying the barrier layer thickness and Al mole fraction in non-recessed depletion-mode GaN HEMTs. The analytical model indicates that the 2DEG charge density in the channel increases from 2.4 × 1012 cm−2 to 1.8 × 1013 cm−2 when increasing the Al mole fraction from x = 0.1 to 0.4 for an experimental non-recessed-gate GaN HEMT. In the recessed-gate GaN HEMT, in addition to these parameters, the recess height can also control the 2DEG to achieve high-performance power electronic devices. The model also calculates the critical recess height for which a normally-ON GaN switch becomes normally-OFF. This model shows good agreement with reported experimental results and promises to become a useful tool for advanced design of GaN HEMTS.

A 3-Phase Resonant Switched-Capacitor Converter for Data Center 48-V Rack Power Distribution

Since the power consumption of data centers keeps increasing, a 48-V rack power distribution system replaces the conventional 12-V power bus to reduce the power delivery IR losses. Meanwhile, the 48 V needs conversion into 1 V or lower at the point-of-load for microprocessors. A 48V-to-12V DC-DC converter can serve as a first stage between the huge gap of 48V-to-1V, in a two-stage voltage regulator module (VRM). Thus, this paper presents a 3-phase resonant switched-capacitor ( 3Φ-ReSC) converter as the first stage of the VRM. Different from the reported solutions, 3-phase resonant operation reduces the current stress across each small-size GaN switch, and thus improves the power conversion efficiency. In addition, we also present the theoretical analysis and design procedures for the 3Φ-ReSC converter. We demonstrated the proposed 3Φ-ReSC converter with a 98% peak power efficiency at an output current of about 1 A, with a maximum output current of 10 A.

Design and Analysis of Single-Inductor Power Converter for Both Battery Balancing and Voltage Regulation

In this article, a highly efficient battery-balancing circuit with output voltage regulation for mobile applications is proposed and analyzed. The proposed circuit performs a cell-balancing operation for series-connected multiple-battery cells and voltage regulation operation simultaneously without additional power conversion stages. Therefore, it can be integrated into a battery pack with maximized power density and efficiency. These features are essential for recent low-power applications with multiple batteries, such as wireless/smart speakers, drones, and electronic point-of-sale, that require high power density. The operational principle, electrical modeling, and tradeoff relationship between the system power losses and balancing characteristics are derived, and the optimal design candidates of the proposed topology are discussed in terms of Pareto optimization. The effectiveness of the proposed system is verified with a prototype board using GaN switches, and 96.05% efficiency is achieved with a 1.8 V, 0.5 A load.

A Multistage Resonant DC–DC Converter for Step-Up Applications

Resonant switched capacitor converters are a viable option for high-efficiency dc-dc converters. This article proposes a generic multistage resonant switched capacitor converter (MSR-SCC) for step-up applications. This article studies the performance of the MSR-SCC using a systematic approach both above and below the resonant frequency, which results in zero-voltage switching and zero-current switching, respectively. It also proposes a control algorithm to regulate the output voltage of the converter in both operating conditions. To validate the proposed analysis and control algorithms, an MSR-SCC is simulated and experimentally implemented with GaN switching devices, which enable converter operation at lower conduction losses and higher switching frequencies.

Identification of ZVS Points and Bounded Low-Loss Operating Regions in a Class-D Resonant Converter

This article presents an analysis of the different loss modes of the switching devices in a class-D series resonant converter operating with either fixed dead time or fixed duty cycle. A feasible operating region where the FETs in the inverter stage only exhibit reverse conduction losses, with no hard-switching, is identified. Furthermore, the impact of using a fixed dead time versus a fixed duty cycle is investigated. We find that using a fixed dead time is superior to using a fixed duty cycle, as a broader operating range can be achieved for the same losses, or the same operating range can be achieved with lower losses. A reduction in the reverse conduction losses of up to 59% or an expansion of the operational frequency range by 33% when using a fixed dead time is found. The modeling approach is validated on a 1-MHz prototype employing GaN switching devices. Lastly, a design example shows how the presented analysis can be used to determine the optimal fixed dead time/duty cycle for use with frequency modulation control such that the losses are minimized.

Analysis of a GaN based PWM AC-AC converter with an improved switch loss model

AC-AC converters have a wide range of use in industrial applications mostly with installed silicon switches. In last decade, GaN’s are served to market with their high efficiency and fast on-off times, hence implementation of GaN’s to any conventional power electronic circuit is a focus of interest. In this paper, effects of both GaN and MOS switches on efficiency of an AC-AC converter were analysed both numerically and practically. A new extended mathematical loss model proposed by the authors was used to calculate the switching-conduction losses. Laboratory tests were performed with same physical circuit for both switches having the same package. Calculated and measured efficiencies and loss results which are in great harmony proved that the proposed mathematical loss model is extraordinarily realistic. All-in-all, it was observed that GaN and MOSFET switches have both high efficiencies (greater than 96.5%) at low switching frequencies, even though they operate at low power levels. With increasing switching frequency, GaN’s efficiency is slightly reduced (less than 0.5%) while that of MOSFET decreased dramatically almost by 3%. Besides, it was seen that at low frequencies, reverse diode has a significant effect on efficiency of a fast switch, although switching-conduction losses are extremely low.

An RF Approach to Modelling Gallium Nitride Power Devices Using Parasitic Extraction

This paper begins with a comprehensive review into the existing GaN device models. Secondly, it identifies the need for a more accurate GaN switching model. A simple practical process based on radio frequency techniques using Vector Network Analyser is introduced in this paper as an original contribution. It was applied to extract the impedances of the GaN device to develop an efficient behavioural model. The switching behaviour of the model was validated using both simulation and real time double pulse test experiments at 500 V, 15 A conditions. The proposed model is much easier for power designers to handle, without the need for knowledge about the physics or geometry of the device. The proposed model for Transphorm GaN HEMT was found to be 95.2% more accurate when compared to the existing LT-Spice manufacturer model. This work additionally highlights the need to adopt established RF techniques into power electronics to reduce the learning curve while dealing with these novel high-speed switching devices.