Zhengyang Liu

Evaluation and Application of 600 V GaN HEMT in Cascode Structure

Gallium nitride high electron mobility transistor (GaN HEMT) has matured dramatically over the last few years. A progressively larger number of GaN devices have been manufactured for in field applications ranging from low power voltage regulators to high power infrastructure base-stations. Compared to the state-of-the-art silicon MOSFET, GaN HEMT has a much better figure of merit and shows potential for high-frequency applications. The first generation of 600 V GaN HEMT is intrinsically normally on device. To easily apply normally on GaN HEMT in circuit design, a low-voltage silicon MOSFET is in series to drive the GaN HEMT, which is well known as cascode structure. This paper studies the characteristics and operation principles of a 600 V cascode GaN HEMT. Evaluations of the cascode GaN HEMT performance based on buck converter at hard-switching and soft-switching conditions are presented in detail. Experimental results prove that the cascode GaN HEMT is superior to the silicon MOSFET, but it still needs soft-switching in high-frequency operation due to considerable package and layout parasitic inductors and capacitors. The cascode GaN HEMT is then applied to a 1 MHz 300 W 400 V/12 V LLC converter. A comparison of experimental results with a state-of-the-art silicon MOSFET is provided to validate the advantages of the GaN HEMT.

Package Parasitic Inductance Extraction and Simulation Model Development for the High-Voltage Cascode GaN HEMT

This paper presents the development of a simulation model for high-voltage gallium nitride (GaN) high-electron-mobility transistors (HEMT) in a cascode structure. A method is proposed to accurately extract the device package parasitic inductance, which is of vital importance to better predict the high-frequency switching performance of the device. The simulation model is verified by a double-pulse tester, and the results match well both in terms of device switching waveform and switching energy. Based on the simulation model, an investigation of the package influence on the cascode GaN HEMT is presented, and several critical parasitic inductances are identified and verified. Finally, a detailed loss breakdown is made for a buck converter, including a comparison between hard switching and soft switching. The results indicate that the switching loss is a dominant part of the total loss under hard-switching conditions in megahertz high-frequency range and below 8~10 A operation current; therefore, soft switching is preferred to achieve high-frequency and high-efficiency operation of the high-voltage GaN HEMT.

Analytical Loss Model of High Voltage GaN HEMT in Cascode Configuration

This paper presents an accurate analytical model to calculate the power loss of a high voltage Gallium Nitride high electron mobility transistor (GaN HEMT) in cascode configuration. The proposed model considers the package and PCB parasitic inductances, the nonlinearity of the junction capacitors and the transconductance of the cascode GaN transistor. The switching process is illustrated in detail, including the interaction of the low voltage Si MOSFET and high voltage GaN HEMT in cascode configuration. The switching loss is obtained by solving the equivalent circuits during the switching transition. The analytical results show that the turn on loss dominates in hard-switching conditions while the turn off loss is negligible, due to the intrinsic current source driving mechanism. The accuracy of the proposed model is validated by numerous experimental results. The results of both the analytical model and experiments suggest that soft-switching is critical for high voltage GaN in high frequency high efficiency applications.

Evaluation and application of 600V GaN HEMT in cascode structure

Gallium nitride high electron mobility transistor (GaN HEMT) has matured dramatically over the last few years. More and more devices have been manufactured and field in applications ranging from low power voltage regulator to high power infrastructure base-stations. Compared to the state of art silicon MOSFET, GaN HEMT has much better figure of merit and is potential for high frequency application. In general, 600V GaN HEMT is intrinsically normally-on device. To easily apply depletion mode GaN HEMT in circuit design, a low voltage silicon MOSFET is in series to drive the GaN HEMT, which is well known as cascode structure. This paper studies the characteristics and operation principles of 600V cascode GaN HEMT. Evaluations of GaN HEMT performance based on Buck converter under hard-switching and soft-switching conditions are presented. Experimental results illustrate that GaN HEMT is superior than silicon MOSFET but still needs soft-switching in high frequency operation due to considerable package and layout parasitic inductors and capacitors. Then GaN HEMT is applied to a 1MHz 300W 400V/12V LLC converter. Comparison of experimental results with state of art silicon MOSFET is provided to validate the advantages of GaN HEMT.

Design and evaluation of GaN-based dual-phase interleaved MHz critical mode PFC converter

This paper presents the design consideration and performance evaluation of gallium nitride (GaN) high electron mobility transistor (HEMT) based dual-phase interleaved MHz critical conduction mode (CRM) power factor correction (PFC) converter. A 1.2kW 1-3MHz interleaved boost PFC converter prototype is built with 97.9% peak efficiency and 120W/in3 power density. The significant impact of MHz frequency is demonstrated as dramatically size reduction of boost inductor and electro-magnetic interference (EMI) filter. Several inductor designs are discussed. The corner frequency of EMI filter is pushed to several hundreds of kHz. Finally, the limitation of conventional boost PFC converter is discussed as high conduction loss on diode rectifier bridge and high switching loss caused by valley switching, which is negligible in other low frequency PFC converter but significant in MHz PFC converter. The totem-pole bridgeless PFC converter is introduced to further improve the efficiency with no rectifier bridge and zero-voltage switching (ZVS) extension strategy.

Avoiding Si MOSFET Avalanche and Achieving Zero-Voltage Switching for Cascode GaN Devices

The cascode structure is widely used for high-voltage normally-on wide-bandgap devices. However, the interaction between the high-voltage normally-on device and the low-voltage normally-off Si MOSFET may induce undesired features. This paper analyzes the voltage distribution principle during the turn-off transition as well as the zero-voltage-switching (ZVS) principle during the turn-on transition for cascode GaN devices. The capacitance mismatch between high-voltage normally-on GaN switch and the low-voltage Si MOSFET causes the Si MOSFET to avalanche, and internal high-voltage GaN switch lose the ZVS condition. This issue must be solved in consideration of both power loss and reliability. A simple and effective solution is proposed by adding an extra capacitor to compensate the capacitance mismatch, thereby avoiding Si MOSFET avalanche and achieving true ZVS for cascode GaN devices. The benefits and small penalty of this solution are analyzed in detail. The theoretical analysis is validated by experiments, which are implemented based on a 600-V cascode GaN device. The experiment shows that the proposed method improves the 600-V cascode GaN devices performance significantly in high-frequency applications. The analysis and proposed solution are also applicable to other cascode devices.

Characterization and Enhancement of High-Voltage Cascode GaN Devices

Gallium nitride (GaN) devices are gathering momentum, with a number of recent market introductions for a wide range of applications such as point-of-load converters, OFF-line switching power supplies, battery chargers, and motor drives. This paper studies the basic characteristics of a 600 V cascode GaN switch, such as voltage distribution during the turn-ON and turn-OFF transition. The switching loss mechanism of the cascode GaN switch is analyzed in detail, including the impact of the package parasitic inductance in both hard- and soft-switching modes. A soft-switching 5 MHz boost converter is developed and shows the advantages and the potential of the cascode GaN.

Evaluation of high-voltage cascode GaN HEMT in different packages

This paper presents the evaluation of high-voltage cascode gallium nitride (GaN) high-electron-mobility transistors (HEMT) in different packages. The high-voltage cascode GaN HEMT in traditional package has high turn-on loss in hard-switching turn-on condition, and severe internal parasitic ringing, which could possibly damage the gate of GaN HEMT, in hard-switching turn-off condition, due to package parasitics. To solve these problems a stack-die package is introduced, which is able to eliminate all the critical common-source inductors in traditional package, avoiding side effects caused by the package, and thus could be more suitable for MHz high frequency operation. A prototype of this stack-die package is fabricated in the lab, experimental results are shown to verify the analysis and to demonstrate the strength of the stack-die package.

Design of GaN-based MHz totem-pole PFC rectifier

Totem-pole bridgeless power factor correction (PFC) rectifier is recently recognized as a promising front end candidate for applications like server and telecommunication power supply. In this paper the advantage, enabled by emerging high-voltage gallium-nitride (GaN) devices, of totem-pole PFC rectifier comparing to traditional PFC rectifier is discussed in the beginning. Critical mode operation is used by the totem-pole PFC rectifier in order to achieve both high frequency and high efficiency. Then several high frequency issues and detailed design considerations are introduced including zero-voltage-switching (ZVS) extension for entire line-cycle ZVS operation; variable on-time strategy for zero-crossing distortion suppression; and interleaving control for ripple current cancellation. The volume reduction of differential-mode (DM) electro-magnetic interference (EMI) filter significantly benefited from MHz high frequency operation and multi-phase interleaving is also presented. Finally, a 1.2kW GaN-based MHz totem-pole PFC rectifier is demonstrated with 99% peak efficiency and 200W/in3 power density.

A New Package of High-Voltage Cascode Gallium Nitride Device for Megahertz Operation

Lateral gallium nitride (GaN)-based high-electron-mobility transistor (HEMT) power devices have high current density, high switching speed, and low on-resistance in comparison to the established silicon (Si)-based semiconductor devices. These superior characteristics make GaN HEMTs ideal for high-frequency, high-efficiency power conversion. Using efficient GaN HEMT devices switched at high frequency in power electronic systems could lead to an increase in power density as well as a reduction in the weight, size, and cost of the system. However, conventional packaging configurations often compromise the benefits provided by high-performance GaN HEMT devices, for example, by increasing the parasitic inductance and resistance in the current loops of the device. This undesirable package-induced performance degradation is prominent in the cascode GaN device, where the combination of a high-voltage depletion-mode GaN semiconductor and low-voltage enhancement-mode Si semiconductor is needed. In this study, a new package is introduced for high-voltage cascode GaN devices and is successfully demonstrated to make the device more suitable for megahertz operation. This packaging prototype for cascode GaN devices is fabricated in a power quad flat no-lead format with the new features of a stack-die structure, embedded external capacitor, and flip-chip configuration. The parasitic ringing in hard-switching turn-off and switching losses in soft-switching transitions are both effectively reduced for this newly packaged device compared with a traditional package using the same GaN and Si devices. Improved thermal dissipation capability is also realized using this new package for better reliability.