Weijing Du

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.

High-Frequency High-Efficiency GaN-Based Interleaved CRM Bidirectional Buck/Boost Converter with Inverse Coupled Inductor

This paper presents a high-frequency high-efficiency GaN device-based interleaved critical current mode (CRM) bidirectional buck/boost converter with an inverse coupled inductor. The switching frequency is continually driven to megahertz range with GaN devices due to their small switching loss and driving loss, which greatly reduces the size of the passive components. The coupled inductor further reduces the core volume due to certain dc flux reductions. The equivalent inductance and the impact of the inverse coupled inductor on the CRM buck-boost converter are analyzed in detail. The resonant period in CRM is less with an inverse coupled inductor than with a noncoupled inductor, which is beneficial for the high-frequency operation. The soft-switching range and the circulating energy are both improved using an inverse coupled inductor in CRM. Experimental results validate the theoretical analysis, and the coupled inductor prototype efficiency is 98.5% at 1 MHz, which is 0.3% higher than a prototype with a noncoupled inductor.

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.

Thermal analysis and improvement of cascode GaN HEMT in stack-die structure

The high-voltage cascode gallium nitride (GaN) high-electron-mobility transistor (HEMT) enables high-frequency and high-efficiency power conversion. The parasitic inductances induced by traditional packages of the cascode GaN HEMT device significantly deteriorate the device switching performance. A new stack-die packaging structure has been introduced and proved to be efficient in reducing package related turn-on loss and turn-off parasitic ringing. However, the thermal dissipation of the device packaged in such structure becomes the limitation for further pushing the operating frequency and the output current level for high-efficiency power conversion. This paper focuses on the analysis of thermal performance of the cascode GaN HEMT device in different packages used in a high-frequency power converter. There exist several possible approaches for improving thermal performance of the device. Nevertheless, it is not convenient and cost-effective to put every method into practice. A thermal model has been built based on the actual structures and materials of the packaged device and the assembled converter in order to evaluate the effectiveness of different thermal performance improving methods. The simulation results of the improved design demonstrate the possibility of further increasing the switching frequency for the converter while maintaining the temperature of the device under 125°C. Finally, the corresponding experiments have been conducted to validate the simulation results using the fabricated stack-die devices. The presented simulation method and result successfully provide a guidance of thermal design improvement for the high-voltage cascode GaN HEMT device in the stack-die structure.

Avoiding Divergent Oscillation of a Cascode GaN Device Under High-Current Turn-Off Condition

Cascode structure is widely used for high voltage normally-on GaN devices. However, the capacitance mismatch between the high voltage GaN device and the low voltage normally-off Si MOSFET may induce several undesired features, such as Si MOSFET reaches avalanche during turn-off, and high voltage GaN device loses ZVS turn-on condition internally during soft-switching turn-on process in every switching cycle. This paper presents another issue associated with the capacitance mismatch in the cascode GaN devices. Divergent oscillation could occur at high current turn-off condition, and eventually destroy the device. The intrinsic reason of this phenomenon is analyzed in detail in this paper. A simple solution is proposed by adding an additional capacitor whose position is critical and should be optimized. Experimental results validate the theoretical analysis, and show that the proposed method improves device performance significantly under high current turn-off condition.

Design consideration of MHz active clamp flyback converter with GaN devices for low power adapter application

With ever increasing demands of smaller size, lighter weight for all forms of consumer electronics, efficient power conversion with higher operating frequency has always being pursued rigorously. This paper demonstrates high frequency, high efficiency and high power density design of active clamp flyback converter for adapter application. Both the primary and secondary switches are gallium nitride (GaN) devices which can significantly reduce the device related conduction and switching loss. The design procedures, including the selection of active clamping capacitor, optimization of flyback transformer, and EMI filter design, are presented in detail. A 65W (19.5V/3.3A) prototype of active clamp flyback front end converter is developed to verify the feasibility of the system design. The prototype efficiency is 1~2% higher than the state of art product and the power density (exclude case) is more than 40W/in3.

Avoiding Si MOSFET avalanche and achieving true zero-voltage-switching for cascode devices

The cascode structure is widely used for high voltage normally-on wide-band-gap 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 devices. The capacitance mismatch between high voltage normally-on devices and the low voltage Si MOSFET causes the Si MOSFET to avalanche, and internal high voltage devices 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 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 600V cascode GaN device. The experiment shows that the proposed method improves the 600V cascode GaN devices performance significantly in high frequency applications.

A novel driving scheme for synchronous rectifier in MHz CRM flyback converter with GaN devices

Synchronous rectifier (SR) is widely used in flyback converter to reduce the output side conduction loss in order to meet the system conversion efficiency requirement. The conventional SR driving methods are not suitable for high frequency (>500kHz) application. This paper proposes a novel SR driving method for MHz flyback converter operating at critical conduction mode (CRM). A RC network is in parallel with SR to emulate the magnetizing current of flyback converter. The sensing signal is clean and immune to the parasitic ringing caused by the leakage inductance and parasitic capacitors. SR gate signal is accurately generated based on a simple signal processing circuit and the conduction loss of SR body diode is minimized. The theoretical analysis is validated by simulation and experiment.

MHz GaN-based interleaved CRM bi-directional buck/boost converter with coupled inductor

This paper presents a high frequency high efficiency GaN device based interleaved critical current mode (CRM) bi-directional buck/boost converter with a coupled inductor. The switching frequency is continually driven to MHz range with GaN devices due to their small switching loss and driving loss, which greatly reduce the size of the passive components. The coupled inductor further reduces the core volume due to certain DC flux reductions. The equivalent inductance and the impact of the coupled inductor on the CRM buck-boost converter are analyzed in detail. The resonant period at CRM is less with a coupled inductor than with a non-coupled inductor, which is beneficial for high frequency operation. The soft-switching range and the circulating energy are both improved using a coupled inductor at CRM. Experimental results validate the theoretical analysis, and the coupled inductor prototype efficiency is 98.5% at 1MHz, which is 0.3% higher than a prototype with a non-coupled inductor.

Avoiding divergent oscillation of cascode GaN device under high current turn-off condition

The cascode structure is widely used for high-voltage normally-on GaN devices. However, the capacitance mismatch between the high-voltage GaN device and the low-voltage normally-off Si MOSFET may induce several undesired features, such as Si MOSFET reaches avalanche during turn-off and the high-voltage GaN device loses zero-voltage switching turn-on condition internally during a soft-switching turn-on process in every switching cycle. This paper presents another issue associated with the capacitance mismatch in the cascode GaN devices. Divergent oscillation could occur at high-current turn-off condition and, eventually, destroys the device. The intrinsic reason of this phenomenon is analyzed in detail in this paper. A simple solution is proposed by adding an additional capacitor whose position is critical and should be optimized. Experimental results validate the theoretical analysis and show that the proposed method improves device performance significantly under high-current turn-off condition.