High Frequency GaN Characterization and Design Considerations by Xiucheng Huang Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Electrical Engineering
Fred C. Lee, Chair
Qiang Li
Dong S. Ha
Jaime De La Ree
Alfred L. Wicks
Sep 6th, 2016
Blacksburg, Virginia
High Frequency GaN Characterization and Design Considerations
Xiucheng Huang
ABSTRACT
The future power conversion system not only must meet the characteristics demanded
by the load, but also have to achieve high power density with high efficiency, high ambient
temperature, and high reliability. Density and efficiency are two key drivers and metrics
for the advancement of power conversion technologies.
Generally speaking, a high performance active device is the first force to push power
density to meet the requirement of modern systems. Silicon has been a dominant material
in power management since the late 1950s. However, due to continuous device
optimizations and improvements in the production process, the material properties of
silicon have increasingly become the limiting factor. Workarounds like the super junction
stretch the limits but usually at substantial cost.
The use of gallium nitride devices is gathering momentum, with a number of recent
market introductions for a wide range of applications such as point-of-load (POL)
converters, off-line switching power supplies, battery chargers and motor drives. GaN
devices have a much lower gate charge and lower output capacitance than silicon
MOSFETs and, therefore, are capable of operating at a switching frequency 10 times
greater. This can significantly impact the power density of power converters, their form
factor, and even current design and manufacturing practices. To realize the benefits of GaN
devices resulting from significantly higher operating frequencies, a number of issues have
to be addressed, such as converter topology, soft-switching technique, high frequency gate
driver, high frequency magnetics, packaging, control, and thermal management.
This work studies the insight switching characteristics of high-voltage GaN devices
including some specific issues related to the cascode GaN. The package impact on the
switching performance and device reliability will be illustrated in details. A stack-die
package is proposed for cascode GaN devices to minimize the impact of package parasitic
inductance on switching transition. Comparison of hard-switching and soft-switching
operation is carried based on device model and experiments, which shows the necessity of
soft-switching for GaN devices at high frequency.
This work also addresses high dv/dt and di/dt related gate drive issues associated with
the higher switching speed of GaN devices. Particularly, the conventional driving solution
could fail on the high side switch in a half-bridge configuration due to relative large
common-mode noise current. Two simple and effective driving methods are proposed to
improve noise immunity and maintain high driving speed.
