Yuanzhe Zhang

High-Frequency PWM Buck Converters Using GaN-on-SiC HEMTs

GaN high electron mobility transistors (HEMTs) are well suited for high-frequency operation due to their lower on resistance and device capacitance compared with traditional silicon devices. When grown on silicon carbide, GaN HEMTs can also achieve very high power density due to the enhanced power handling capabilities of the substrate. As a result, GaN-on-SiC HEMTs are increasingly popular in radio-frequency power amplifiers, and applications as switches in high-frequency power electronics are of high interest. This paper explores the use of GaN-on-SiC HEMTs in conventional pulse-width modulated switched-mode power converters targeting switching frequencies in the tens of megahertz range. Device sizing and efficiency limits of this technology are analyzed, and design principles and guidelines are given to exploit the capabilities of the devices. The results are presented for discrete-device and integrated implementations of a synchronous Buck converter, providing more than 10-W output power supplied from up to 40 V with efficiencies greater than 95% when operated at 10 MHz, and greater than 90% at switching frequencies up to 40 MHz. As a practical application of this technology, the converter is used to accurately track a 3-MHz bandwidth communication envelope signal with 92% efficiency.

100 MHz, 20 V, 90% efficient synchronous buck converter with integrated gate driver

This paper describes a synchronous buck converter based on a GaN-on-SiC integrated circuit, which includes a halfbridge power stage, as well as a modified active pull-up gate driver stage. The integrated modified active pull-up driver takes advantage of depletion-mode device characteristics to achieve fast switching with low power consumption. Design principles and results are presented for a synchronous buck converter prototype operating at 100 MHz switching frequency, delivering up to 7 W from 20 V input voltage. Measured power-stage efficiency peaks above 91%, and remains above 85% over a wide range of operating conditions. Experimental results show that the converter has the ability to accurately track a 20 MHz bandwidth LTE envelope signal with 83.7% efficiency.

High-frequency integrated gate drivers for half-bridge GaN power stage

This paper describes three driver options for integrated half-bridge power stage using depletion-mode GaN-on-SiC 0.15μm RF process: an active pull-up driver, a bootstrapped driver, and a modified active pull-up driver. The approaches are evaluated and compared in 5 W, 20 V synchronous Buck converter prototypes operating at 100 MHz switching frequency over a wide range of operating points. Measured efficiency peaks above 91% for the designs using the bootstrap and the modified active pull-up integrated drivers.

Very High Frequency PWM Buck Converters Using Monolithic GaN Half-Bridge Power Stages With Integrated Gate Drivers

Integration is a key step in utilizing advances in GaN technologies and enabling efficient switched-mode power conversion at very high frequencies (VHF). This paper addresses design and implementation of monolithic GaN half-bridge power stages with integrated gate drivers optimized for pulsewidth-modulated (PWM) dc-dc converters operating at 100 MHz switching frequency. Three gate-driver circuit topologies are considered for integration with half-bridge power stages in a 0.15-μm depletion-mode GaN-on-SiC process: an active pull-up driver, a bootstrapped driver, and a novel modified active pull-up driver. An analytical loss model is developed and used to optimize the monolithic GaN chips, which are then used to construct 20 V, 5 W, 100 MHz synchronous buck converter prototypes. With the bootstrapped and the modified pull-up gate-driver circuits, power stage efficiencies above 91% and total efficiencies close to 88% are demonstrated. The modified active pull-up driver, which offers 80% reduction in the driver area, is found to be the best-performing approach in the depletion-mode GaN process. These results demonstrate feasibility of high-efficiency VHF PWM dc-dc converters based on high levels of integration in GaN processes.

High efficiency 20–400 MHz PWM converters using air-core inductors and monolithic power stages in a normally-off GaN process

This paper presents high efficiency dc-dc converters based on monolithic normally-off GaN half-bridge power stages with integrated gate drivers. A new gate driver circuitry is introduced, which enhances both the power stage efficiency and the converter overall efficiency. While using only n-type transistors in the GaN process, the proposed gate driver maintains low quiescent power consumption by emulating the complementary operation commonly employed in CMOS processes. Level shifting is accomplished using a bootstrap technique, with the bootstrap capacitor and the bootstrap diode integrated on the same chip. A family of monolithic GaN chips has been designed, targeting operation from up to 45 V, delivering up to 16 W of output power, and operating at 20-400 MHz switching frequencies. The GaN chips are verified in synchronous buck converters, demonstrating record peak power stage efficiencies of 95.0% at 20 MHz, 94.2% at 50 MHz, 93.2% at 100 MHz, 86.5% at 200 MHz, and 72.5% at 400 MHz.

Envelope tracking GaN power supply for 4G cell phone base stations

This paper introduces an envelope tracking (ET) power supply for 4G cell phone base stations using EPC eGaN® FETs. An analytical loss model is developed for design optimization and verified by a single phase synchronous buck converter using the zero-voltage switching (ZVS) technique. The model is then extended to four phases and is used to design a 60 W ET power supply with 20 MHz large signal bandwidth (BW). At 25 MHz per-phase switching frequency, measured static power stage efficiency peaks at 96.5% with 68 W output power delivered from 30 V. Experimental results demonstrate accurate tracking of 20 MHz 7 dB Peak-to-Average Power Ratio (PAPR) LTE envelope with 92.3% total efficiency, delivering 67 W average power from 30 V.

Output filter design in high-efficiency wide-bandwidth multi-phase buck envelope amplifiers

Supply modulation is a system efficiency improvement technique in radio-frequency (RF) transmitters, where a power converter operating as an envelope tracking (ET) amplifier serves as the power supply for an RF power amplifier. This paper presents design of higher-order output filters in multi-phase buck ET amplifiers to achieve wide-bandwidth tracking with high efficiency using zero-voltage switching (ZVS) technique. The approach is verified by experiments on two-phase GaN-based ET prototypes operating at 10 MHz and 50 MHz per-phase switching frequency, and having fourth-order filters with 5 MHz and 20 MHz tracking bandwidth, respectively. Experimental results demonstrate accurate tracking of 2.4 MHz and 20 MHz bandwidth LTE envelope signals with 93.2% and 89.7% power-stage efficiency, respectively.

Voltage regulation and efficiency optimization in a 100 MHz series resonant DC-DC converter

This paper describes a closed-loop regulated dual half-bridge series resonant dc-dc converter operated at up to 100 MHz switching frequency using GaN half-bridge stages with integrated gate drivers. A digital online efficiency optimization technique together with burst-mode control are developed and applied, to achieve output voltage regulation and efficiency improvements. An experimental prototype is designed and constructed to convert an input de voltage of 20 V to a 12 V output at 100 MHz switching frequency and up to 1.75 W of output power, with up to 82 % efficiency.

High frequency synchronous Buck converter using GaN-on-SiC HEMTs

GaN-on-SiC technology has been widely used in radiofrequency (RF) power amplifiers to achieve high power density at very high frequencies due to the enhanced power handling capabilities provided by the SiC substrate and very high transition frequencies of GaN High Electron Mobility Transistors (HEMTs). Along with other well-known advantages of GaN, such as high breakdown voltage and high temperature operation, these characteristics make GaN-on-SiC technology suitable for power electronics applications. This paper explores the use of GaN-on-SiC depletion-mode HEMTs in switched-mode power converters, targeting high efficiencies at switching frequencies in the tens of Megahertz range. The fundamental limits of the technology are analyzed, and design principles and guidelines are given to exploit the capabilities of the devices. Results are presented for a 10 W, 20 V synchronous Buck converter prototype operating at 10 to 40 MHz. Measured efficiency peaks above 96% at 10 MHz and remains above 90% over a wide range of operating conditions. To demonstrate the applicability of the technology, the Buck converter switching at 20 MHz is used as a supply modulator to track a 3 MHz envelope signal with greater than 90% average efficiency.

100 MHz isolated DC-DC resonant converter using spiral planar PCB transformer

This paper describes a very high frequency (VHF) isolated dc-dc resonant converter operating at 100 MHz switching frequency with a rated output power of 2 W. The power stage consists of two half-bridge GaN chips with integrated gate drivers, one operating as a dc-ac inverter on the primary side, the other operating as an ac-dc synchronous rectifier on the secondary side. Isolation is provided by a spiral planar printed circuit board (PCB) transformer designed and optimized using 3D finite element modeling (FEM) tools and radio-frequency (RF) circuit design software. Simulation and experimental results are presented for a prototype step-down converter operating from 20 V input voltage. Experimental results show a peak efficiency of 80.5% at 1.8 W output power.