Due to its remarkably high Johnson figure of merit, gallium nitride (GaN) has become the material of choice for applications requiring high output power at high frequencies. In this paper, we have reviewed high-speed switching work to date and we demonstrate a 28 V, 865-MHz switching speed step-down converter that can be applied to envelope tracking (ET). We report a GaN monolithic microwave integrated circuit technology utilized to fabricate a gate driver IC at a switching speed > 1 GHz with <;95-ps transitions, monolithically integrated with a 100 V breakdown GaN power switch. The on-chip integration of the power switch and gate driver allows in-circuit characterization of the power switch switching speed. Using the integrated gate driver and switch, we report a slew rate of the power switch of 152 V/ns at 50 V, which enables a down converter IC capable of ET with over 50-MHz tracking bandwidth. This paper shows the first demonstration of a GaN low-side gate driver capable of > 1-GHz switching, monolithic integration with a 0.15-μm T-gate suitable for up to Ka-band operation, and a > 100 V breakdown voltage power switch on the same chip.

Envelope tracking (ET) is an appealing alternative to the widely used Doherty power amplifier (PA) due to its potential to increase efficiency, particularly for high data rate transmissions. In this work, we demonstrate a 28 V, 865 MHz switching speed step-down converter that can be applied to envelope tracking. The converter's ET capability is demonstrated at up to 865 MHz PWM carrier frequency. An integrated RF GaN HEMT and 100 V GaN power switch process is reported, which enables fabrication of a power switch and gate driver on the same IC, necessary for achieving the 865 MHz switching speed. We report a slew rate of the main power switch of 152 V/ns at 50 V, which enables ET with up to 20% less power than that required without ET. To the authors' best knowledge, this is the first demonstration of a GaN MMIC process that integrates a GaN HEMT suitable for X through Ka-band power amplifiers and a high voltage power switch on the same IC.

Thermal management of power electronics is an extremely challenging problem in the harsh environment of military hybrid vehicles, where the local air and liquid coolant's temperature exceed 100 degrees C under regular operating conditions. This paper presents the development work of a high heat flux, jet impingement-cooled heat exchanger for a 600-V/50-A silicon carbide (SiC) power module (rated at 175 degrees C device junction temperature), used for bidirectional power conversion between a 28-V battery and a 300-V dc bus. A total of 50 volume% mixture of water-ethylene glycol (WEG) coolant at 100 degrees C inlet temperature is the only available coolant. An array of WEG coolant microjets impinges on the base plate of the SiC module. The jet impingement cooling system has been optimized by experimental studies on a surrogate module, along with a high-fidelity computational model, to accurately estimate the SiC device junction temperature in relevant operating conditions. Results indicate that at the design heat load of 151 W (worst-case scenario), the SiC device junction temperature is reduced from 290 degrees C with commercial-off-the-shelf (COTS) cold plate cooling and 215 degrees C with COTS microchannel heat exchanger cooling, to 169 degrees C with a jet impingement-cooled heat exchanger, using the same flow rate.

De-rating of power electronics is a common practice in harsh environment operating conditions. Advanced cooling solutions are needed to overcome the thermal limitations and prevent the de-rating of power components. This study investigates a silicon carbide power module for 28 V DC to 300 V DC power conversion (or reverse) in military hybrid vehicle applications. A combined experimental and numerical modeling effort is reported, to accurately predict the silicon carbide switch (device) junction temperature. The results show that jet impingement cooling at the module base plate increases the module heat dissipation capability by 2.5X and 1.5X, compared to the commercial-off-the-shelf cold plate cooling and micro-channel cooling solutions respectively.

A high-temperature, SiC JFET power module was designed for use in a bi-directional, single-stage DC-DC converter. The converter is the interface between a 28 V battery and a 300 V bus, and is required to operate in a harsh environment with an ambient temperature of 120 °C and available 100 °C water-ethylene glycol (WEG) coolant. The power module consists of two JFETs in a single pole configuration with SiC Schottky diodes used as freewheeling diodes. The power module has been subjected to standard characterization tests and also extended (tens of hours) operational tests to confirm its suitability for the application. To the best of our knowledge this is the first demonstration of long term reliability of high temperature (175 °C device junction) SiC power electronics power conversion, with tens of hours of operation well past its infant mortality period.

An electrical and thermal optimization study is conducted for a high-temperature gate drive circuit, developed to drive custom-designed, 50 A, 600 V silicon-carbide (SiC) power modules consisting of multiple normally-off JFET dice (SemiSouth SJEC120R100) in parallel and rated at 175 °C device junction temperature. The gate drive and power modules are intended for use in a bi-directional DC-DC converter. The gate drive circuit is designed for operation in an enclosure in 120°C ambient air. Primary cooling of the gate drive is through the back of the circuit board to an aluminum plate, the base of which is cooled with engine coolant (water-ethylene glycol mixture) at an inlet temperature of 100°C. The power module consists of a single pole, with the close-coupled gate drive circuit providing independent and isolated drive for the two switches. The gate drive circuit is capable of operating the switches at PWM carrier frequencies up to 50 kHz, with duty cycles ranging from 0 to 98%.

Gallium Nitride is an attractive material for a high voltage handling, high current drive power switch technology. More precisely the high electron mobility and large bandgap of GaN offers a technology with a low Ron*Qg product of 50 mOhm-nC with ~600V blocking. However, the most mature device is a lateral GaN high electron mobility transistor (HEMT) that presents scaling and packaging challenges. In addition, enhancement mode operation remains a challenge in a manufacturable process that retains the two key benefits of breakdown and drive current.