Jeremy Junghans

Power Conversion With SiC Devices at Extremely High Ambient Temperatures

This paper evaluates the capability of SiC devices for operation under extremely high ambient temperatures. To this end, the authors packaged SiC JFET and Schottky barrier diodes (SBD) in thermally stable packages and built a high-temperature inductor to be evaluated in a DC-DC buck converter. The DC characteristics of the SiC JFET devices were first measured at ambient temperatures ranging from room temperature up to 450 degC. The experimental results show that the device can operate at 450 degC, which is impossible for conventional Si devices, but as expected the current capability of the SiC JFET diminishes with rising temperatures. A DC-DC converter was then designed and built in accordance with the static characteristics of the SiC JFETs that were measured under extremely high ambient temperatures. The converter was tested up to an ambient temperature of 400 degC. The conduction loss of the SiC JFET increases slightly, as predicted from its DC characteristics, but its switching characteristics hardly change with increasing temperatures. Thus, SiC devices are well suited for operation in harsh temperature environments

Power Conversion with SiC Devices at Extremely High Ambient Temperatures

This paper evaluates the capability of SiC devices for operation under extremely high ambient temperatures. To this end, the authors packaged SiC JFET and Schottky barrier diodes (SBD) in thermally stable packages and built a high-temperature inductor to be evaluated in a DC-DC buck converter. The DC characteristics of the SiC JFET devices were first measured at ambient temperatures ranging from room temperature up to 450 degC. The experimental results show that the device can operate at 450 degC, which is impossible for conventional Si devices, but as expected the current capability of the SiC JFET diminishes with rising temperatures. A DC-DC converter was then designed and built in accordance with the static characteristics of the SiC JFETs that were measured under extremely high ambient temperatures. The converter was tested up to an ambient temperature of 400 degC. The conduction loss of the SiC JFET increases slightly, as predicted from its DC characteristics, but its switching characteristics hardly change with increasing temperatures. Thus, SiC devices are well suited for operation in harsh temperature environments

Switching characteristics of SiC JFET and Schottky diode in high-temperature dc-dc power converters

This paper reports on SiC devices operating in a dc-dc buck converter under extremely high ambient temperatures. To this end, the authors packaged SiC JFET and Schottky diodes in thermally stable packages and built a high-temperature inductor. The converter was tested at ambient temperatures up to 400°C. Although the conduction loss of the SiC JFET increases slightly with increasing temperatures, the SiC JFET and Schottky diode continue normal operation because their switching characteristics show minimal change with temperature. This work further demonstrates the suitability of the SiC devices for high-temperature power converter applications.

Liquid metal heat sink for high-power laser diodes

We report on the development of a novel, ultra-low thermal resistance active heat sink (AHS) for thermal management of high-power laser diodes (HPLD) and other electronic and photonic components. AHS uses a liquid metal coolant flowing at high speed in a miniature closed and sealed loop. The liquid metal coolant receives waste heat from an HPLD at high flux and transfers it at much reduced flux to environment, primary coolant fluid, heat pipe, or structure. Liquid metal flow is maintained electromagnetically without any moving parts. Velocity of liquid metal flow can be controlled electronically, thus allowing for temperature control of HPLD wavelength. This feature also enables operation at a stable wavelength over a broad range of ambient conditions. Results from testing an HPLD cooled by AHS are presented.

Next-Generation Microchannel Coolers

A next-generation microchannel cooler has been developed for packaging laser diode arrays that eliminates many of the problems associated with typical copper-based cooling designs. The coolers are built on well-established Low-Temperature Cofired Ceramic technology and provide excellent thermal performance. They do not require the use of deionized water. This work highlights the strengths of the new cooler technology. The results of a long-term, high-flow-rate test which demonstrates the excellent erosion resistance of these coolers are presented. Three devices have been tested for 2500 hours at a flow rate of 0.25 GPM and show minimal signs of erosion. This data is compared to a similar test conducted with copper coolers. Several design parameters are also addressed for the ceramic coolers. The available form and fit characteristics are addressed, as is the custom-configurable nature of the devices.

Low-Cost Diode Arrays for the LIFE Project

One of the primary challenges of the Laser Inertial Fusion Engine (LIFE) project is the cost and availability of the laser diode arrays needed to pump the solid-state laser gain media in the system. Current projections indicate that the arrays need to be available for approximately one cent per Watt of output power, which is one to two orders of magnitude cheaper than currently available. This work focuses on potential manufacturing approaches to meet the projected specifications of the LIFE project. Special attention will be paid to requirements related to power density (25 kW/cm2), bar pitch (150 - 400 microns), output wavelength (87x), and fast-axis divergence (+/- 4 degrees). A summary of the supply limitations and cost ramifications of each requirement is presented. Also discussed are potential supply chain limitations that are anticipated as a result of the immense size of the LIFE project.

A novel packaging methodology for spray cooling of power semiconductor devices using dielectric liquids

The heat flux requirements of present power electronics systems are exceeding 100 W/cm2 and are predicted to reach the 1000 W/cm2 range in the near future. Spray cooling is a cooling technology that can provide such enormous cooling demands. Direct spray cooling of power devices (e.g., IGBTs) that are conventionally wire-bonded creates long-term reliability problems. So in order to achieve the expected future heat load demands and improve the reliability of the system there is a need for novel packaging methodologies. This paper reports on an innovative packaging methodology and a test vehicle that demonstrates the potential of spray cooling power electronics. This paper describes the proposed methodology as well as the spraying parameters that affect the cooling

VBG controlled narrow bandwidth diode laser arrays

Northrop Grumman Cutting Edge Optronics has developed large kilowatt class lensed laser diode arrays with subnanometer spectral width using Volume Bragg Grating (VBG) reflectors. Using these CW arrays with 100W bars at 885nm, excellent absorption in Nd:YAG is achieved, with lower thermal aberration than can be attained with 808nm pumps. The additional cost of the VBG reflectors and their alignment is partially offset by the much broader wavelength tolerance that is allowed in the unlocked array enhancing bar yield. Furthermore, the center wavelength of the arrays exhibit lower temperature sensitivity allowing the arrays to be operated over a wider current or temperature range than arrays without wavelength control. While there is an efficiency penalty associated with the addition of VBGs of 5-8%, it is more than compensated for by enhanced absorption, especially when used with narrowband absorption lines, such as 885nm in Nd:YAG. An overview of the design and manufacturing issues for arrays that are wavelength-locked with VBGs is presented along with the effect of post-construction hard UV exposure.

Testing of active heat sink for advanced high-power laser diodes

We report on the development of a novel active heat sink for high-power laser diodes offering unparalleled capacity in high-heat flux handling and temperature control. The heat sink employs convective heat transfer by a liquid metal flowing at high speed inside a miniature sealed flow loop. Liquid metal flow in the loop is maintained electromagnetically without any moving parts. Thermal conductance of the heat sink is electronically adjustable, allowing for precise control of diode temperature and the laser light wavelength. This paper presents the principles and challenges of liquid metal cooling, and data from testing at high heat flux and high heat loads.

QCW diode array reliability at 80x and 88x nm

Northrop Grumman Cutting Edge Optronics (NGCEO) has recently developed high-power laser diode arrays specifically for long-life operation in quasi-CW applications. These arrays feature a new epitaxial wafer design that utilizes a large optical cavity and are packaged using AuSn solder and CTE-matched heat sinks. This work focuses on life test matrix of multiple epitaxial structures, multiple wavelengths, and multiple drive currents. Particular emphasis is given to the 80x and 88x wavelength bands running at 100-300 Watts per bar. Reliable operating points are identified for various applications including range finding (product lifetimes less than 1 billion shots) and industrial machining (product lifetimes greater than 20 billion shots). In addition to life test data, a summary of performance data for each epitaxial structure and each bar design is also presented.