Ljubisa Dragoljub Stevanovic

Recent advances in silicon carbide MOSFET power devices

Emerging silicon carbide (SiC) MOSFET power devices promise to displace silicon IGBTs from the majority of challenging power electronics applications by enabling superior efficiency and power density, as well as capability to operate at higher temperatures. This paper reports on the recent progress in development of 1200V SiC power MOSFETs. Two different chip sizes were fabricated and tested: 15A (0.225cm×0.45cm) and 30A (0.45cm×0.45cm) devices. First, the 30A MOSFETs were packaged as discrete components and static and switching measurements were performed. The device blocking voltage was 1200V and typical on-resistance was less than 50 mΩ with gate-source voltages of 0V and 20V, respectively. The total switching losses were 0.6 mJ, over five times lower than the competing devices. Next, a buck converter was built for evaluating long-term stability of the MOSFETs and typical switching waveforms are presented. Finally, the 15A MOSFETs were used for fabrication of 150A all-SiC modules. The module on-resistance values were in the range of 10 mQ, resulting in the best-in-class on-state voltage values of 1.5V at nominal current. The module switching losses were 2.3 mJ during turn-on and 1 mJ during turn-off, also significantly better than competing designs. The results validate performance advantages of the SiC MOSFETs, moving them a step closer to power electronics applications.

Direct comparison of silicon and silicon carbide power transistors in high-frequency hard-switched applications

RECENT progress in wide bandgap power (WBG) switches shows great potential. Silicon carbide (SiC) is a promising material for power devices with breakdown voltages of several hundred volts up to 10 kV. SiC Schottky power diodes have achieved widespread commercial acceptance. Recently, much progress has been made on active SiC switches, including JFETs, thyristors, BJTs, IGBTs, and MOSFETs. Many a great promise has been made, and wondrous claims abound, but the question remains: will they live up to the hype? We explore this question for the class of high-frequency, hard-switched converters with input voltages of up to 600 VDC and power throughputs in the kilowatt range. Experimental evidence shows that both superior efficiency and higher power density may be obtained via the use of SiC MOSFETs. A direct comparison is made using silicon power devices (IGBTs and MOSFETs) and SiC MOSFETs in a 200 kHz, 6 kW, 600 V hard-switched converter. The losses are measured and conduction and switching losses of the active devices are estimated. Total losses can be reduced by a factor of 2–5 by substitution of SiC MOSFETs for Si active power devices.

Low inductance power module with blade connector

A novel single-switch power module has been developed, featuring a laminated blade connector for low inductance interconnect to a busbar. The module was designed, optimized and experimentally validated as part of a high frequency three-phase converter, demonstrating parasitic inductances of less than one nano henry for the module and as low as five nano henries for the converter phase-leg commutation loop. The flexible plug-in hardware facilitated direct comparison of switching performance between three different chipsets, including a 150A and a 300A hybrid designs using the fastest 1200V silicon IGBTs with silicon carbide (SiC) Schottky diodes, as well as a 150A all-SiC module with emerging SiC MOSFETs. The results were also compared with switching performance of standard modules. First, the impact of parasitic inductance on switching performance was quantified by testing the same 300A hybrid chipset in an industry-standard module. Compared to the low inductance blade POL module, the standard module had 65% higher voltage overshoot and 30% higher total switching losses. Second, the switching performance of the 150A, 1200V fast IGBT, in either standard silicon or the hybrid blade module, was compared with the all-SiC blade module under the same test conditions. The IGBT switching losses of the standard silicon module were 3.5 times higher, while the hybrid blade module losses were 2.5 times higher than those of the all-SiC module. The new low inductance blade module is an excellent package for the new generation of fast silicon IGBTs and the emerging SiC power devices. The module will enable efficient power conversion at significantly higher switching frequencies and power densities.

Performance and Reliability of SiC MOSFETs for High-Current Power Modules

We address the two critical challenges that currently limit the applicability of SiC MOSFETs in commercial power conversion systems: high-temperature gate oxide reliability and high total current rating. We demonstrate SiC MOSFETs with predicted gate oxide reliability of >106 hours (100 years) operating at a gate oxide electric field of 4 MV/cm at 250°C. To scale to high total currents, we develop the Power Overlay planar packaging technique to demonstrate SiC MOSFET power modules with total on-resistance as low as 7.5 m. We scale single die SiC MOSFETs to high currents, demonstrating a large area SiC MOSFET (4.5mm x 4.5 mm) with a total on-resistance of 30 m, specific on-resistance of 5 m-cm2 and blocking voltage of 1400V.

Integral micro-channel liquid cooling for power electronics

A novel integral micro-channel heat sink was developed, featuring an array of sub-millimeter channels fabricated directly in the back-metallization layer of the direct bond copper or active metal braze ceramic substrate, thus minimizing the material between the semiconductor junction and fluid and the overall junction-to-fluid thermal resistance. The ceramic substrate is bonded to a baseplate that includes a set of interleaved inlet and outlet manifolds for uniform fluid distribution across the actively cooled area of the heat sink. The interleaved manifolds greatly reduce the pressure drop and minimize temperature gradient across the heat sink surface. After performing detailed simulations and design optimization, a 200 A, 1200 V IGBT power module with the integral heat sink was fabricated and tested. The junction-to-fluid thermal resistivities for the IGBTs and diodes were 0.17°C⋆cm2/W and 0.14°C⋆cm2/W, respectively. The design is superior to all reported liquid cooled heat sinks with a comparable material system, including the micro-channel designs. It is also easily scaleable to larger heat sink surfaces without compromising the performance.

3.3kV SiC MOSFETs designed for low on-resistance and fast switching

This paper discusses the latest developments in the optimization and fabrication of 3.3kV SiC vertical DMOSFETs. The devices show superior on-state and switching losses compared to the even the latest generation of 3.3kV fast Si IGBTs and promise to extend the upper switching frequency of high-voltage power conversion systems beyond several tens of kHz without the need to increase part count with 3-level converter stacks of faster 1.7kV IGBTs.

Overview of 1.2kV – 2.2kV SiC MOSFETs targeted for industrial power conversion applications

This paper presents the latest 1.2kV-2.2kV SiC MOSFETs designed to maximize SiC device benefits for high-power, medium voltage power conversion applications. 1.2kV, 1.7kV and 2.2kV devices with die size of 4.5mm × 4.5mm were fabricated, exhibiting room temperature on-resistances of 34mOhm, 39mOhm and 41mOhm, respectively. The ability to safely withstand single-pulse avalanche energies of over 17J/cm2 is demonstrated. Next, the 1.7kV SiC MOSFETs were used to fabricate half-bridge power modules. The module typical onresistance was 7mOhm at Tj=25°C and 11mOhm at 150°C. The module exhibits 9mJ turn-on and 14mJ turn-off losses at Vds=900V, Id=400A. Validation of GEs SiC MOSFET performance advantages was done through continuous buck-boost operation with three 1.7kV modules per phase leg exhibiting 99.4% efficiency. Device ruggedness and tolerance to terrestrial cosmic radiation was evaluated. Experimental results show that higher voltage devices (2.2kV and 3.3kV) are more susceptible to cosmic radiation, requiring up to 45% derating in order to achieve module failure rate of 100 FIT, while 1.2kV MOSFETs require only 25% derating to deliver similar FIT rate. Finally, the feasibility of medium voltage power conversion based on series connected 1.2kV SiC MOSFETs with body diode is demonstrated.

Micro-channel thermal management of high power devices

Heat fluxes in semiconductor power devices have been steadily increasing over the past two decades, now approaching 500 W/cm2 . This dissipation requires advanced thermal management in order to maintain device maximum junction temperatures below the Si limit of 150degC. Micro-channel cooling shows great promise for high heat flux removal, with the potential for greater than 750 W/cm2 performance. As flow passages decrease in size to sub-millimeter scales, the surface area-to-volume ratio increases, allowing greater potential heat transfer area. However, the correspondingly higher pressure losses across the channel can quickly exceed the maximum pump performance at these small dimensions. A novel micro-channel heat sink was developed, featuring micro-channel passages fabricated directly into the active metal braze (AMB) substrate, minimizing the junction-to-fluid thermal conduction resistance. The heat sink performance was simulated using computational fluid dynamics models and the results show that heat fluxes above 500 W/cm2 could be achieved for a 50degC device junction-to-coolant temperature rise. The heat sink was fabricated and tested using an array of power diodes, and infrared thermography measurements validated the simulation results. The demonstrated thermal performance is superior to any existing micro-channel heat sink with a comparable electrical assembly

1.2kV class SiC MOSFETs with improved performance over wide operating temperature

In this paper, we report on 1.2kV SiC MOSFETs rated to T_{j, max}=200°C, exhibiting improved performance characteristics across operating temperature. Our devices show stable, rugged and reliable operation when subjected to industry standard qualification tests. Low on-resistance of 35mOhm/79mOhm at T_j=25°C and 47mOhm/103mOhms at T_j=150°C are shown for 0.1cm² and 0.2cm² die. 1000 hour High-Temperature Gate-Bias (HTGB) tests at T_j=200°C show excellent threshold stability with less than 5% parametric shift observed. High-Temperature Reverse Bias (HTRB) at T_j=200°C/V_DS=960V also show stable and reliable operation. Single-pulse avalanche energies of over E_Av=1.75J are obtained with 0.1cm² MOSFETs.

DC and Transient Performance of 4H-SiC Double-Implant MOSFETs

SiC vertical MOSFETs were fabricated and characterized, achieving blocking voltages around 1 kV and specific on-resistances as low as RSP,ON=8.3 mOmegamiddotcm2. DC and transient characteristics are shown. Room and elevated temperature (up to 200degC) 600 V/5 A inductive switching performance of the SiC MOSFETs are shown with turn-on and turn-off transients of approximately 20-40 ns.