Christian Uhrenfeldt

Influences of Device and Circuit Mismatches on Paralleling Silicon Carbide MOSFETs

This paper addresses the influences of device and circuit mismatches on paralleling the silicon carbide (SiC) MOSFETs. Comprehensive theoretical analysis and experimental validation from paralleled discrete devices to paralleled dies in multichip power modules are first presented. Then, the influence of circuit mismatch on paralleling SiC MOSFETs is investigated and experimentally evaluated for the first time. It is found that the mismatch of the switching loop stray inductance can also lead to on-state current unbalance with inductive output current, in addition to the on-state resistance of the device. It further reveals that circuit mismatches and a current coupling among the paralleled dies exist in a SiC MOSFET multichip power module, which is critical for the transient current distribution in the power module. Thus, a power module layout with an auxiliary source connection is developed to reduce such a coupling effect. Finally, simulations and experimental tests are carried out to validate the analysis and effectiveness of the developed layout.

Broadband photocurrent enhancement and light-trapping in thin film Si solar cells with periodic Al nanoparticle arrays on the front.

Plasmonic resonances in metal nanoparticles are considered candidates for improved thin film Si photovoltaics. In periodic arrays the influence of collective modes can enhance the resonant properties of such arrays. We have investigated the use of periodic arrays of Al nanoparticles placed on the front of a thin film Si test solar cell. It is demonstrated that the resonances from the Al nanoparticle array causes a broadband photocurrent enhancement ranging from the ultraviolet to the infrared with respect to a reference cell. From the experimental results as well as from numerical simulations it is shown that this broadband enhancement is due to single particle resonances that give rise to light-trapping in the infrared spectral range and to collective resonances that ensure an efficient in-coupling of light in the ultraviolet-blue spectral range.

Design of low impedance busbar for 10 kV, 100A 4H-SiC MOSFET short-circuit tester using axial capacitors

This paper discusses the design of a setup for short-circuit (SC) testing of 10 kV 10A 4H-SiC MOSFETs. The setup can achieve voltages up to 10 kV and currents in excess of 100A. The main objective during the design was to obtain low parasitic inductance throughout the setup, while at the same time, reduce the complexity and size of the setup by avoiding series connection of DC-link capacitor and by employing capacitors with voltage ratings above 10 kV. Obtaining a low inductance at such voltage levels is challenging, considering the required clearance distances, the lack of radial style capacitor rated for 10 kV on the market, the package design of CREE 10 kV 10 A 4H-SiC MOSFETs and the required space for the device heater. Ansys Q3D is used in order to extract the parasitic components from the design. Custom designed aluminum cans for 15 kV axial capacitors are used in order to minimize the inductance, with a symmetrical arrangement in order to provide optimal current sharing distribution. Busbar measurements verify the low inductive design of the DC-link. The measured inductance is also validated by means of Finite Element Method analysis and by experimental validation.

Diffractive coupling and plasmon-enhanced photocurrent generation in silicon

Arrays of metal nanoparticles are considered candidates for improved light-coupling into silicon. In periodic arrays the coherent diffractive coupling of particles can have a large impact on the resonant properties of the particles. We have investigated the photocurrent enhancement properties of Al nanoparticles placed on top of a silicon diode in periodic as well as in random arrays. The photocurrent of the periodic array sample is enhanced relative to that of the random array due to the presence of a Fano-like resonance not observed for the random array. Measurements of the photocurrent as a function of angle, reveal that the Fano-like enhancement is caused by diffractive coupling in the periodic array, which is accordingly identified as an important design parameter for plasmon-enhanced light-coupling into silicon.

Vce as early indicator of IGBT module failure mode

A double point monitoring concept of the on-state voltage is presented as a means to identify the failure mode early in a power module lifetime. The monitoring approach is carried out in a simple accelerated test setup, however, it is argued that the concepts should be possible in field. Additionally, by carrying out the monitoring in calibration conditions it is shown that it is possible to identify the fracture rate of the bond wire fatigue failure mechanism. The curve shape of the degradation rate is in accordance with expected fracture theory.

Failure mechanism analysis of a discrete 650V enhancement mode GaN-on-Si power device with reverse conduction accelerated power cycling test

A commercial discrete enhancement mode gallium nitride power component employing advanced package technology without conventional bond wire possesses the ability for bidirectional conduction. The gallium nitride power components can provide not only higher forward conductivity but also superior reverse conductivity. For the most part recent critical debates about the reverse conductivity of a GaN device have tended to center around their performance. However, the reliability of the device under reverse operation should be assessed in order for the advantages to be fully utilized in real applications. We present the results of reverse conduction power cycling test of a discrete 650-volt gallium nitride power device with novel package technology at temperature swings of 100 K. The result shows degradation of thermal conductivity and raising leakage current drain to source as reaching the number of cycles to failure. In physical failure analysis, delamination of a solder joint between a chip and a copper layer of an aluminum print circuit board is observed with a scanning acoustic microscope.

Conduction, reverse conduction and switching characteristics of GaN E-HEMT

In this paper switching and conduction characterization of the GS66508P-E03 650V enhancement mode gallium nitride (GaN) transistor is described. GaN transistors are leading edge technology and as so, their characteristics are less than well documented. The switching characteristics are found using a simulated double pulse test (DPT) whereas the conduction characteristics are measured in a curvetracer. The reverse conduction was found to be similar to the forward conduction with a voltage drop of Vth-Vgs(OFF). To decrease the parasitic impedance some considerations has been taken. These considerations are described and a model of the double pulse test is formulated.

10kV SiC MOSFET split output power module

The poor body diode performance of the first generation of 10kV SiC MOSFETs and the parasitic turn-on phenomenon limit the performance of SiC based converters. Both these problems can potentially be mitigated using a split output topology. In this paper we present a comparison between a classical half bridge and a split-output power module. It is found that the peak current during turn-on is reduced significantly, however some additional challenges arise during implementation.

Modeling of Short-Circuit-Related Thermal Stress in Aged IGBT Modules

In this paper, the thermal stress on bond wires of aged insulated gate bipolar transistor modules under short-circuit conditions has been studied with respect to different solder delamination levels. To ensure repeatable test conditions, ad-hoc direct bond copper samples with delaminated solder layers have been purposely fabricated. The temperature distribution produced by such abnormal conditions has been modeled first by means of finite-element method simulations and then experimentally validated by means of a nondestructive testing technique, including an ultrafast infrared camera. Results demonstrate a significant imbalance in the surface temperature distribution, which confirms the hypothesis that short-circuit events produce significantly uneven stresses on bond wires.

Test bench for thermal cycling of 10 kV silicon carbide power modules

This paper presents a test bench for lifetime investigation of 10 kV silicon carbide power modules. The test bench subjects high voltage switching operation to the modules while power cycling. Thus both a thermal and electrical operating point is emulated. The power cycling setup features offline measurement of on-state voltages and direct real-time measurement of die surface temperatures, enabled by fiber optical sensors, which are built into the power modules. A thermal model of the module prototypes, based on the temperature measurements, is established. Independent verification steps have been made to validate the performance of the on-state voltage measurement and the thermal model. Issues are revealed in the form of common mode currents in gate drive supply, which should be remedied. Finally a new operating point for power cycling is suggested to better stress the power modules.