Dean P. Hamilton

A Fast Loss and Temperature Simulation Method for Power Converters, Part I: Electrothermal Modeling and Validation

Simulation of power converters has traditionally been carried out using simplified models to shorten simulation time. This will compromise the accuracy of the results. A proposed fast simulation method for simulating converter losses and device temperatures over long mission profiles (load cycles) is described in this paper. It utilizes accurate physics-based models for the device losses, and is validated with experimentally obtained results.

The Impact of Parasitic Inductance on the Performance of Silicon–Carbide Schottky Barrier Diodes

1200V/300A silicon carbide Schottky barrier diode (SiC SBD) and Si pin diode modules have been tested as free-wheeling diodes under conditions of clamped inductive switching over a temperature range between -40 °C and 125 °C. Over the temperature range, the turn-OFF switching energy increases by 100% for the Si pin diode, whereas that of the SiC diode is temperature invariant and is 50% less than that of the Si pin diode at 125°C. However, the SiC SBD suffers from ringing/oscillations due to an underdamped response to an RLC circuit formed among the diode depletion capacitance, parasitic inductance, and diode resistance. These oscillations contribute to additional power losses that cause the SiC SBDs to be outperformed by the Si pin diodes at -40 °C and 0 °C. The higher depletion capacitance and lower series resistance of the SiC SBD contribute to a lower damping factor compared to the Si device. Furthermore, the positive temperature coefficient of the ON-state resistance in silicon contributes to better damping at high power levels, whereas the temperature invariance of the ON-state resistance in SiC means the oscillations persist at high temperatures. SPICE simulations and experimental measurements have been used to validate analytical expressions that have been developed for the circuit damping and oscillation frequency.

On the Ti3SiC2 metallic phase formation for robust p-type 4H-SiC ohmic contacts

This paper presents a detailed physical and electrical analysis of 4H-SiC ohmic contacts to p-type material, the main aim being to examine their ruggedness under high temperature conditions. XRD, FIB-TEM and SEM are techniques that have been utilized to examine the microstructure and interface properties respectively. A detailed physical study revealed the presence of a crystalline hexagonal Ti layer orientated in the same direction as the 4H-SiC epitaxial layer. This factor seems to be important in terms of electrical performance, having the lowest measured specific contact resistivity of 1x10-6 Ωcm2. We attribute this to the optimized formation of Ti3SiC2 at the metal/SiC interface. An initial high temperature study shows thermionic emission occurring across the metal/semiconductor junction.

Highly integrated power modules based on copper thick-film-on-DCB for high frequency operation of SiC semiconductors — Design and manufacture

This paper encompasses the design and the manufacture of a full-SiC module based on copper thick-film. Both DC-link capacitors as well as gate drives are implemented onto the substrate in order to minimise parasitic inductances. Thus, the module is especially suitable for high-frequency operation such as inductive energy transfer and inverter systems for renewable energies and electrical vehicles. In order to maintain high mechanical strength of the modules substrate, a Direct Copper Bond (DCB) provides the basis for multiple thick-film layers. The used thick-film dielectric insulates the gate-drive islands and also works as solder-stop material. The heat-spreading capabilities of DCB substrates are investigated by simulations.

Improved performance of 4H-SiC PiN diodes using a novel combined high temperature oxidation and annealing process

In this paper, the application of a novel combined high temperature thermal oxidation and annealing process to mesa-isolated epitaxial-anode 4H-SiC PiN diodes with thick (110 μm) drift regions is presented, the aim of which was to increase the carrier lifetime in the 4H-SiC. Diodes were fabricated using 4H-SiC material having undergone this process, which consisted of a thermal oxidation in dry pure O2 at 1550°C followed by an argon anneal at the same temperature. Forward current-voltage characterization showed that the oxidised/annealed samples typically showed around 15% lower forward voltage drop and around 40% lower differential on-resistance (at 100 A/cm2 and 25°C) compared to control sample PiN diodes, whilst reverse recovery tests indicated a carrier lifetime increase also of around 40%. These findings illustrate that the use of this process is a highly effective and efficient way of improving the electrical characteristics of high voltage 4H-SiC bipolar devices.

High-Temperature Electrical and Thermal Aging Performance and Application Considerations for SiC Power DMOSFETs

The temperature dependence and stability of three different commercially-available unpackaged SiC Dmosfets have been measured. On-state resistances increased to 6 or 7 times their room temperature values at 350 °C. Threshold voltages almost doubled after tens of minutes of positive gate voltage stressing at 300 °C, but approached their original values again after only one or two minutes of negative gate bias stressing. Fortunately, the change in drain current due to these threshold instabilities was almost negligible. However, the threshold approaches zero volts at high temperatures after a high temperature negative gate bias stress. The zero gate bias leakage is low until the threshold voltage reduces to approximately 150 mV, where-after the leakage increases exponentially. Thermal aging tests demonstrated a sudden change from linear to nonlinear output characteristics after 24–100 h air storage at 300 °C and after 570–1000 h in N2 atmosphere. We attribute this to nickel oxide growth on the drain contact metallization which forms a heterojunction p-n diode with the SiC substrate. It was determined that these state-of-the-art SiC mosfet devices may be operated in real applications at temperatures far exceeding their rated operating temperatures.

Degradation and Reliability of Bare Dies Operated up to 300°C

In this paper, we demonstrate the degradation of commercially available 1.2kV SiC MOSFET bare dies subjected to long periods of isothermal heating at 300°C in air. Periodic electrical measurements indicated an increase in on-state resistance to different extents for three different vendor designs, and the discovery of a progressive rectifying type forward characteristic at low drain-source voltages. Subsequent investigations to determine the cause of the degraded electrical characteristics including sectioning and SEM/TEM analysis revealed some mechanical degradation within the device gate-source cross-sections and backside drain contact metal layers. While one vendor device was severely degraded after approximately 24 hours of heating, another vendor device was only just beginning to degrade after 100 hours, indicating that these devices may be used successfully in real applications at 300°C junction temperatures for relatively long periods.

Evaluation of commercially available SiC devices and packaging materials for operation up to 350°C

The characteristics of commercially available silicon carbide power devices and packaging technologies have been measured up to 350°C in order to obtain their reliability and suitability for use in a hybrid electric vehicle application. Electro-thermal simulations of representative power module packaging structures, using measured conduction losses, revealed the respective temperature profiles of the devices and packaging. By correlating lifetime data found from our passive thermal cycling of candidate packaging technologies, with the magnitude and number of thermal cycles extracted from simulated temperature profiles, the lifetime of high temperature power module packages has been predicted. It was found that the limiting factor for high temperature thermal cycled operation is the silicon nitride substrate material, followed closely by the pressure-less silver sinter die attach. In this case, no aluminum wirebond failures were observed.

Conduction and switching loss comparison between an IGBT/Si-PiN diode pair and an IGBT/SiC-Schottky diode pair

Two insulated-gate bipolar-transistors (IGBTs) inverter leg modules of identical power rating have been manufactured and tested. One module has silicon-carbide (SiC) Schottky diodes as anti-parallel diodes and the other silicon PiN diodes. The power modules have been tested in an inductive switching circuit and curve tracer at a range of temperatures. Static and dynamic characteristics of both IGBTs and diodes have been used in loss comparisons between the two power modules. The results demonstrate the superior electrothermal performance of the SiC Schottky diode over the Si PiN diode leading to a reduction in the power module switching and conduction losses.

Connector-less SiC power modules with integrated shunt - low-profile design for low inductance and low cost

This paper presents the design, manufacture and characterization of connector-less 1200 V SiC MOSFET half-bridge power modules based on AlN DCB substrate. The modules contain four MOSFETs and no external antiparallel diodes. They are rated for a current of 40 A and include a shunt. Static and dynamic measurement results are presented. Multiphysics simulations are used to validate the measured data. The modules show a power path inductance below 3 nH. The power rating of the implemented chip shunt resistors is sufficient for the performed characterizations but requires revision. The switching loss at turn-on is 340 μJ at 23 A, 800 V, the turn-off loss is well below 50 μJ, principally allowing MHz operation in resonant mode.