Saeed Jahdi

Temperature and Switching Rate Dependence of Crosstalk in Si-IGBT and SiC Power Modules

The temperature and dV/dt dependence of crosstalk has been analyzed for Si-IGBT and SiC-MOSFET power modules. Due to a smaller Miller capacitance resulting from a smaller die area, the SiC module exhibits smaller shoot-through currents compared with similarly rated Si-IGBT modules in spite of switching with a higher dV/dt and with a lower threshold voltage. However, due to high voltage overshoots and ringing from the SiC Schottky diode, SiC modules exhibit higher shoot-through energy density and induce voltage oscillations in the dc link. Measurements show that the shoot-through current exhibits a positive temperature coefficient for both technologies, the magnitude of which is higher for the Si-IGBT, i.e., the shoot-through current and energy show better temperature stability in the SiC power module. The effectiveness of common techniques of mitigating shoot-through, including bipolar gate drives, multiple gate resistance switching paths, and external gate-source and snubber capacitors, has been evaluated for both technologies at different temperatures and switching rates. The results show that solutions are less effective for SiC-MOSFETs because of lower threshold voltages and smaller margins for negative gate bias on the SiC-MOSFET gate. Models for evaluating the parasitic voltage have also been developed for diagnostic and predictive purposes. These results are important for converter designers seeking to use SiC technology.

An Evaluation of Silicon Carbide Unipolar Technologies for Electric Vehicle Drive-Trains

Voltage sourced converters (VSCs) in electric vehicle (EV) drive-trains are conventionally implemented by silicon Insulated Gate Bipolar Transistors (IGBTs) and p-i-n diodes. The emergence of SiC unipolar technologies opens up new avenues for power integration and energy conversion efficiency. This paper presents a comparative analysis between 1.2-kV SiC MOSFET/Schottky diodes and silicon IGBT/p-i-n diode technologies for EV drive-train performance. The switching performances of devices have been tested between -75 °C and 175 °C at different switching speeds modulated by a range of gate resistances. The temperature impact on the electromagnetic oscillations in SiC technologies and reverse recovery in silicon bipolar technologies is analyzed, showing improvements with increasing temperature in SiC unipolar devices whereas those of the silicon-bipolar technologies deteriorate. The measurements are used in an EV drive-train model as a three-level neutral point clamped VSC connected to an electric machine where the temperature performance, conversion efficiency and the total harmonic distortion is studied. At a given switching frequency, the SiC unipolar technologies outperform silicon bipolar technologies showing an average of 80% reduction in switching losses, 70% reduction in operating temperature and enhanced conversion efficiency. These performance enhancements can enable lighter cooling and more compact vehicle systems.

An Analysis of the Switching Performance and Robustness of Power MOSFETs Body Diodes: A Technology Evaluation

The tradeoff between the switching energy and electro-thermal robustness is explored for 1.2-kV SiC MOSFET, silicon power MOSFET, and 900-V CoolMOS body diodes at different temperatures. The maximum forward current for dynamic avalanche breakdown is decreased with increasing supply voltage and temperature for all technologies. The CoolMOS exhibited the largest latch-up current followed by the SiC MOSFET and silicon power MOSFET; however, when expressed as current density, the SiC MOSFET comes first followed by the CoolMOS and silicon power MOSFET. For the CoolMOS, the alternating p and n pillars of the superjunctions in the drift region suppress BJT latch-up during reverse recovery by minimizing lateral currents and providing low-resistance paths for carriers. Hence, the temperature dependence of the latch-up current for CoolMOS was the lowest. The switching energy of the CoolMOS body diode is the largest because of its superjunction architecture which means the drift region have higher doping, hence more reverse charge. In spite of having a higher thermal resistance, the SiC MOSFET has approximately the same latch-up current while exhibiting the lowest switching energy because of the least reverse charge. The silicon power MOSFET exhibits intermediate performance on switching energy with lowest dynamic latching current.

Improved Electrothermal Ruggedness in SiC MOSFETs Compared With Silicon IGBTs

A 1.2-kV/24-A SiC-MOSFET and a 1.2-kV/30-A Si-Insulated gate bipolar transistor (IGBT) have been electrothermally stressed in unclamped inductive switching conditions at different ambient temperatures ranging from -25 °C to 125 °C. The devices have been stressed with avalanche currents at their rated currents and 40% higher. The activation of the parasitic bipolar junction transistor (BJT) during avalanche mode conduction results from the increased body resistance causing a voltage drop between the source and body, greater than the emitter-base voltage of the parasitic BJT. Because the BJT current and temperature relate through a positive feedback mechanism, thermal runaway results in the destruction of the device. It is shown that the avalanche power sustained before the destruction of the device increases as the ambient temperature decreases. SiC MOSFETs are shown to be able to withstand avalanche currents equal to the rated forward current at 25 °C, whereas IGBTs cannot sustain the same electrothermal stress. SiC MOSFETs are also shown to be capable of withstanding avalanche currents 40% above the rated forward current though only at reduced temperatures. An electrothermal model has been developed to explain the temperature dependency of the BJT latchup, and the results are supported by finite-element models.

The Impact of Temperature and Switching Rate on the Dynamic Characteristics of Silicon Carbide Schottky Barrier Diodes and MOSFETs

Silicon carbide Schottky barrier diodes (SiC-SBDs) are prone to electromagnetic oscillations in the output characteristics. The oscillation frequency, peak voltage overshoot, and damping are shown to depend on the ambient temperature and the metal-oxide- semiconductor field-effect transistor (MOSFET) switching rate (dIDS/dt). In this paper, it is shown experimentally and theoretically that dIDS/dt increases with temperature for a given gate resistance during MOSFET turn-on and reduces with increasing temperature during turn-off. As a result, the oscillation frequency and peak voltage overshoot of the SiC-SBD increases with temperature during diode turn-off. This temperature dependence of the diode ringing reduces at higher dIDS/dt and increases at lower dIDS/dt. It is also shown that the rate of change of dIDS/dt with temperature (d2IDS/dtdT) is strongly dependent on RG and using fundamental device physics equations, this behavior is predictable. The dependence of the switching energy on dIDS/dt and temperature in 1.2-kV SiC-SBDs is measured over a wide temperature range (-75 °C to 200 °C). The diode switching energy analysis shows that the losses at low dIDS/dt are dominated by the transient duration and losses at high dIDS/dt are dominated by electromagnetic oscillations. The model developed and results obtained are important for predicting electromagnetic interference, reliability, and losses in SiC MOSFET/SBDs.

Compact Electrothermal Reliability Modeling and Experimental Characterization of Bipolar Latchup in SiC and CoolMOS Power MOSFETs

In this paper, a compact dynamic and fully coupled electrothermal model for parasitic BJT latchup is presented and validated by measurements. The model can be used to enhance the reliability of the latest generation of commercially available power devices. BJT latchup can be triggered by body-diode reverse-recovery hard commutation with high dV/dt or from avalanche conduction during unclamped inductive switching. In the case of body-diode reverse recovery, the base current that initiates BJT latchup is calculated from the solution of the ambipolar diffusion equation describing the minority carrier distribution in the antiparallel p-i-n body diode. For hard commutation with high dV/dt, the displacement current of the drain-body charging capacitance is critical for BJT latchup, whereas for avalanche conduction, the base current is calculated from impact ionization. The parasitic BJT is implemented in Simulink using the Ebers-Moll model and the temperature is calculated using a thermal network matched to the transient thermal impedance characteristic of the devices. This model has been applied to CoolMOS and SiC MOSFETs. Measurements show that the model correctly predicts BJT latchup during reverse recovery as a function of forward-current density and temperature. The model presented, when calibrated correctly by device manufacturers and applications engineers, is capable of benchmarking the robustness of power MOSFETs.

Accurate Analytical Modeling for Switching Energy of PiN Diodes Reverse Recovery

PiN diodes are known to significantly contribute to switching energy as a result of reverse-recovery charge during turn-off. At high switching rates, the overlap between the high peak reserve-recovery current and the high peak voltage overshoot contributes to significant switching energy. The peak reverse-recovery current depends on the temperature and switching rate, whereas the peak diode voltage overshoot depends additionally on the stray inductance. Furthermore, the slope of the diode turn-off current is constant at high insulated-gate bipolar transistor (IGBT) switching rates and varies for low IGBT switching rates. In this paper, an analytical model for calculating PiN diode switching energy at different switching rates and temperatures is presented and validated by ultrafast and standard recovery diodes with different current ratings. Measurements of current commutation in IGBT/PiN diode pairs have been made at different switching rates and temperatures and used to validate the model. It is shown here that there is an optimal switching rate to minimize switching energy. The model is able to correctly predict the switching rate and temperature dependence of the PiN diode switching energies for different devices.

Analytical Modeling of Switching Energy of Silicon Carbide Schottky Diodes as Functions of dI

SiC Schottky Barrier diodes (SiC SBD) are known to oscillate/ring in the output terminal when used as free-wheeling diodes in voltage-source converters. This ringing is due to RLC resonance among the diode capacitance, parasitic resistance, and circuit stray inductance. In this paper, a model has been developed for calculating the switching energy of SiC diodes as a function of the switching rate (dIDS/dt of the commutating SiC MOSFET) and temperature. It is shown that the damping of the oscillations increases with decreasing temperature and decreasing dIDS/dt. This in turn determines the switching energy of the diode, which initially decreases with decreasing dIDS/dt and subsequently increases with decreasing dIDS/dt thereby indicating an optimal dIDS/dt for minimum switching energy. The total switching energy of the diode can be subdivided into three phases namely the current switching phase, the voltage switching phase, and the ringing phase. Although the switching energy in the current switching phase decreases with increasing switching rate, the switching energy of the voltage and ringing phase increases with the switching rate. The model developed characterizes the dependence of diodes switching energy on temperature and dIDS/dt, hence, can be used to predict the behavior of the SiC SBD.

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Using the Fourier series solution to the ambipolar diffusion equation, the robustness of the body diodes of SiC MOSFETs during reverse recovery has been studied. Parasitic bipolar latch-up during the reverse recovery of the body diode is a possible if there is sufficient base current and voltage drop across the body resistance to forward bias the parasitic BJT. SiC MOSFETs have very low carrier lifetime and thin epitaxial drift layers, which means that the dV/dt during the recovery of the body diode can be quite high. This dV/dt coupled with the parasitic drain-to-body capacitance can cause a body current. The paper introduces a new way of assessing the reliability of SiC MOSFETs during the reverse recovery of the body diode. The impact of switching rates, parasitic inductances and carrier lifetime on the activation of the parasitic BJT has been studied.

/dt and Temperature

This paper investigates the physics of device failure during avalanche for 1.2 kV SiC MOSFETs, silicon MOSFETs and silicon IGBTs. The impact of ambient temperature, initial conditions of the device prior to avalanche breakdown and the avalanche duration is explored for the different technologies. Two types of tests were conducted namely (i) constant avalanche duration with different peak avalanche currents and (ii) constant peak avalanche current with different avalanche durations. SiC MOSFETs are shown to be the most rugged technology followed by the silicon IGBT and the silicon MOSFET. The material properties of SiC suppress the triggering of the parasitic BJT that causes thermal runaway during avalanche.