Recent advancements in semiconductor technology have demonstrated how silicon carbide (SiC) has several unique properties that make it a promising material for semiconductor devices with high temperature, high speed, and high voltage operation capabilities. SiC devices have several advantages over traditional Silicon (Si)-based devices, including higher efficiency and power density, making them suitable for use in harsh environments such as those in the aerospace, automotive, and energy industries.

However, developing SiC-based devices for high-temperature applications presents challenges in packaging and integration due to the need for thermally stable materials for die attach, wire bonds, and auxiliary circuit components that can operate at elevated temperatures. When it comes to power devices like the JFET and SBD, testing typically takes place at high temperatures of up to 450℃ using thermally stable packages and an automated setup. Nonetheless, the main conversion circuit's ability to operate under locally hot temperatures after testing with SiC devices at 400°C significantly lowers the overall temperature.

Components Used During Experimentation

The DC-DC buck converter is a popular power electronics circuit used for stepping down a DC voltage level to a lower level. It consists of a switching device (usually a MOSFET or a BJT), a diode, an inductor, and input and output capacitors. The inductor plays a crucial role in the operation of the converter by storing energy during the on-time of the switching device and releasing it during the off-time. The inductor helps to maintain a smooth and constant output voltage by filtering the output ripple caused by the switching action.

The JEDEC TO-258 package is a type of through-hole metal package used for high-power semiconductor devices which were Ni-plated to provide additional thermal and electrical conductivity, beneficial for high-temperature operation. The lack of suitable capacitors capable of operating at such high temperatures made it impossible to be utilized in the converter circuit. As a result, the circuit did not include any filter capacitors, which may have affected its performance and stability.

DC Characterization of SiC at High Temperatures

● Measurement Setup

Fig 1: An experimental setup used to measure the DC characteristics of a SiC JFET at various ambient temperatures.

Fig. 1 is a schematic figure illustrating the configuration of the experimental setup used. The SiC device under test (DUT) was placed inside a temperature-controlled oven to regulate the ambient temperature surrounding the DUT. This allows it to measure the device's performance under different temperature conditions.

The setup includes two power sources, one to supply the drain-source voltage (V_ds) and the other to supply the gate-source voltage (V_gs). The drain-source voltage  (V_ds)  was gradually increased from 0 V to 20 V while keeping different values for the gate-source voltage  (V_gs). To supply the gate voltage, the power supply provides two voltage levels, with the higher level used for the gate voltage and the lower one used to switch off the JFET. An optically-isolated gate driver, the TLP-250 from Toshiba Corp, was utilized to impose the gate voltage on the DUT. Furthermore, a pulse generator capable of producing gate pulses with arbitrary widths was set to output 40 s ON pulses to prevent self-heating of the device due to the conduction current.

The digital storage oscilloscope was then triggered by the gate signal from the pulse generator using an external trigger terminal (extTrig) which was further used for data acquisition. In order to process and store the data, a PC with LabView was used.

● SiC JFET Characteristics

The DC characteristics of SiC JFET play an important role in the functioning of SiC devices at high temperatures, where it is seen that threshold gate voltage varies with temperature. This can be illustrated in Figure 2 where the graphs show V_ds and I_ds characteristics concerning a JFET SiC with gate voltage V_gs at 25℃, 200℃, and 450 ℃.

Figure 2: DC characteristics of SiC JFET at 25℃.

Figure 3: DC characteristics of SiC JFET at 200℃.

Here, the SiC JFET has dc characteristics like pentode at room temperature, moreover, the saturation current at V_gs at 0 comes out as 3.5A. However, due to the -12 V threshold gate voltage and low drain-source resistance of 1.33, the slope comes out as steep in the linear region. On the other hand, Figure 3 shows that the threshold gate voltage is -13 V for a temperature of 200℃.

Figure 4: DC characteristics of SiC JFET at 400℃.

Figure 5: Forward blocking voltage ability of SiC JFET at different temperatures.

At extremely high temperatures of 450℃, the pinch-off drain voltage cannot be concluded at V_gs=0V, which is presented in Figure 4, however, the output current saturates at around 20 percent of the original value at 25℃. The threshold voltage becomes more negative at around -15V. Alternately when a threshold voltage of -20V is applied the leakage current rises exponentially high as shown in Figure 5. This simply shows that  SiC JFET has the ability to remain in “OFF” condition at 400℃, but with high leakage current.

● SiC SBD Characteristics

The I_ak-V_ak characteristics of SiC SBD are very peculiar and vary from low to high ambient temperatures. Figure 6 shows the slope of the characteristic curve which becomes shallower as the temperature increases, due to the reduction of Schottky barrier height. This ultimately results in an increase in series resistance with rising temperatures.

Figure 6: V_ak-I_ak characteristics of SiC SBD at different temperatures.

On the other hand, it is seen that due to the changes in series resistance and cut in voltage with respect to temperature, the forward voltage drop changes non-linearly. Although this voltage drop is insignificant when compared to the absolute value of the forward voltage drop and the fluctuations in temperature in SiC JFET, it is still noteworthy.

One of the most important aspects of designing SiC SBD is the consideration of the effect of reverse voltage on the device. Traditionally, when the temperature exceeds 300℃, the reverse leakage current rises sharply. This in turn reduces the reverse voltage capability from  660 V at 25℃ to 100 V at 400℃.

Buck Converter Performance at High Temperatures

Buck converters are widely used in power electronics as they offer high efficiency and excellent transient response. However, the performance of buck converters can be affected by high temperatures, which can cause degradation in the performance of the device. The most common issue is an increase in the on-resistance of the switching devices, which results in increased power dissipation and reduced efficiency. The increased temperature can also lead to changes in the characteristics of passive components such as inductors and capacitors, which can further degrade the performance of the device. These characteristics are studied in detail with the help of an experimental setup.

● Setup Used During the Experiment

The general setup is illustrated in Figure 7, where it is clearly seen that only the SBD, inductor, and SiC JFET are exposed to the variations in ambient temperature produced in the oven.

Figure 7: Experimental Setup for a dc-dc buck converter to be tested at different temperatures.

Here, the converter is working at an input voltage of 100V, 100 load resistance, and a switching frequency of 100 kHz. However, the impact of parasitic inductance is somewhat prominent, as the measurements were taken outside the oven with the help of long wires. On the other hand, the “ON” condition has a gate driver voltage of 0V whereas, the “OFF” condition has a voltage of -18 V which is lower than the threshold voltage of -15V at 450℃. In the setup, the voltage is determined with the help of the reverse voltage limitation of the SBD and the forward blocking voltage limitation of the JFET.

● Switching Characteristics of SiC Systems at High Temperatures

The drain-source voltage V_ds of SiC JFET devices in ambient temperature indicates that it can remain in “OFF” condition when a gate voltage of -18V is applied, however this drain-source voltage drop increases at “ON” condition. This can be clearly illustrated clearly in Figure 8. Moreover, the turn-off behavior is dominated by SiC JFET itself, and it maintains a high-speed switching ability at the ambient temperature of 400℃. The circuit parasitic inductance results in the reduction of peak value in the transient current due to rising temperatures which ultimately results in a reduction of saturated drain current.

Figure 8: Vds-based experimental results for SiC-based dc-dc buck converter.

On the other hand, Figure 9 illustrates the current passing through the SiC SBD which turns on as soon as the current flows through SiC SBD, which is accompanied by a small forward recovery and small oscillations. However, it produces a large reverse current during turn-off. Moreover, a large dv/dt is maintained as the SiC SBD does not produce reverse recovery due to carrier combination which results in a fast turn-on speed of SiC JFET.

Figure 9: Ids-based experimental results for SiC-based dc-dc buck converter.

Conclusion

The traditional plastic packaging for electronic components cannot withstand high temperatures, therefore, packages that can withstand thermal stress were used to contain SiC bare die. The SiC JFET and SBD were tested at various ambient temperatures to assess their performance. The results showed that the available current ratings of the SiC JFET decreased as the temperature increased. At 450℃, the current rating dropped to 20% of the rating at room temperature. The threshold gate voltage was slightly lower with increasing temperature although it could be managed by adjusting the gate voltage a few volts higher in the negative direction. The SiC SBD showed increased series resistance with temperature, and the forward voltage drop interacted with the cut-in voltage drop as the temperature rose.

On the other hand, experimental results have shown that the SiC JFET possesses excellent switching characteristics within the tested temperature range. The inductive switching characteristics of both the SiC JFET and SBD did not significantly deteriorate with increasing temperatures. However, the extremely fast turn-on operation of the SiC JFET resulted in reverse displacement currents in the SiC SBD, which must be addressed in the design of the gate driver circuit. Moreover, a  dc-dc buck converter was designed and experimented with for high temperatures using SiCJFET, SiCSBD, and an inductor. The dc-dc buck converter was assessed which was operating at temperatures ranging from 25℃ to extremely high ambient temperatures of 450℃. While the drain-source resistance increased and power conversion efficiency declined with temperature, the decrease in efficiency was not substantial. Additionally, the increase in voltage drop between the drain and the source was relatively minor in comparison to the converter's output voltages.