Designing Robust SiC Power Devices in Extreme Thermal Conditions

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

Achieving maximum power density is an increasingly important requirement for applications like DC/DC power converters, onboard chargers (OBC) in electric vehicles, industrial motor drives, solar inverters, and traction inverters. This requirement increases system operating temperatures, necessitating the use of components that can function safely at temperatures up to 175 °C. Devices based on wide bandgap materials like silicon carbide (SiC) meet this requirement and are increasingly popular in such applications. However, at elevated temperatures, even SiC MOSFETs exhibit complex behavior that can be attributed to subtle variations in critical parameters like V_GS(th) (gate threshold voltage), R_DS(on) (on-resistance), I_DSS (drain-source leakage current), and I_GSS (gate-source leakage current). These variations, if not carefully accounted for, can cause power electronics systems to fail unexpectedly. Manufacturer’s device datasheets typically do not contain information detailing the interdependence of these various parameters, especially at higher operating temperatures. This article addresses this deficiency by providing guidelines on using these critical parameters when designing a SiC-based DC/DC power converter required to operate at temperatures up to 175 °C.

Image used courtesy of Adobe Stock

The SiC Advantage

SiC MOSFETs offer significant advantages over traditional Silicon MOSFETs and Insulated Gate Bipolar Transistors (IGBT) in high voltage and temperature, making them ideal for automotive, renewable energy, and industrial applications.

Engineers usually test their devices in application conditions and try to push the performance boundaries of the device to get maximum performance, keeping all the derating factors; the thermal design is one such boundary.

Table 1. Measured R_DS(on) drift versus junction temperature variations. Image used courtesy of Bodo’s Power Systems [PDF]

Nexperia comprehensively tests performance parameters using industry-standard methods. Figure 1(a) is one such setup of double pulse setup used to test the device parameters such as RDSON, V_GS(th), I_GSS, and I_DSS, and evaluates switching performance.

Figure 1. R_DS(on) comparison with competitors. Image used courtesy of Bodo’s Power Systems [PDF]

IV curves are generated using Keysight 505 Power Analyzer. To push the converter operation at a high temperature, the first parameter to consider for design is the R_DS(on) of the device. The below section will compare Nexperia devices with regards to a few competitors and variations of R_DS(on) parameters within its tight control manufacturing process to show the superior R_DS(on) stability of Nexperia devices. Figure 1 shows the variation of R_DS(on) with regard to temperature and compares it with industry competitors to understand the variation. The red line, representing the Nexperia component, shows a 38% increase in R_DS(on), while the blue lines, corresponding to competitors C and E, indicate increases of over 180% and 210%, respectively. An increase in R_DS(on) directly correlates with higher conduction power loss, as expressed by the following equation,

P_{Conduction Loss} = I² × R_DS(on) (1)

If R_DS(on) doubles, the conduction losses also double, resulting in greater heat generation within the device, potentially driving the device closer to its thermal limits and heightening the risk of failure.

Table 1 shows the R_DS(on) experimental measurement results of several 1200 V, 40 mΩ SiC MOSFETs, including Nexperia and five competitors (Comp A-E). The data reveals that Nexperia’s 40 mΩ SiC MOSFET demonstrates the most stable R_DS(on) performance across a temperature range from 25 °C to 175 °C, with increases of 1.27 and 1.55 times—lower than those of its top five competitors.

From a practical perspective, significant increases in R_DSON at elevated temperatures can greatly affect a system’s power loss and efficiency, as shown in Figure 2, efficiency measurement at high temperatures, thereby impacting its overall reliability. This R_DS(on) stability underscores Nexperia’s components' ability to maintain higher efficiency under demanding conditions.

Figure 2. Efficiency comparison between Nexperia and competitor. Image used courtesy of Bodo’s Power Systems [PDF]

Figure 3 (a) illustrates R_DS(on) behavior across temperatures, with the x-axis showing R_DS(on) in milliohms and the y-axis indicating the percentage change from the 2nd to the 98th percentile. Tests were conducted on 25 DUTs at V_GS = 15 V, covering temperatures from -55 °C to 175 °C. Each line represents a specific temperature, highlighting R_DS(on) variability. At higher temperatures, R_DS(on) stability improves, with standard deviations around 1.20 mΩ from 125 °C to 175 °C, ensuring consistent performance under thermal stress and reducing power loss risks. This high-temperature R_DS(on) stability improves power efficiency, as shown in Figure 2.

The second parameter of interest is V_GS(th). A tight control of this parameter translates into static and dynamic current sharing between different devices. Figure 3(b) offers a detailed visualization of V_GS(th) behavior across a wide temperature range (-55 °C to 175 °C), with the x-axis representing V_GS(th) values in millivolts and the y-axis depicting the percentage change in V_GS(th) from the 2nd to the 98th percentile. Each colored line in the graph corresponds to a specific temperature result, clearly comparing how R_DS(on) varies with temperature. The average value and standard deviation are labeled as Av and S. The more stable threshold voltage was found at the 175 °C test with the lowest standard deviation, S = 56.26 mV. The highest variation in V_GS(th) happened at -55 ^oC with a standard deviation of S = 85.78 mV.

Figure 3. (a) R_DSON measurement and (b) V_GS(th) measurement at -55 °C and 175 °C. Image used courtesy of Bodo’s Power Systems [PDF]

A more detailed explanation will be given in the full paper. Figure 4(a) and (b) show the ongoing testing of I_DSS and I_GSS with 75 DUTs and the notable differences of test data between the lower temperatures (-55 °C, 25 °C and up to 125 °C) and higher temperatures (150 °C or 175 °C) due to the temperature dependence of the leakage currents. At temperatures up to 150°C tests, I_DSS values are very low, < 200 nA among 72 samples, and at 175 °C, I_DSS values are between 400 nA and 800 nA, which are within device ratings. Similarly, I_GSS test data at 175 °C are <10 nA, which is within device ratings.

Figure 4. (a) I_DSS measurement and (b) I_GSS measurement from -55 °C to 175 °C. Image used courtesy of Bodo’s Power Systems [PDF]

Analyzing the dynamic switching behavior is crucial for evaluating the performance of the devices at 175 °C. To achieve this, the devices listed in Table 1 were tested using a double pulse configuration, with their respective recommended gate-to-source voltage levels and external gate resistances (RGext) as specified in the datasheets. Figure 5 presents typical turn-on and turn-off waveforms for Nexperia’s 40 mΩ device.

Figure 5. DPT Turn-on and turn-off transitions of Nexperia’s devices. Image used courtesy of Bodo’s Power Systems [PDF]

Conclusions and Future Work

At elevated temperatures, mainly 150 °C or 175 °C, Nexperia’s 1200 V SiC MOSFETs demonstrate R_DS(on) stability, low variations in V_GS(th), I_GSS, and I_DSS, lower switching losses, and higher efficiency in the DC/DC converter shown in Figure 2. This consistency is particularly beneficial in demanding applications such as electric vehicle traction inverters, aerospace power systems, power grids, industrial motor drives, and other high-temperature scenarios where performance stability is paramount.

Ongoing testing for 17, 30, 60, and 80 mΩ, 1200 V SiC MOSFETs includes static characteristics, dynamic switching, and DC/DC converter testing to show efficiency improvement at 175 °C. The goal is to build a comprehensive static and dynamic performance dataset. This analysis will guide further optimization of these devices, making them suited for high-temperature applications where consistent performance, power efficiency, and reliability are paramount.

This article originally appeared in Bodo’s Power Systems [PDF] magazine.