Power Electronics Course with QSPICE (Episode 20): SiC MOSFETs and Temperature

In recent years, silicon carbide (SiC) MOSFETs have transformed the power electronics industry thanks to their high efficiency, fast switching speed, and ability to operate at high voltages. Due to their high switching speed, they reduce the size of inductors and transformers, increasing the overall power density of the system. They are widely used in charging stations, high-power DC/DC converters, power supplies, inverters, and drives. However, SiC MOSFETs are affected by operating temperature as well. This affects several electrical parameters, including conduction resistance, losses, switching speed, and thermal noise. Understanding and simulating these effects is important for designing reliable, efficient converters.the

How temperature affects electrical circuits

Temperature is one of the main factors that negatively affect the operation of electrical circuits. Some adverse effects related to increases in temperature include:

Changing semiconductor characteristics
Varying resistivity of materials
Increased electrical noise
Increased losses
Decreased reliability and safety

For all of these reasons, thermal simulation has become an integral and indispensable part of power circuit design. Now, we will examine some cases in which temperature affects not only the properties of the electronic component but also the operation of the entire system.

Temperature effects on R_DS(on)

One of the most important parameters of a MOSFET is its resistance during R_DS(on) conduction. Measuring R_DS(on) is quite simple, especially under static operating conditions. The circuit diagram in Figure 1 uses the UF3C065030T3S MOSFET, a powerful device with the following characteristics:

N-channel FET
TO-220-3 package
Drain-source voltage: 650 V
R_DS(on): 27 mΩ
Gate-source voltage: −25 V to 25 V
Continuous drain current (I_d) (case temperature, or T_c, = 25°C): 85 A
I_d (T_c = 100°C): 62 A
Pulsed drain current (T_c = 25°C): 230 A
Power dissipation (T_c = 25°C): 441 W
Maximum operating temperature: 175°C

*Figure 1: A generic application of the SiC MOSFET (Source: Giovanni Di Maria)*

The electrical diagram describes a static test of the SiC MOSFET to analyze its conduction behavior and losses. This circuit is fairly useful for measuring the R_DS(on) of the MOSFET, studying the effect of temperature, and comparing SiC MOSFETs. It is powered at 48 V with a 4-Ω resistive load connected to the drain of the MOSFET. The latter, as mentioned, is the UF3C065030T3S model.

The gate drive is 25 V, as specified in the official datasheet. With a gate-source voltage of 25 V, the MOSFET is fully turned on. The current on the MOSFET (and on the load) is approximately:

Because the MOSFET’s R_DS(on) is not zero, the actual current will be slightly lower, about 11.8 A. The diagram uses the following SPICE directives:

.tran 1 starts the 1-second transient simulation.
.option SAVEPOWERS=1 displays the power dissipated by the components.
.option MAXSTEP=1µ sets an integration step of 1 µs.
.option TEMP=27 sets the simulation temperature to 27°C.
.plot V(drain)/i(r1) displays the ratio of drain voltage to load current. In practice, this corresponds to R_DS(on), which, according to the simulation results, is fully consistent with the official datasheet.

Based on these settings, the circuit measurements are as follows:

Temperature: 27°C
V_CC: 48 V
V(drain): 789.166 mV
V(gate): 25 V
I(drain) (I(R1)) = 11.8027 A
P(R1): 557.216 W
P(MOSFET): 9.31429 W only
Efficiency: 99.36%

At approximately 12 A, even a few milliohms of R_DS(on) can produce dissipation that becomes significant, depending on the application.

The next simulation allows us to observe the R_DS(on) value as a function of temperature, ranging from –55°C to 150°C. As temperature increases, R_DS(on) also increases, reducing circuit efficiency because conduction losses rise. Figure 2 shows the graph of the R_DS(on) of this specific MOSFET as the operating temperature changes.

*Figure 2: Graph of R_DS(on)* *and circuit efficiency as a function of operating temperature (Source: Giovanni Di Maria)*

The circuit efficiency can also be observed, which, as expected, decreases as the temperature increases. At 25°C, the R_DS(on) matches the manufacturer-specified value, and at 150°C, it can even double. The graph, therefore, highlights the influence of temperature on the performance of a UF3C065030T3S SiC MOSFET under static conduction conditions.

The upper graph shows the V(drain)/I(R1) ratio, which represents the equivalent drain-source resistance and corresponds to the R_DS(on) of the device. Resistance increases almost linearly with temperature, from about 12 mΩ at –55°C to over 50 mΩ at 175°C. This behavior is typical of MOSFETs, where charge-carrier mobility decreases as temperature increases, increasing conduction resistance.

The lower graph shows the efficiency of the circuit as a function of temperature. As the temperature increases, the percentage decreases, going from approximately 99.7% to about 98.8%. This is because R_DS(on) increases, so the MOSFET dissipates more power as heat. While this efficiency decrease may seem small, when thousands of watts are involved, the issue could become quite significant.

The results confirm that the device operates with excellent performance even at the most extreme temperatures. Although R_DS(on) increases more than fourfold between –55°C and 175°C, the MOSFET continues to dissipate only a small fraction of the total power, leaving over 98% of the available energy to the load. This confirms the high efficiency of SiC MOSFETs and their suitability for power applications. To perform the analysis over a range of temperatures, the following SPICE directives were used:

.STEP LIN PARAM TT -55 175 1
.option MAXSTEP=10µ
.option TEMP=TT

The first directive performs a linear parametric sweep of the TT variable from –55°C to 175°C in 1°C increments. A total of 231 consecutive simulations are executed, one for each temperature value. This allows for a simple, high-resolution observation of the evolution of the MOSFET’s electrical parameters across its entire operating range.

The second directive sets the transient simulation resolution to 10 µs. The third directive links the simulation temperature to the TT parameter defined by the .STEP directive. For each simulation, the circuit is recalculated at the corresponding temperature of the current cycle. All models that include thermal dependence automatically update their internal parameters accordingly.

Thermal models

In a traditional MOSFET “.subckt” model, the usual “d,” “g,” and “s” terminals represent the “drain,” “gate,” and “source” pins, respectively. Recently, models that represent the component’s temperature as an electrical voltage have also been implemented. This is the only way to obtain this parameter using the SPICE language, which does not natively include temperature.

The new models include two additional terminals, “T_j” and “T_c,” which monitor the junction and case temperature as if they were voltage. It is assumed that 1 V corresponds to 1°C. Below is an excerpt from the MOSFET SPICE model. Note the presence of the five terminals on the first line:

.subckt C2M0025120D d g s Tj Tc
xgmos	d1 g1 s1 Tj Tc gmos_C2M0025120D 
Ls 		s	s1	9n
R_Ls 		s	s1      10
Rg 		g1	g2	1.1
Lg 		g	g3	15n
R_Lg		g	g3	10
Rgg		g1	s1	1E6

The circuit diagram in Figure 3 implements this model in QSPICE. The MOSFET used is a C2M0025120D. The T_c pin is connected to a constant voltage representing the case temperature. The goal is to observe how the circuit affects T_j. Resistor R2 represents the thermal resistance between the case and the environment, and its value is set low to complete the simulation quickly before reaching equilibrium. The MOSFET is driven by a 1-kHz rectangular voltage.

*Figure 3: The circuit diagram uses a thermal model. (Source: Giovanni Di Maria)*

Figure 4 reveals what happens during the simulation transient. The graphs show the thermal model’s behavior. In the upper graph, the gate voltage switches between 0 V and 15 V at 1 kHz with a 50% duty cycle. The MOSFET therefore alternates between conduction and cutoff phases. In the lower graph, T_c is shown in green, while T_j is shown in blue. Note that T_j is always greater than T_c because the heat originates in the junction and then flows to the case.

With the load used, the difference between the two temperatures is quite small, about 0.3°C. Both temperatures start at about 31°C and gradually increase until they stabilize. The small sawtooth oscillations on the blue curve are due to the MOSFET’s on/off cycle: During the on state, the junction heats up slightly, and during the off state, it cools down.

Temperature graph of the MOSFET’s case and junction. — *Figure 4: Temperature graph of the MOSFET’s case and junction (Source: Giovanni Di Maria)*

Reducing the load resistance significantly changes the operating conditions, and the MOSFET is obviously subjected to greater stress (see FEM simulation in Figure 5). The table below confirms this. Up to a load of 8 Ω, the MOSFET remains cool. Then, as the load decreases and the current increases significantly, the junction temperature rises rapidly.

Note that the case temperature has been “forced” to remain stable, but in reality, it rises quickly, depending on the thermal resistance between the case and the heatsink and between the heatsink and the environment.

Load (Ω)	Current (A)	T_j (°C)	T_c (°C)	Temperature Difference (°C)
1,000	0.024	31	31	0
470	0.051	31	31	0
100	0.236	31	31	0
22	1	31	31	0
8	2.94	31.1	31	0.1
4	5.9	31.5	31	0.5
2	11.70	33.2	31.08	2.12
1	22.9	40.75	31.35	9.4
0.5	43.41	79.78	32.77	47.01

With this example, we have seen how T_j varies over time by applying a voltage to the T_c terminal. If, however, we want to observe the MOSFET’s electrical effects at different junction temperatures, we can apply a fixed voltage to the T_j terminal and leave the T_c terminal disconnected. In any case, it is always essential to connect one of the T_c or T_j terminals; otherwise, the model will not work.

FEM simulation of the temperature rise of a MOSFET in a TO-220 package. — *Figure 5: FEM simulation of the temperature rise of a MOSFET in a TO-220 package (Source: Giovanni Di Maria)*

Monitoring junction heating

From the analysis presented in this installment of the course, we have clearly shown that temperature is not a simple environmental parameter but a dynamic variable that significantly affects the performance of the SiC MOSFET (as well as other components). R_DS(on) can quadruple between –55°C and 175°C, reducing system efficiency and increasing losses in high-power applications, where small percentage changes translate into significant power losses. Simulation with QSPICE thus confirms its importance as an indispensable tool, not only for controlling electrical operation but also for predicting the device’s thermal behavior. Advanced models with thermal terminals let designers monitor junction heating in real time.

How temperature affects electrical circuits

Temperature effects on RDS(on)

Thermal models

Monitoring junction heating

Temperature effects on R_DS(on)