A SiC Diode Test Procedure

In comparison to conventional silicon-based semiconductors, the new silicon carbide (SiC) semiconductor material provides improved thermal conductivity.

The fundamental components of power electronics will change over the next few years as wide bandgap semiconductor technologies gain in popularity. In comparison to conventional silicon-based semiconductors, the new silicon carbide (SiC) semiconductor material provides improved thermal conductivity, a higher switching speed, and physically smaller devices. In this article, We will learn how to measure a SiC MOSFET.

SiC components

More than ten years ago, the first SiC Schottky diodes for commercial use were released. Since then, these components have been used in numerous power supply systems. The SiC power switches, Jfet, Bjt, and Mosfet, were placed after the diodes. There are currently SiC switches available with breakdown voltages of 600-1700 V and rated currents of 1 A-60 A. Designers are becoming more and more fond of SiC Mosfets.

SiC diodes

Simple diodes were the only SiC devices that were first commercially available, but as the technology of the material has advanced, JFETs, MOSFETs, and bipolar transistors have been made possible. Schottky diodes made of silicon carbide (SiC) provide better switching performance, greater efficiency, greater power density, and reduced system costs. These diodes have a positive temperature coefficient, 0% reverse recovery, low forward voltage drop, current stability, and strong surge voltage capacity. The new diodes are intended for power converter designers working on projects such as photovoltaic solar inverters, EV chargers, power supply, and automotive applications. The majority of silicon carbide diodes are Schottky diodes. In comparison to what silicon is capable of, it has a significantly lower leakage current and a significantly higher doping. Behavior at high temperatures is a crucial trait since it affects how silicon behaves directly as temperature rises. SiC, on the other hand, is still restricted to a confined area. A strong and dependable substance is silicon carbide.

Let’s examine a SiC diode

A good SiC diode to start testing is the SCS205KG model, a SiC Schottky Barrier Diode, by ROHM (figure 1). Its most important characteristics are:

• Vr: 1200 V;
• If: 5 A (at +150° C);
• Surge nonrepetitive forward current: 23 A (PW=10ms sinusoidal, Tj=+25°C);
• Surge nonrepetitive forward current: 17 A (PW=10ms sinusoidal, Tj=+150°C);
• Surge nonrepetitive forward current: 80 A (PW=10us square, Tj=+25°C);
• Total power dissipation: 88 W;
• Junction temperature: +175° C;
• TO-220AC package.

It’s a very robust component, with a short recovery time and high-speed switching. Its official SPICE model allows to simulate the component at any conditions.

* SCS205KG
* SiC Schottky Barrier Diode model
* 1200V 5A
* Model Generated by ROHM
* All Rights Reserved
* Date: 2015/11/16
*****************A C
.SUBCKT SCS205KG 1 2
.PARAM T0=25
.FUNC R1(I) {40.48m*I*EXP((TEMP-T0)/155.8)}
.FUNC I1(V) {2.102f*(EXP(V/0.02760/EXP((TEMP-T0)/405.3))-1)*
+ EXP((TEMP-T0)/7.850*EXP((TEMP-T0)/-601.3))}
.FUNC I2(V) {TANH(V/0.1)*(710.4p*EXP(-V/198.3)*EXP((TEMP-T0)/54.40)+
+ 26.02f*EXP(-V/63.22/EXP((TEMP-T0)/178.9))*
+ EXP((TEMP-T0)/8.493*EXP((TEMP-T0)/-600)))}
V1 1 3 0
E1 3 4 VALUE={R1(MIN(MAX(I(V1)/0.5,-500k),500k))}
V2 4 5 0
C1 5 2 0.5p
G1 4 2 VALUE={0.5*(I1(MIN(MAX(V(4,2),-5k),5))+I2(MIN(MAX(V(4,2),-5k),5)))+
+ I(V2)*(913.9*(MAX(V(4,2),0.5607)-0.5607)+
+ 727.2*(1-360.9*TANH(MIN(V(4,2),0.5607)/360.9)/1.121)**-0.4987)}
R1 4 2 1T
.ENDS SCS205KG

Forward voltage

The first measurement operation concerns the forward voltage of the SiC diode. Let’s take a look at figure 2, in which we can see the simple electric circuit of the test, its three-dimensional representation and the excerpt of the datasheet of the component concerning the forward voltage, at the various temperatures.

The Schottky SCS205KG SiC diode is linked in series with a fairly strong resistance of roughly 6.7 ohms in the test wiring diagram, which is sized to allow a current of 5 A to flow through the circuit. A 36 V supply voltage has been specified. In order to imitate a single 6.7 ohm resistor, we used 10 67 ohm resistors connected in parallel in order to better optimize the dissipation and distribute the heat. Each resistance must have a minimum power of 20 W. The following voltages across the component, at the various working temperatures, are determined by the Schottky diode SCS205KG’s datasheet:

If=5A, Tj=+25° C: 1.4 V;
If=5A, Tj=+150° C: 1.8 V;
If=5A, Tj=+175° C: 1.9 V.

These properties make it possible to see how the temperature has a significant impact on the voltage across the diode. As a result, the designer must make every effort to keep this voltage fluctuation under control because doing otherwise could alter the system’s behavior. Let’s use the SPICE directive to run a DC Sweep simulation, which measures the voltage across the power diode at a temperature between 0 and 200 degrees Celsius:

.DC temp 0 200 25

The simulation returns the following voltage values on the diode at different temperatures, fully confirming the indications provided by the datasheet. The colored cells contain the temperature of test reported in the document.

According to figure 3, the green graph displays the diode’s anode’s fixed 36 V voltage whereas the yellow graph displays the cathode’s voltage in relation to temperature. The “Forward voltage” is this potential difference. The algebraic difference between the voltages of the anode and the cathode allows us to view the difference in the component’s potential on the same figure. Only a few seconds must be used for this test.

Reverse current

The third measurement operation concerns the reverse current of the SiC diode. Let’s take a look at figure 4, in which there are the simple electric circuit of the test, its three-dimensional representation and the excerpt of the datasheet of the component that concerns the reverse current, at the various temperatures.

The electric diagram contains the Schottky SCS205KG SiC diode connected in series with a very low resistance of about 0.1 ohms. The supply voltage is a sinusoidal source set to 1200 V. The diode is connected in reverse mode. For this test we can perform a DC simulation (sweep), which involves the measure of the reverse current flowing across the diode, in the domain on the temperatures between +20° C and +200° C, using the SPICE directive:

.DC TEMP 20 200 1

In figure 5, the graph shows the very little reverse current flowing on the diode, according to the temperature.

Conclusions

The SiC diode is unique in that it recovers very quickly. As a result, it is possible to achieve higher switching rates and smaller magnetic and other passive component sizes. In final items, higher power densities are achievable. For power switching applications, they also offer significant advantages in terms of efficiency and thermal performance. These components can operate in hotter conditions. The temperature, which is an important factor, considerably affects the operating conditions of the electronic components. To evaluate the efficiency and worth of the simulators, especially the SPICE models, real tests (using real SiC components) and simulations can be interesting to run.

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