Talk about the short circuit capability of SiC MOSFET

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Talk about the short circuit capability of SiC MOSFET

Posted Date: 2024-01-31

In many applications of power electronics, such as motor drives, short circuit conditions sometimes occur. This requires the power device to have a certain short-circuit capability, that is, it can withstand the short-circuit current within a certain period of time without being damaged.

Currently, most IGBTs on the market have short-circuit capabilities marked in their data sheets, most of which are between 5 and 10 us. For example, the short-circuit time of Infineon IGBT3/4 is 10 us, and the short-circuit time of IGBT7 is 8 us.

Most SiC MOSFETs are not marked with short-circuit capability, and even if they are, it is relatively short. For example, the nominal short-circuit time of Infineon's CoolSiCTM MOSFET single-tube packaged devices is 3us, and the nominal short-circuit time of EASY packaged devices is 2us.

Why are the short circuit capabilities of IGBT and SiC MOSFET so different? Is this an inherent defect of SiC? Today we will briefly analyze it.

Let's take IGBT as an example. Let's take a look at what happens inside the power device when there is a short circuit?

When the power device operates normally, it is in the saturation zone and the CE voltage is very low. At this time, the device current increases as the CE voltage increases. As the CE voltage further increases, the inversion layer channel is pinched off, and the device current remains relatively stable and no longer rises as the CE voltage rises. This is called exiting the saturation zone. On the output characteristic curve of IGBT, we can see obvious desaturation phenomenon.

(For a more detailed analysis of IGBT desaturation characteristics, please refer to How to Understand IGBT Desaturation Phenomenon and Safe Working Area)

(a) IGBT works in saturation zone

(b) IGBT exits the saturation zone and the channel is pinched off

IGBT output characteristic curve

Some SiC MOSFETs do not have short circuit capability. Is it because they have no desaturation characteristics? No, SiC MOSFET also has desaturation characteristics, but for MOSFET, the naming method of the working area is exactly opposite to that of IGBT. The normal working state is the linear area. When the voltage between DS rises to a certain level, the channel is pinched off, and the current becomes smaller as the DS voltage rises. At this time, the MOSFET enters the saturation zone. However, judging from the output characteristics, for SiC MOSFET, the inflection point into saturation is not obvious. The inflection point of SiC MOSFET entering the saturation region is not very obvious, and it is related to DIBL (Drain Induced Barrier Lowering Effect). Readers who are interested in learning about it can click on this article: Short Channel Effect of SiC MOSFET

Let’s take the following figure as an example to illustrate the first-class short-circuit process of SiC MOSFET. This is the short circuit waveform of two 45mΩ 1200V CoolSiC™ MOSFETs: one in a 4-pin TO-247 package and the other in a 3-pin TO-247 package. The figure shows the situation of the two under a DC voltage of VDS=800V.

When a short circuit first occurs, the drain current rises rapidly and reaches a peak value soon. Due to the reduced feedback loop in the Kelvin source design, the current in the 4-pin TO-247 packaged MOSFET rises faster and it also shows less self-heating at the onset of a short circuit event, with a high peak current of over 300A. In contrast, the device in the 3-pin TO-247 package shows smaller peak current. The main reason for this situation is that di/dt acts on the stray inductance in the power loop of the 3-pin component, and the instantaneous voltage generated produces negative feedback to VGS, thus reducing the switching speed. Subsequently, the short-circuit current caused the junction temperature of the SiC MOSFET chip to rise, and the channel mobility μn decreased accordingly. At the same time, the JFET effect was superimposed, causing the short-circuit current to begin to decrease after the peak value, and the drain current dropped to about 150A until it was turned off. Test waveforms demonstrate the typical 3µs short-circuit capability of TO-247 CoolSiC™ MOSFETs in both packages. For power modules, the current short-circuit capability is up to 2μs, depending on the relevant target application requirements. Our CoolSiC™ MOSFETs are the first devices to have a guaranteed short-circuit withstand time in the datasheet.

In the TO247 3pin package IMW120R030M1H, the definition of short circuit time:

In the FF33MR12W1M1H packaged in EASY, the definition of short-circuit time is:

The short circuit time of most IGBTs is 5~10μs, and the short circuit time of SiC MOSFET devices is relatively low. The main reasons are as follows:

1. Through the above analysis, we can see that when the power device is in a short-circuit state, the short-circuit current is relatively constant. For IGBT, the short-circuit current is generally 4 to 6 times the rated current, while the short-circuit current of SiC MOSFET can generally reach 10 times the rated current. This can be seen from the output characteristic curves of the two.

2. When the power device is short-circuited, the device withstands the bus voltage and the electric field is distributed throughout the drift zone. Because the critical electric field strength of SiC material is about 10 times that of Si material, to achieve the same withstand voltage level, the SiC MOSFETI drift region only needs one-tenth that of SiIGBT. This means that when the SiC MOSFET is short-circuited, the heat generated is more concentrated and the temperature is higher.

3. The chip area of ​​SiC MOSFET is smaller than that of IGBT with the same current level, the current density is higher and the heat is more concentrated.

To sum up, the characteristics of SiC MOSFET such as small area, high short-circuit current, and thin drift layer lead to concentrated heat generation during short circuit. Compared with IGBT, the short-circuit time is relatively shorter.

Is the short circuit capability of SiC MOSFET necessarily inferior to IGBT? Not really. The short-circuit capability of power devices is designed, and the short-circuit capability needs to be compromised with other properties. For example, if the device channel density is increased, the on-resistance of MOSFET will decrease, but correspondingly, the current density will be higher and the short-circuit current will be larger, so the short-circuit time will decrease.

In addition to on-resistance, SiC MOSFET short-circuit capability design also needs to consider factors such as withstand voltage, loss, and lifespan. You can design a device with extremely low loss but no short-circuit capability, or you can sacrifice a little performance to make the device have short-circuit capability, thereby improving the reliability of the overall system. Which direction to choose and what kind of performance the device ultimately exhibits are all the result of trade-offs for the target application.

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