Silicon carbide operates at higher voltages than silicon
SiC MOSFETs enable high-speed switching at high voltages
Infineon SiC MOSFETs have higher threshold voltage than competitors


As power consumption increases, the demand for small, high-efficiency inverters is gradually increasing. Accordingly, developers designing inverters are looking for power devices with low losses and high current density per area.

Silicon carbide (SiC) is a semiconductor material composed of silicon (Si) and carbon (C).

It operates at high voltages because its dielectric breakdown field strength is 10 times higher than that of Si, and its band gap is 3 times wider so it maintains its properties regardless of temperature. Its thermal conductivity is 3 times higher so less energy is required for cooling.

Based on the above features, SiC MOSFETs can replace Si-based MOSFETs as well as IGBTs.

SiC MOSFETs, which are being given priority review in fields such as solar power generation (PV) power storage devices (ESS) and electric vehicle chargers that emphasize efficiency, are seeing their merits increase as their prices fall due to the advancement of related technologies and the expansion of production facilities.

We met with Geon-Ho Lee, Deputy Manager in charge of industrial semiconductor technology support at Infineon Technologies Korea, and asked him about what developers need to know before applying SiC MOSFETs to inverter designs.


Q. What methods is the industry devising to design high-efficiency inverters?
A. Since the loss of semiconductor devices used in inverter systems is directly related to the efficiency of the system, Infineon is focusing on developing high-efficiency devices with low losses. Infineon’s CoolSiC MOSFET based on SiC is a high-efficiency inverter device with low losses.


Q. What are the advantages of SiC MOSFETs over Si-based MOSFETs and IGBTs?
A. Si IGBTs have a breakdown voltage of 600 V, 1200 V, or even higher, but their switching loss is high, making it difficult to use high switching frequencies. Si MOSFETs have no problem with high-speed switching, but there are no products that support very high breakdown voltages. As far as I know, there are currently only product groups that support breakdown voltages up to 900 V.

SiC MOSFETs can perform high-speed switching at high voltages such as 1200 V. They have lower switching losses than IGBTs, so they can use high switching frequencies. Although SiC devices are more expensive than Si devices, as the system frequency increases, the size of other analog devices such as reactors can be reduced, which can actually save more costs for the entire system.


Q. What are the things to keep in mind when designing a high-efficiency inverter using SiC MOSFETs?
A. Basically, high-speed switching devices have fast di/dt and dv/dt issues. dv/dt can cause gate side malfunction, and di/dt generates high spike voltage at turn-off. Therefore, these aspects should be given special consideration when designing.


Q. How does Infineon overcome the Miller effect that occurs in high-speed switching devices?
A. Generally, three methods are used to overcome the Miller effect.

The first method is to increase the gate on-resistance and decrease the off-resistance. However, the off-resistance also needs to be increased for the turn-off spike voltage, so the overall gate resistance tends to increase to satisfy both requirements.

The second method is to use a negative voltage. In addition to the usual method of using 15V, 0V, you can also use -15V to -8V or a higher voltage, but SiC MOSFETs have an issue where the threshold voltage shifts when a large negative voltage is used. This is a common issue not only with Infineon devices but with all SiC MOSFETs. Therefore, you cannot simply increase the negative voltage.

The third method is to use a driver IC with active clamp miller function. This method helps to some extent in reducing the miller effect, but it is not a solution, so usually all three methods should be used.


Q. What are the strengths of Infineon’s CoolSiC portfolio compared to other companies?
A. There are three main types. The first is a short circuit guarantee. Infineon guarantees the time that the device can withstand a short circuit in the data sheet. There is a big difference in whether the data sheet guarantees a short circuit or not.

The second is that the threshold voltage is higher than that of other companies. If the threshold voltage is 2V, any parasitic noise will turn on again when it exceeds 2V, but Infineon products are generally 4.5V, which is 1~2V higher than other companies.

Because the threshold voltage is high, the three methods of overcoming the Miller effect described above can be used less frequently. By using a small resistor instead of a large one, you can gain from the loss, and the lifespan is also advantageous when using a small negative voltage.

Thirdly, all SiC MOSFETs have a threshold voltage shift issue. The higher the frequency, the larger the negative voltage, the more acceleration occurs, so Infineon suggests a safe area.

A third advantage is that it is documented up front that the desired lifetime can be met using a certain number of V at a certain switching frequency, so that the designer can take this into account.


Q. What benefits can engineers gain by using CoolSiC MOSFETs and EiceDRIVER ICs together?
A. Basically, there are two ways to protect power devices: using a shunt resistor and using the Desat function of the driver IC. Shunt resistors are not suitable for designs that aim for high efficiency because of the losses that occur in the resistor itself.

For the DSET function, most drive ICs have a filter time of 300 to 400 ns, so if noise of 0.3 ms or 0.4 ms occurs, the protection function may malfunction. On the other hand, Infineon's EiceDRIVER IC allows users to select the desired filter time from 0.5 ms to 4 ms, making it suitable for SiC MOSFETs and high-speed switching elements.


Q. Please tell us about Infineon’s future goals and plans in the field of SiC MOSFETs.
A. Infineon is constantly releasing high-efficiency, low-loss devices and driver ICs that match them. Infineon will continue to provide technical support online and offline so that those who are new to these devices can design products without difficulty.