The SiC MOSFET (or SiC Power MOSFET) is gaining popularity due to its properties that make it very useful for many applications. It has an extremely low Rdson (turn-on resistance) and incredible switching speed which makes it possible to achieve very high switching frequencies, reducing power losses and overall system size and costs.

 

However, the SiC MOSFET also has one less desirable property:its gate driver needs to be fast and able to deliver high output current to charge the gate of the SiC MOSFET quickly.

 

The gate driver is responsible for turning on and off the MOSFET by applying the right voltage signal at the gate. That said, what are the most important parameters that you should consider when choosing a SiC MOSFET gate driver?

 

Things To Consider When Choosing A Gate Driver

 

First of all, you have to check the datasheet of the SiC MOSFET you want to use for the optimum gate source voltage. Now, the most common scenario is when you want to use a SiC MOSFET with an n-channel enhancement mode channel (n-CHIP), then your gate source voltage should be 15V or 18V.

 

Now, let’s say that you understand what are the tolerable limits for your application, how can you then choose the best SiC driver?

 

Voltage rating or current rating for the gate driver is meaningless without some parameters that relate to the switching frequency of your application.

 

For instance, if you plan on using a SiC MOSFET in some low power consumer electronics device (e.g., an amplifier), you will not need to drive it at high enough frequency, so the worst case will be when you are driving it to turn on or off once per 2s or more.

 

Then you should consider how much power your driver needs to dissipate in order to keep itself cool during operation. The permissible dissipation depends largely on the ambient temperature.

 

For example, if you are using a gate driver that dissipates, let’s say 100mW at room temperature (25°C), then it needs to be able to dissipate more than 200mW if you are operating the same MOSFET sample in an environment with 50°C. The same driver should be able to further withstand more than 500mW of dissipation if you are operating it at 100°C.

 

If the ambient temperature is high, remember that a heat sink compound can significantly reduce the thermal resistance between your MOSFET’s TO-247 package and the heat sink, which will lower the chip junction temperature, allowing for more power dissipation before something bad happens to your part. Similarly for the gate driver, ground planes can be used to help reduce the thermal resistance between the gate driver and ambient.

 

Now, let’s say that you have chosen your SiC MOSFET and its gate driver,  how can you ensure that both of them are safe in your application?

 

One thing to look for is the gate driver’s protection capability when the SiC MOSFET goes into fault condition. In this case, the driver must not be destroyed, but instead would react in a certain manner to protect MOSFET.

 

To do that, your SiC driver may need some additional circuitry for overcurrent or short circuit protection, to improve the performance and reliability of the system. As SiC MOSFET switches much faster than conventional power switches, the rule of thumb is that it should be shut down with 1 to 2s during short circuit. Else the short circuit current will increase 4 to 5 times its operating ratings very quickly and lead to catastrophic failure.

 

A SiC driver with protection feature is able to detect the fast increase short circuit current beyond its operating rating. This is usually done by measuring the drain and source voltage of the MOSFET and relating it to the operating drain current. When the drain increases beyond its rating, an abnormal drain and source voltage will be able to trigger a fault threshold voltage defined by the gate driver and trigger a control manner of shut down.

 

A control manner of shut down is important as the SiC MOSFET is usually connected to an inductor load. If the high short circuit current is shutdown immediately, it will cause other complications like ringing which can exceed the maximum voltage rating of MOSFET.

 

If you follow these design tips, then you can be sure that both your SiC MOSFET and gate driver will operate properly throughout the entire life of the application.

 

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