Third-Generation Power Electronics Semiconductor SiC MOSFET: Focus on Efficient Driving Solutions

Compared to traditional silicon MOSFETs, SiC MOSFETs can achieve high-frequency switching under high voltage. In fields such as new energy, electric vehicles, and industrial automation, SiC MOSFETs (Silicon Carbide-Metal Oxide Semiconductor Field Effect Transistors) are highly regarded for their outstanding performance in high frequency, high power, and low losses, making SiC MOSFET driving solutions highly sought after. However, the unique characteristics of SiC MOSFETs also mean that they have special requirements for gate drive circuits.

This article will focus on understanding the driving solutions for SiC MOSFETs, including over-current and over-voltage protection, and how to select the right driver chip for SiC MOSFETs.

SiC MOSFET Driving Protection

1. Overvoltage Protection

a) Drain-Source Overvoltage Protection

In practical applications of SiC MOSFETs, overvoltage may occur at the drain-source, typically in the following two scenarios:

The first scenario is in applications such as electric vehicles and power systems. When the bus voltage is high and unstable, the voltage of the main circuit of the power electronic converter may exceed the rated voltage of the SiC MOSFET drain-source, leading to breakdown and damage of the device.

Therefore, in practical applications, to ensure safety, it is necessary to consider reserving a certain margin.

The second scenario occurs when the SiC MOSFET is turned off. The rate of change of drain current (di/dt) is high, which will generate a voltage on the circuit loop parasitic inductance, superimposed with the bus voltage at the drain-source of the SiC MOSFET, resulting in a large voltage overshoot at the drain-source, which can exceed the device's safe voltage, leading to device damage.

Therefore, unstable DC bus voltage and voltage overshoot at the drain-source are the main factors causing drain-source overvoltage.

To protect the device and ensure the safe operation of the converter, in some high-power applications, the commonly used protection measures for drain-source overvoltage are:

1. For unstable DC voltage: reduce the rated voltage method;

2. For overvoltage caused by stray inductance in the circuit and large current changes: passive buffer circuits or active clamping circuits are often used for protection. (The diagram shows an RC buffer circuit)

b) Gate-Source Overvoltage Protection

The main causes of gate-source overvoltage of SiC MOSFETs (Silicon Carbide Metal-Oxide-Semiconductor Field-Effect Transistors) can be divided into the following two points:

1. Instability of the driving circuit, resulting in the output voltage exceeding the gate-source voltage;

2. When SiC MOSFETs are used in bridge circuits, under the transient of one switch tube, the gate-source voltage of the other switch tube may exceed the gate-source turn-on voltage or the reverse safe voltage.

To ensure the normal operation of SiC MOSFETs, the gate-source voltage should generally be controlled within the range of -10V to 25V. If the voltage exceeds this range, it may cause permanent damage to the SiC MOSFET.

To avoid such situations, the gate-source of the SiC MOSFET should be equipped with protection measures: for example, using the traditional method of parallel capacitors at the gate-source to ensure that the gate-source voltage remains within the allowable range.

2. Overcurrent Protection

Overcurrent fault refers to the situation where the leakage current of the SiC MOSFET exceeds the rated value due to abnormal control signals at the load end, causing device damage.

Based on the overcurrent fault of the SiC MOSFET, the current value is a multiple of the rated current. Overcurrent faults can be divided into overload faults and short-circuit faults.

a. Overload Fault

Refers to the fault that occurs when the output of the electronic device where the SiC MOSFET is located exceeds the rated value of the load, and the current value of the SiC MOSFET is about 1.4 times the rated current.

When the SiC MOSFET is in an overload and overcurrent fault condition, the current change is small, and the device can withstand a relatively long time.

b. Short-Circuit Fault

Refers to the fault that occurs when the load is short-circuited or in a bridge circuit structure, and the upper and lower tubes are almost simultaneously conductive, at which time the current value of the SiC MOSFET will rapidly increase to about 9 times its rated current.

In this case, due to the large current quickly passing through the device, the SiC MOSFET can only withstand a short time. Therefore, a safe, reliable, and fast current protection scheme is required:

The simplest method is to use a current detection method with a shunt resistor in the circuit, which detects the current by connecting a resistor in series in the circuit. This method is simple and can be freely selected for use in any system. At the same time, in order to minimize the impact on the circuit and reduce its own power dissipation, the impedance value of the shunt resistor is generally very low.

So, how to choose the right driver chip for SiC MOSFETs?

Several aspects need to be considered:

1. Requirements for driving voltage level and driving current

When selecting SiC MOS devices, priority should be given to driver chips with a larger peak output current. At the same time, if the output pulse has a faster rise and fall speed, the driving effect will be better. This means that the rise and fall time parameters of the driver chip should be small.

2. Meet the short dead time to ensure that the inverter system has higher output voltage quality;

3. The protection functions of the chip: short-circuit protection & active Miller clamp

a. Utilize the short-circuit tolerance protection function of the SiC MOSFET to improve system reliability;

b. Use a driver chip with an active Miller clamp function to ensure that it does not mis-trigger during turn-off due to the Miller effect;

4. Chip anti-interference (CMTI) performance. In high-frequency application environments, the chip itself requires high anti-interference ability.

Of course, selecting SiC MOSFETs with better performance is also an important factor in high-frequency drive applications. VBsemi's new SiC MOSFET products have very low switching and conduction losses, making low-loss characteristics possible, thanks to the relatively stable relationship between RDS (on-state resistance) and temperature. In particular, suppressing gate mis-triggering caused by parasitic capacitance enhances the robustness of the device, not only beneficial for reducing switching losses but also improving the usability of the product.