SiC features 10x the breakdown field strength compared with silicon, resulting in breakdown voltages in the thousands of volts. In addition, ON resistance per unit area is considerably lower, significantly improving power loss. In order to minimize the increase in ON resistance associated with higher breakdown voltages, silicon IGBTs are primarily used. However, SiC has been shown to provide superior high-speed switching performance.
A reference board for the gate drive circuit is available (drive board designed for direct attachment to SiC modules). When connected in parallel with SiC MOSFETs, the external gate resistance is connected to each MOSFET to achieve gate signal balance.
The effects of parasitic capacitance and inductance on the board can be considered a type of LC resonance. Please confirm the following items. 1) External resistance connected to the gate drive circuit 2) Output capacitance of the gate drive circuit 3) Parasitic inductance of the gate drive circuit wiring 4) SiC MOSFET gate capacitance 5) Internal gate resistance of hte SiC MOSFET etc. When the resistance is small the overshoot/undershoot peak value will increase and prolong the ringing decay time. Also, when the capacitance is large the peak value deceases, slowing down the switching speed. Plus, a larger inductance will cause the inductance to rise.
The effects of parasitic capacitance and inductance on the board can be considered a type of LC resonance. Please confirm the following conditions. 1) Increased external gate resistance connected to the gate drive circuit 2) Smaller gate drive circuit output capacitance 3) Reduced wiring parasitic inductance of the gate drive circuit When the resistance is small the overshoot/undershoot peak value will increase and prolong the ringing decay time. Also, a higher capacitance will slow down the switching speed. And we recommend that the inductance be as small as possible.
If the drive gate voltage while ON is less than 15V it may be impossible to maintain ON operation, and when less than 14V the temperature characteristics of the ON resistance will change from positive to negative. As a result, the ON resistance will decrease at high temperatures, increasing the risk of thermal runaway. Therefore, please ensure that the voltage is greater than 15V. ROHM's TZDB series features a withstand voltage greater than 40V, preventing gate breakdown and ensuring worry-free operation. However, continuing to provide more than the rated voltage (-6V/+22V) will gradually change the threshold value due to the effects of the traps that exist on the gate oxide film interface. However, because the effects from threshold voltage fluctuations during momentary voltage surges (under 300nsec) are small, the acceptable range is actually -10V to +26V.
When the drain potential increases in the FET OFF state, there is a chance that the gate potential will rise due to the effects of AC coupling of the gate-drain capacitance. A typical example is a bridge drive connected in series. In order to prevent short-circuit damage due to erroneous ON, we recommend using negative bias. Gate potential rise can also be mitigated by adding gate-source capacitance. In addition, connecting a Miller clamp MOSFET between the gate and source can prevent an increase in gate potential through reliable short-circuit operation. However, please note that malfunctions may occur when driving the Miller clamp MOSFET due to noise.
Although specific guidelines are not offered, please note that the wiring length from the device gate terminal to the gate drive circuit PC terminal has the greatest impact. The wiring inductance from the device source pin to the board ground pattern must also be taken into account.
Rg is inversely proportional to the chip size, so as the chip size decreases (for example when using SiC) Rg will increase. We are also using a material with high sheet resistance for the gate electrode. Connecting an external gate resistance of 0Ω to the gate drive circuit should be ok, but factors such as the driver current capacity and surge must be considered.