In lower power applications, to simplify the circuit and reduce the cost, a shunt resistor can be used in series in the power loop and measure the current directly. Since the resistor is in series in the power loop, it directly measures the current and can have high accuracy by using a high precision resistor. The resistance needs to be small to reduce the power loss, and should have large enough voltage resolution for the protection. Since the sensing resistor is also in series in the gate driver loop, the voltage drop on the sensing resistor can cause the voltage drop on the gate voltage of the IGBT or SiC MOSFET modules. The parasitic inductance of the sensing resistor and the PCB trace of the sensing loop will also cause a noise voltage source during switching transient, which makes the gate voltage oscillate. Thus, this method is not recommended for high power application, or when dI/dt is high. To use it in low power application, the shunt resistor loop should be designed to have the optimal voltage drop and minimum noise injection to the gate loop.

Figure 9-9 Overcurrent and Short Circuit Protection Based on Shunt Resistor