Power semiconductor devices, such as, insulated gate bipolar transistors (IGBTs) are used in a variety of applications that require high frequency switching of electrical power. For instance, IGBTs may be used to generate pulse-width modulation (PWM) signals for industrial and traction applications. In such applications, the IGBTs are turned on and off at a high frequency. During operation, when inductive loads are switched off, the IGBTs become non-conducting, and a transition to a high voltage state is executed in a very short period of time.
For instance, when a control signal is fed to the gate (G) of the IGBT, the IGBT may be turned on and the voltage between the collector (C) and the emitter (E) of the IGBT is approximately zero volts. During this period of time load current will flow between the collector (C) and the emitter (E) of the IGBT. When the control signal is switched off, the IGBT may become non-conducting between the collector C and the emitter E, and the voltage between the collector C and the emitter E of the IGBT rapidly increases at the initial period of the switch off period. As a result of the inductance in the wiring to the IGBT terminals, a voltage spike may occur, which can, in certain cases, result in damage to the IGBT and a power loss imbalance. To reduce the overvoltage of the IGBT during the turn off period, a voltage clamp circuit was developed.
As shown in FIG. 2, a conventional voltage clamp circuit 110 may include at least one resistor RG and one or more zener diodes, ZD1, ZD2. The zener diodes, ZD1, ZD2, may be connected in series between the collector C and the gate G of the IGBT Q1, and the at least one resistor RG may be connected to the gate G of the IGBT Q1. During operation, when the voltage output between the collector C and the emitter E of the IGBT Q1 reaches the zener voltage, a current flows to the gate G and turns on the IGBT Q1 in order to limit the output voltage of the IGBT Q1 to the value of the zener voltage.
While the prior art voltage clamp circuit 110 of FIG. 2 limits the output voltage of the IGBT, the zener protection is known to retard the time profile of the high voltage transition during the switching off period due to a time delay, due to parasitic inductances and capacitances in the wiring loop comprising the components in the conventional voltage clamp circuit. As shown in FIG. 3, when the load is switched off, at time 122, the voltage output VCE between the collector C and the emitter E of the IGBT increases rapidly and may overshoot above the zener voltage. Thereafter, the cut-in of limiting the output voltage VCE to the zener voltage is time delayed as represented at time point 120. This may result in a voltage overshoot of the IGBT at the start of the turn off cycle, which may cause damage to the IGBT.