The forward dv/dt rating of an SCR:-
Right Answer is:
Increase with an increase in the junction temperature
dv/dt is the rate of change of anode to cathode voltage with respect to time.
The dv/dt rating of a thyristor indicates the maximum rate of rise of anode voltage that will not trigger the device without any gate signal. If the rate of rise of forward voltage is higher than the specified maximum value, it may cause switching from the OFF-state to the ON state. The mechanism for this phenomenon can be explained in terms of the internal capacitance that the thyristor exhibits.
Whenever the SCR is in forward blocking state, only forward leakage current flows through the SCR. In such state, an equivalent internal capacitor is formed inside the SCR from an anode to gate and gate to cathode. Fig. shows such an internal circuit. Whenever the voltage applied across the SCR changes rapidly, a transient current flows through the SCR This transient current flows due to rapid voltage variations (dv/dt) and internal capacitance. This current adds to the forward leakage current. And hence the SCR turns on even if anode to cathode voltage (VAK) < Break overvoltage (VBO) or gate drive is not applied.
The dv/dt turn-on makes false triggering (unwanted) of the SCR. It is never used for triggering. Every SCR has dv/dt rating. It is expressed in volts per microseconds (V /μs). The voltage variations across the SCR must be kept less than the permissible value of dv/dt to avoid false triggering. Normally a small resistance is connected between gate and cathode to avoid false triggering of SCR due to dv/dt.
Hence when the voltage across the device is increased, charges flow through it in a manner analogous to the charging current of a capacitor (i = C dv/dt ). The greater the rate of rise of applied voltage, the greater will be this flow of charges. As the rate of rise is increased. Efficient charges will eventually flow to act as a gate current o turn on the SCR even without any positive gate signal.
Effect of temperature on dv/dt rating.
The dv/dt rating of SCR depends upon the temperature. dV/dT decreases with increasing temperatures for the semiconductor diodes. As temperature increases, the forward bias voltage of a diode decreases. For every 1° increase in temperature, the forward voltage decreases approximately by 2 mV. An increase in temperature causes an increase in intrinsic carrier density, a decrease in band gap causes an increased value of diode current. All these factors lead to a net result that is with an increase in temperature, the diode current increases. An increasing diode current leads to power dissipation in the diode, which ultimately leads to a further increase in temperature.