Dynamic Characteristics of an SCR

Turn-on and turn-off characteristics of an SCR are called the dynamic characteristics of the SCR.

(a)    Turn-On Character­istic.

The turn-on time charac­teristic shows the variation of current and voltage during turn-on. Turn-on time is defined as the time from the initiation of triggering, when the SCR offers infinite impedance to the flow of anode current, to the time when an equilibrium charge distribution is established throughout the device together with a steady state forward voltage drop. The turn-on time is about 1 to 3 micro seconds for the SCR readily available commercially.

Figure shows the form of current rise during the transi­tion from the non-conducting to the fully conducting state. The time t_O indicate the initiation of turn-on which is caused by a step function of voltage applied to the gate. The period t_d is the delay time between the front of  the gate pulse and the beginning of a rapid rate of increase of anode current. During the initial interval of turn-on (t_d) only a small area near the gate electrode conducts anode current. t_d is due to the charge build up time and decreases with the increase in gate current. The rise time, t_r depends on the speed at which charges build up to support the final anode current, I_A. t_r is independent of gate current but depends upon the widths of base an construction of SCR. The sum of t_d and t_c gives turn-on time t_on. The turn-on time i_on depends on the anode circuit parameters, gate current amplitude and rise time. The turn-on time is important in pulse triggering.

(b)    Turn-off Characteristic.

Turn-off means that all forward conduction has ceased and the re-application of a positive voltage to the anode will not cause flow of current without there being a gate signal. The turn-off characteristic is shown in figure. The time t₀ indi­cates the instant of application of reverse voltage, t_rr is the duration for which the reverse recovery current flows after application of reverse voltage and t is the time required for the recombination of all excess carriers in the inner two layers of the device. The sum of time duration t_rr and time duration t gives the turn-off time t_0ff, that is t_0ff = t_rr + t_gr . At the end of turn-off time, a depletion layer gets developed across junction J₂ and now SCR is capable of withstanding forward voltage. The turn-off time depends upon anode current, magnitude of reverse voltage and the rate of application of forward voltage.