新型电子元器件第6章.pptVIP

Chapter 6 phase shifting an SCR 6.1 phase shifting an SCR 1. the purpose of phase shifting In order for an SCR to gain complete control of the waveform of AC voltage it must be phase shifted. 2. the definition of phase shifting phase shifting means to change or shift the phase of one thing in reference to another. (2) In this case, the concern is with shifting the phase of the voltage applied to the gate with respect to the voltage applied to the anode. (3) an SCR which has not been phase shifted has the voltage which is applied to the gate locked in phase with the voltage applied to the anode. 3. Practical Application Recall that an SCR which has not been phase shifted can control only half of the positive waveform. An SCR which has not been phase shifted has the voltage which is applied to the gate locked in phase with the voltage applied to the anode. Assume that the SCR in this example fires when the gate current reaches a 5ma level. The variable resistor in figure below is adjusted so that the gate current will not reach the 5 ma level until the applied voltage reaches its peak value. The SCR fires when the voltage applied to the anode is at the peak level also. If the variable resistor is decreased in value, the gate current can reach 5 ma sooner, and the SCR will fire sooner during the cycle. Each decrease of the variable resistor will cause the gate current to reach 5 ma earlier in the cycle and cause the SCR to fire sooner. 4. Separating voltage Notice that the gate controls the firing of the SCR during only half of the positive cycle. As long as the voltage applied to the gate of the SCR is locked in phase with the voltage applied to the anode, the SCR cannot be fired after the AC voltage applied to the anode has reach 90°. To obtain full control of the AC half cycle applied to the anode, the voltage applied to the gate must be out-of-phase with the voltage applied to the anode. To shift the phase of the voltage applied to the gate, the gat