SCR or Silicon Controlled Rectifier is one of power semiconductor devices which is popular for ac to dc converter, ac chopper, and ac phase control applications. As shown in figure 1, the SCR (sometimes called thyristor) is a three-junction P-N-P-N device, where P-N-P and N-P-N component transistors are connected in regenerative feedback mode. The SCR blocks voltage in both the forward and reverse directions; so that is a symmetric blocking. In the SCR characteristic curve or SCR volt-ampere characteristic or SCR static characteristic (figure 2), the SCR blocking voltage are: (i) forward break over voltage (VBO), (ii) reverse break down voltage (VBD).
Figure 1. SCR (left: symbol; right: characteristic)
A several techniques can be used to turn-on SCR. In this article, we only consider the normal method to turn-on SCR. If the anode (A) is positive, the SCR can be triggered into conduction by a short positive gate current pulse. But once SCR is conducting, the gate loses its control to turn off the device. The i-v characteristics of SCR indicate that at gate current IG = 0, if forward voltage is applied on the device (mean that the anode is positive respect to the cathode), there will be a leakage current due to blocking of the middle junction. If the voltage exceeds its critical limit or equal to forward breakover voltage, VBO), then SCR is ON-STATE. By increasing IG, VBO is reduced, and if IG = IG3, SCR behaves like a diode with the entire forward blocking region removed. SCR will turn on successfully if a minimum current, called a latching current (IL), is maintained. During ON-STATE, if the IG =0 and the anode current (IA) falls below a critical limit, called the holding current (IH), SCR reach its forward blocking state. With reverse voltage, the end P-N junctions of the device become reverse-biased and the i-v curve becomes similar to diode.
In summary, SCR is turn-on by the positive gate current, and turn of naturally when the anode voltage drop to zero. And, each SCR has its own i-v characteristics. So, in practice we must consider the datasheet of SCR to get the important parameters of this device. There are, IT(rms), VBO , VBD and IG (see my other articles)
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