To study the characteristics of switching device in Power Electronics.
An SCR is a unidirectional device as it can conduct from anode to cathode only and not from cathode to anode. A triac can, conduct in both the directions. A triac is the bidirectional thyristor with three terminals. It is used extensively for control of power in AC circuit. When in operation, a triac is equivalent to two SCRs connected in antiparallel. The device symbol and V1 characteristics are shown in figure.
As the triac can conduct in both the directions, the term anode and cathode are not applicable to triac. It is three terminals are usually designated as MT1 (main terminal 1), MT2 and the gate by G as in thyristor.
With no signal the gate, the triac will block both half cycles of applied voltage in case peak value of this voltage is less than the brake over voltage of VBO1 or VBO2 of triac. The triac can however turns ON in each half cycle of the applied voltage by applying a positive or negative voltage to the with respect to terminal MT1. For convenience terminal MT1 is taken as the point for measuring the voltage and current at the gate and MT2 terminal.
1) MT2 is Positive and Gate Current is Positive.
In this mode MT2 is positive with respect to MT1, junctions P1N1, P2N2 are forward biased but junction N1P2 is reverse biased. When gate terminal is positive with the respect to MT1, gate current flows mainly through P2N2 junction like an ordinary SCR, Fig.6(a). When gate current has injected sufficient charge into P2 layer, reverse biased junction N1P2 breaks down just as in a normal SCR. As a result, triac starts conduction through P1N1P2N2 layers. This shows the when MT2 and gate terminals are positive with respect to MT1, triac turns on like a conventional thyristor. Under this condition, triac operates in the first quadrant. The device is more sensitive in this mode. It is recommended method of triggering if the conduction is desired in the first quadrant.
2) MT2 is Positive and Gate Current is Negative.
In this mode gate terminal is negative with respect to MT1, gate current flows through P2N3 junction Fig 6(b) and reverse biased junction N1P2 is forward biased as in a normal thyristor. As a result, triac starts conducting through P1N1P2N3 layers initially. With the conduction of P1N1P2N3, the voltage drop across this path falls but potential of layer between P2N3 rises towards the anode potential of MT2. As the right hand portion of P2 is clamped at the cathode potential of MT1, a potential gradient exists across layer P2, its left hand region being at higher potential than its right hand region. A current shown dotted is thus established in layer P2 from left to right. This current is similar to conventional gate current of an SCR. As a consequence, right-hand part of triac consisting of main structure P1N1 P2N2 begins to conduct. The device structure P1N1P2N3 may be regarded as pilot SCR and the structure P1N1P2N2 as the main SCR. It can then be stated that anode current of pilot SCR serves as the gate current for the main SCR. As compared with turn-on process discussed in (I) above, the device with MT2 positive but gate current negative is less sensitive and therefore, more gate current is required.
3) MT2 is Negative and Gate Current is Positive
The gate current Ig forward biases P2N2 junction. Layer N2 injects electrons into P2 layer as shown by dotted arrows. As a result, reverse biased junction N1P1 breaks down as in a conventional thyristor. Eventually the structure P2N1P1N4 is completely turned on. As usual, the current after turn-on is limited by the external load. As the triac is turned on by remote gate N2, the device is less sensitive in the third quadrant with positive gate current.
4) MT2 is negative and Gate Currents is Negative
In this mode, N3 acts as a remote gate. The gate current Ig flows from P2 to N3 as in a normal thyristor. Reverse-biased junction N1P1 is broken and finally, the structure P2N1P1N4 is turned on completely. Though the triac is turned on by remote gate N3 in third quadrant, yet the device is more sensitive under this condition compared with turn-on action with positive gate current discussed in (iii) above.
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