A silicon controlled switch (SCS) is essentially an SCR equipped with a second gate terminal. Like a conventional switch, and unlike an SCR, an SCS can be switched either on or off through external gate control. Figure lR-16 shows the structure, symbol, and two-transistor equivalent of an SCS. Note that the gate terminal at the base of 02 is now called the cathode gate, and the second gate, at the base of 0″ is called the anode gate.
The silicon controlled switch can be turned on in exactly the same way as an SCR: by supplying a positive pulse of current to the cathode gate, which drives O! on and initiates regenerative brcakovcr. It call also he turned on by supplying a negative current pulse to the anode gate. Notice that negative current turns on PNP transistor 0″ so 0, initiates the regeneration in this case. The SCS can be turned off either by supplying negative current to the cathode gate, which turns off Q!, or by supplying positive current to the anode gate, which turns off 0,. Figure 18-17 summarizes the methods by which an SCS can be turned on and off. Silicon controlled switches can be turned on and off faster than SCRs, but they arc not available with the high power, high current ratings of SCRs.
DIACs and TRIACs
A DJAC is a four-layer device whose “top” and “bottom” layers contain both N . and P material, as shown in Figure 18-18. The right side of the stack can be regarded as a PNPN structure with the same characteristics as a garcless SCR, while the left side is an inverted SCR having an NPNP structure. A four-transistor equivalent circuit of a DIAC, resembling an SCR in parallel with an inverted SCR, is shown in the figure. Note that the terminals are labeled AI (anode 1) and A2 (anode 2). There are no gates. The DIAC can conduct current in either direction: from AI to A2 through Q, and Q2, or from A2 to AI through Q3 and Q4.1f AI is made sufficiently positive with respect to A2 to induce brcakover in Q, .and Q:, then conciuction occurs in that path, while Q3 anci Q4 remain off. Similarly, if A2 is sufficiently positive with respect to AI, then Q3 and Q4 conduct while QI and Q2 remain off. The two cases are illustrated in Figure IH-18(b) and (c). Part (d) shows the schematic’
symbols.
Figure 18-19 shows the I-V characteristic of a DIAC. We may arbitrarily assume one direction through the DIAC as positive (from A, to A2, for example), and the opposite direction as negative. The positive portion of the characteristic is the same as that of an SCR with zero gate current. The negative portion shows that breakover occurs when the reverse voltage reaches at which time a large reverse current flows. To turn the DIAC off, the current must be reduced below the posiuvc holding current I” if conducting in the forward direction. or below the negative holding current :-1/1 if conducting, in the reverse direction.
A TRIAC is equivalent to a DIAC having a gate terminal, as shown in Figure H~-20. Note that the gate is connected to the base of 01 and to the base of OJ. both. of which arc NPN transistors. Therefore, a pulse of current flowing into the gate will induce current Ilow through 01 and Q1 if AI is positive with respect to A1 or will induce current flow through OJ and O~if A1 is positive with respect tv AI
The gate must be made positive with respect to 11.2to turn on 01 and 02. and must be made positive with respect to AI to turn on 0″ and Q~. Modern TRIACs have an additional N layer connected to one gate. This layer serves as the emitter of an NPN transistor that increases gate sensitivity on one side. but negative gate currentis required to trigger that side on
Figure lX-21 shows a family of characteristic curves for a TRIAC. Like the SCR. the magnitudes of the hrcukover voltage and holding current become smaller as the values of gate current increase. Whatever the direction of the current through the TRIAC, its value must be reduced below the holding current to turn the TRIAC oft
Figure 18-22 shows a power controller utilizing 11 ‘I RJAC that is triggered by positive gate current when III is positive with respect to A2 and by negative gate current when AI is negative with respect to 112, Notice that a DIAC is connected between the gate and an RC circuit. When the capacitor voltage rises far enough to overcome the sum of the breakpower voltage of the DIAC and the forward drop of the gate, the DIAC fires. the capacitor discharges rapidly into the gate, and the TRIAC is triggered into conduction. Since R,. controls the time constant at which the capacitor charges, it is used to control the firing angle. (Jr: Large values of R; delay the charging and therefore increase the firing angle
respect to A2, the polarities of all voltages are reversed, and the TRIAC is triggered into conduction during a portion of the negative half-cycle. The firing angle, can be adjusted in practical circuits from near 0° to near 180°, so load current can be made to flow for nearly an entire cycle of input or, at the other extreme, for a very small portion of an input cycle. Since the positive and negative areas of the current waveform are equal, the average current is O. The rms load current is
