Introduction • As in SCR gate control, using breakover device will overcome the firing inconsistency from one device to another and the temperature independency. • In addition, it will deliver a pulse of gate current rather than sinusoidal as RC circuits does.
1. The Diac • The diac is a two terminal, three layer, bidirectional thyristor. • The diac has no gate. • It is a bidirectional because current can flow in both directions. • Can be connected in a circuit regardless of polarity. • Available in various break-over voltage ratings. • Advantages of diac over other breakover devices are that it has excellent temprature stability and symmetrical triggering characteristics. • Fig.6-5(a) shows typical gate control circuit using a Diac (bidirectional trigger diode, symmetrical trigger diode) as a breakover device. • Fig.6-5(b) shows the characteristics of the diac. It shows that for applied forward voltages less than the forward breakover voltage (+VBO) the diac permits no current to flow. If VBO is reached, the diac switches ON and its current surges up as its terminal’s voltage declines. This surge in the current accounts for pulsing ability of diac • In thenegative region the behavior is identical. When the applied voltage is less than (in magnitude) the reverse breakover voltage (-VBO) the Diac is OFF. If the (-VBO) is reached the diac will ON.
Figure 6–5(a) Triac gate control circuit containing a diac (bidirectional trigger diode). This triggering method has several advantages over the methods shown in Fig. 6–4. (b) Current versus voltage characteristic curve of a diac. (c) Another schematic symbol for a diac.
Diacs are relatively temperature stable and • There is very little difference (<1V) in magnitude between (+VBO) and (-VBO). This enables the triggering circuit to maintain nearly equal firing delay angles for both half cycles. • The operation of circuit in figure 6-5(a) is the same as that of the figure 6-4(a) except that the capacitor voltage must build up to the VBO of the diac in order to deliver gate current to the triac. This normally higher than needed value (+-32V). Thus the charging time constant should be decreased by decreasing the values of R1, R2 and C.
2. Silicon Bilateral Switches (SBS) Figure 6–6(a) Schematic symbol and terminal names of an SBS (silicon bilateral switch). (b) Current-voltage characteristic curve (gate terminal is disconnected) of an SBS, with important points indicated.
SBS is popular in low-voltage trigger circuits for triac due to its lower VBO (+-8V). The I-V characteristic curve of an SBS is similar to that of diac, but the SBS has larger negative resistance region as shown in Fig. 6-6 (b). • The SBS (without using the gate lead) can be used in place of the diac in Fig. 6-5 (b). • SBS is more temperature stable than the diac, more symmetrical (+VBO-(-VBO)~0.3V) and less batch spread(<0.1V) than a diac (~4V). • The gate terminal can be used to change the VBO as shown in Fig.6-7(a).
Figure 6–7(a) SBS combined with a zener diode to alter the breakover point in the forward direction. (b) Characteristic curve of the SBS-zener diode combination. The forward breakover voltage is lower, but the reverse breakover voltage is unchanged.
3.Unilateral Breakover Devices • Diac and SBS are bilateral (bidirectional) breakover devices. Why? • The four-layer diode and silicon-unilateral switch (SUS) are unilateral (unidirectional) breakover devices because they break over in only one direction. Their I-V characteristics (Fig. 6-9 (c)) are similar to that of SBS except that only forward breakover is possible. Reverse breakdown can happen at much greater voltage than +VBO. • SUS like SBS has a gate terminal which can be used to change the basic breakover voltage using zener diode similar to figure 6-7(a) for SBS. • SUSs are low-voltage, low current devices (VBO around 8V, current limit less than 1A).
Figure 6–9 (a) Schematic symbol of a four-layer diode. (b) Symbol and terminal names of an SUS (silicon unilateral switch). (c) Current-voltage characteristic curve of an SUS.
4. SUS For Triac Triggering Circuit Figure 6–10(a) Complete schematic diagram of a triac control circuit containing an SUS and a pulse transformer. (b) Ac supply voltage waveform. (c) Full-wave rectified voltage (Vbridge) which is applied to the triggering circuit. (d) Capacitor voltage waveform, shown reaching the VBO of the SUS. (e) Pulse transformer primary current (f) Inverted pulses of secondary current from the pulse transformer. (g) Load voltage waveform.
Fig. 6-10(a) shows a triggering circuit using SUS. • Since SUS is a unilateral, Bridge rectifier is used. • Pulse transformer is used to avoid shorting the lower right-hand diode of the bridge. • A negative gate current is generated for firing the triac in order to avoid mode 4 in triggering the triac; the four modes are as follows: • 1. positive main terminals voltage, positive gate current. • 2. positive main terminals voltage, negative gate current. • 3. negative main terminals voltage, negative gate current. • 4. negative main terminals voltage, positive gate current. Mode 4 is the most unsuitable and difficult to trigger the triac • The RC circuit across the triac is to limit the rise of off-state triac voltage (dv/dt) to its rated value(100v/us, medium sized triac) which is the maximum rate of rise of the main terminal voltage a triac can withstand. If this rate is exceeded the triac may fire ON without any gate signal. The resistance R in this RC circuit is to limit the discharge current of the capacitor thru the main terminals of the triac when it is ON.
Hysteresis in triac can be described as follows: a single value of R2 (see Fig.6-5(a)) can cause two completely different results, depending on the direction in which R2 is changing. The Flash-on of a triac is a specific example of this hysteresis. Triac hysteresis can be almost completely eliminated with circuit in Fig. 6-8 (a). Suppose that R2 is set so that C voltage cannot reach ±8V, so the SBS will not breakover and the triac will not fire and the lamp will be extinguished. During the positive half cycle, C charges (positive top, negative bottom). At the end of the positive half cycle, the top of R3 tends to zero V (Why?). Eliminating Triac Hysteresis
This will forward bias D1 and very small current (iG) will flow from C thru A2-G-D1, R3. With this small current, even very low voltage from A2 to A1 will break over the SBS (see Fig 8-8(c)). Hence, As long as VC is greater than 1 V, the SBS will break over. • When the SBS breaks over, it dumps the capacitor charge thru R4. the negative half cycle of the ac supply therefore starts with the capacitor almost completely discharged. Hence the capacitor starts charging with the same initial charge (0V) no matter whether the triac is firing or not firing. Thus, the triac hysteresis is eliminated. • The function of D2 in the circuit is to maintain VR3=Vac during the negative half cycle in order to prevent firing the SBS in the reverse direction.
Figure 6–8(a) A more complex triac triggering circuit. Triac flash-on can be eliminated with this circuit. (b) Direction of gate current through the SBS as the ac supply approaches its zero crossover. (c) The very low forward breakover voltage when gate current is flowing in the SBS.
UJT as Trigger Device for Triac • So far, the firing delay angles for SCR and Triac circuit we saw were set by a potentiometer. This is an open loop control system. • In practical, usually the firing angle is set by a closed loop (feed back) control system. • The actual voltage of the load (lamp, motor…) is measured and compared with the desired one; if they are different, the control system will adjust the firing delay angle automatically, so that this difference is eliminated. • Sometimes the feedback signal takes the form of varying resistance (resistive feedback) instead of varying a voltage (voltage feedback). • Whenever, a feedback system is needed a UJT is used in the triggering circuit.
Resistive Feedback Triggering Circuit • Fig 6-11(a) shows a triac triggering circuit using UJT with resistive feedback. T1 is an isolation transformer (1:1). • RF is a variable resistance which varies with the load voltage. • R1 and RF form a voltage divider (with correct sizing of the resistance, IB of Q1 is very small). • The rate of charging of C1 is determined by the ratio of RF to R1. • The greater VR1 the faster charging rate and the earlier the UJT and triac fire (the smaller delay angle). • The smaller VR1 the slower charging rate and the later the UJT and triac fire (the bigger delay angle).
c d Of T2 Vz e f Figure 6–11(a) Complete schematic diagram of a triac control circuit. The triggering circuit uses a UJT and a constant-current source, which is controlled by resistance feedback. (b) The same triggering circuit as part (a) except that the current source is controlled by voltage feedback. (c) The zener-clipped full-wave voltage which drives the triggering circuit. (d) The capacitor voltage waveform. It rises at a constant slope until it hits VPof the UJT (15 V in this case). (e) Secondary current pulse from the pulses transformer. (f) load voltage waveform
Voltage Feedback Triggering Circuit • Fig 6-11(b) shows a triac triggering circuit using UJT with voltage feedback. • It is similar to the circuit in (a) except that the RF is replaced by a variable feedback voltage VF which represents the variation in the load voltage. • VF controls the firing delay angle by controlling the rate of charging C1 thru determination of VR1 as follows • The greater VR1 the faster charging rate and the earlier the UJT and triac fire (the smaller delay angle). • The smaller VR1 the slower charging rate and the later the UJT and triac fire (the bigger delay angle). • Solve Example 6-4 page 240.
b Figure (b) The same triggering circuit as part (a) except that the current source is controlled by voltage feedback