THYRISTOR TURN-ON METHODS
With anode positive with respect to cathode, a thyristor can be turned on by any one of the following techniques :
(a) Forward voltage triggering (b) gate triggering
(c) dv/dt triggering (d)Temperature triggering
These methods of turning-on a thyristor are now discussed one after the other.
(a) Forward Voltage Triggering: When anode to cathode forward voltage is increased with gate circuit open, the reverse biased junction J2 will break. This is known as avalanche breakdown and the voltage at which avalanche occurs is called forward breakover voltage VB0. At this voltage, thyristor changes from off-state (high voltage with low leakage current) to on-state characterised by low voltage across thyristor with large forward current. As other junctions J1, J3 are already forward biased, breakdown of junction J2 allows free movement of carriers across three junctions and as a result, large forward anode-current flows. As stated before, this forward current is limited by the load impedance. In practice, the transition from off-state to on-state obtained by exceeding VB0 is never employed as it may destroy the device.
The magnitudes of forward and reverse breakover voltages are nearly the same and both are temperature dependent. In practice, it is found that VBR is slightly more than VB0. Therefore, forward breakover voltage is taken as the final voltage rating of the device during the design of SCR applications.
After the avalanche breakdown, junction J2 looses its reverse blocking capability. Therefore, if the anode voltage is reduced below VB0 SCR will continue conduction of the current. The SCR can now be turned off only by reducing the anode current below a certain value called holding current (defined later).
(6) Gate Triggering : Turning on of thyristors by gate triggering is simple, reliable and efficient, it is therefore the most usual method of firing the forward biased SCRs. A thyristor with forward breakover voltage (say 800 V) higher than the normal working voltage (say 400 V) is chosen. This means that thyristor will remain in forward blocking state with normal working voltage across anode and cathode and with gate open. However, when turn-on of a thyristor is required, a positive gate voltage between gate and cathode is applied. With gate current thus established, charges are injected into the inner p layer and voltage at which forward breakover occurs is reduced. The forward voltage at which the device switches to on-state depends upon the magnitude of gate current. Higher the gate current, lower is the forward breakover voltage
When positive gate current is applied, gate P layer is flooded with electrons from the cathode. This is because cathode N layer is heavily doped as compared to gate P layer. As the thyristor is forward biased, some of these electrons reach junction J2. As a result, width of depletion layer around junction J2 is reduced. This causes the junction J2 to breakdown at an applied voltage lower than forward breakover voltage VB0. If magnitude of gate current is increased, more electrons will reach junction J2 ,as a consequence thyristor will get turned on at a much lower forward applied voltage.
Fig. 4.2 (b) shows that for gate current Ig = 0, forward breakover voltage is VB0. For Igl , forward breakover voltage, or turn-on voltage is less than VB0 For Ig2 > Ig1 , forward breakover voltage is still further reduced. The effect of gate current on the forward breakover voltage of a thyristor can also be illustrated by means of a curve as shown in Fig. 4.4. For Ig < oa, forward breakover voltage remains almost constant at VB0. For gate currents Ig1 , Ig2 and Ig3 the values of forward breakover voltages are ox, oy and oz, respectively as shown. In Fig. 4.2 (b), the curve marked Ig = 0 is actually for gate current less than oa. In practice, the magnitude of gate current is more than the minimum gate current required to turn on the SCR. Typical gate current magnitudes are of the order of 20 to 200 mA.
Once the SCR is conducting a forward current, reverse biased junction J2 no longer exists. As such, no gate current is required for the device to remain in on-state. Therefore, if the gate current is removed, the conduction of current from anode to cathode remains unaffected. However, if gate current is reduced to zero before the rising anode current attains a value, called the latching current, the thyristor will turn-off again. The gate pulse width should therefore be judiciously chosen to ensure that anode current rises above the latching current. Thus latching current may be defined as the minimum value of anode current which it must attain during turn-on process to maintain conduction when gate signal is removed.
Once the thyristor is conducting, gate loses control. The thyristor can be turned-off (or the thyristor can be returned to forward blocking state) only if the forward current falls below a low-level current called the holding current. Thus holding current may be defined as the minimum value of anode current below which it must fall for turning-off the thyristor. The latching current is higher than the holding current. Note that latching current is associated with turn-on process and holding current with turn-off process. It is usual to take latching current as two to three times the holding current . In industrial applications, holding current (typically 10 mA) is almost taken as zero.
(c) dv/dt Triggering : This method is discussed further in separate post.
(d) Temperature Triggering : During forward blocking, most of the applied voltage appears across reverse biased junction J2. This voltage across junction J2 associated with leakage current may raise the temperature of this junction. With increase in temperature, leakage current through junction J2 further increases. This cumulative process may turn on the SCR at some high temperature.
(e) Light Triggering: For light-triggered SCRs, a recess (or niche) is made in the inner p-layer as shown in Fig. 4.5 (a). When this recess is irradiated, free charge carriers (holes and electrons) are generated just like when gate signal is applied between gate and cathode. The pulse of light of appropriate wavelength is guided by optical fibres for irradiation. If the intensity of this light thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a thyristor is known as light-activated SCR (LASCR).
LASCR may be triggered with a light source or with a gate signal. Sometimes a combination of both light source and gate signal is used to trigger an SCR. For this, the gate is biased with voltage or current slightly less than that required to turn it on, now a beam of light directed at the inner p-layer junction turns on the SCR. The light intensity required to turn-on the SCR depends upon the voltage bias given to the gate. Higher the voltage (or current) bias, lower the light intensity required.
Light-triggered thyristors have now been used in high-voltage direct current (HVDC) transmission systems. In these several SCRs are connected in series-parallel combination and their light-triggering has the advantage of electrical isolation between power and control circuits.