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Thyristor (Control Device ON-OFF Control)

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Thyristor (Control Device ON-OFF Control) Thyristor (control device ON-OFF control) ➢ Thyristor is a power switch made of semiconductor material having 4 layers and 3 terminals. ➢ It is a static switch latching from ONOFF & OFFON. ➢ It is semi-controlled switch, because ON state can be controlled by gate signal but after thyristor is turn ON, it remain latched in ON state due to internal regenerative action and gate losses control. ➢ Thyristors are very much in use. In some cases transistors are replaced by power transistors because it is very fast operating device so some extra harmonics are introduced. power transistors has lesser switching speed but almost same control. ➢ Thyristors may be used in power-switching circuits, relay-replacement circuits, inverter circuits, oscillator circuits, level-detector circuits, chopper circuits, light-dimming circuits, low-cost timer circuits, logic circuits, speed-control circuits, phase-control circuits, etc. Thyristor family Thyritsor Circuit symbol Voltage/current rating Upper frequency range SCR 7000V/5000A 1.0 kHz LASCR 6000V/3000A 1.0 kHz ASCR/RCT 2500V/400A 2.0 kHz GTO 5000V/3000A 2.0 kHz SITH 2500V/500A 100 kHz MCT 1200V/40A 20 kHz Triac 1200V/1000A 0.50 kHz Basic Construction of Thyristor – SCR A high- resistive, n-base region, presents in every thyristor. As it is seen in the figure, this n-base region is associated with junction, J2. This must support the large applied forward voltages that occur when the switch is in its off- or forward-blocking state (non-conducting). This n-base region is typically doped with impurity phosphorous atoms at a concentration of 1013 to 1014 per cube centimeter. This region is typically made 10 to 100 micrometer thick to support large voltages. High- voltage thyristors are generally made by diffusing aluminum or gallium into both surfaces to create p- doped regions forming deep junctions with the n-base. The doping profile of the p-regions ranges from about 1015 to 1017 per cube centimeter. These p-regions can be up to tens of micrometer thick. The cathode region (typically only a few micrometer thick) is formed by using phosphorous atoms at a doping density of 1017 to 1018 cube centimeter. For higher forward-blocking voltage rating of thyristor, the n-base region is made thicker. But thicker n – based high-resistive region slows down on off operation of the device. This is because of more stored charge during conduction. A device rated for forward blocking voltage of 1 kV will operate much more slowly than the thyristor rated for 100 V. Thicker high-resistive region also causes larger forward voltage drop during conduction. Impurity atoms, such as platinum or gold, or electron irradiation are used to create charge-carrier recombination sites in the thyristor. The large number of recombination sites reduces the mean carrier lifetime (average time that an electron or hole moves through the Si before recombining with its opposite charge-carrier type). A reduced carrier lifetime shortens the switching times (in particular the turn-off or recovery time) at the expense of increasing the forward-conduction drop. There are other effects associated with the relative thickness and layout of the various regions that make up modern thyristors, but the major trade off between forward-blocking voltage rating and switching times and between forward-blocking voltage rating and forward-voltage drop during conduction should be kept in mind. V-I Characteristics of a Thyristor Thyristor has 3 basic modes of operation. 1) Forward blocking mode (OFF state). 2) Forward conduction mode (ON state). 3) Reverse blocking mode. Forward Blocking Mode Considering the anode is positive w.r.t cathode, with gate kept in open condition. The thyristor is now said to be forward biased. the junctions J1 and J3 are now forward biased but junction J2 goes into reverse biased condition. In this particular mode, a small current, called forward leakage current is allowed to flow initially as shown in the diagram for characteristics of thyristor. Now, if we keep on increasing the forward biased anode to cathode voltage.In this particular mode, the thyristor conducts currents from anode to cathode with a very small voltage drop across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward break over voltage (VBO) or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending beyond a certain point, then the reverse biased junction J2 will have an avalanche breakdown at a voltage called forward break over voltage VB0 of the thyristor. Forward Conduction Mode When the anode to cathode forward voltage is increased, with gate circuit open, the reverse junction J2 will have an avalanche breakdown at forward break over voltage VBO leading to thyristor turn on.In this mode of operation, the thyristor conducts maximum current with minimum voltage drop, this is known as the forward conduction forward conduction or the turn on mode of the thyristor. Reverse Blocking Mode of Thyristor The anode is negative w.r.t cathode, with gate kept in open condition. Here Junctions J1 and J3 are reverse biased whereas the junction J2 is forward biased. The behavior of the thyristor here is similar to that of two diodes are connected in series with reverse voltage applied across them. As a result only a small leakage current of the order of a few μAmps flows. This is the reverse blocking mode or the off-state, of the thyristor. If the reverse voltage is now increased, then at a particular voltage, known as the critical breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. A large current associated with VBR gives rise to more losses in the SCR, which results in heating. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a thyristor does not exceed VBR. When reverse voltageapplied across a thyristor is less than VBR, the device offers very high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as open circuit. • Once the thyristor is turned on by a gate signal and its anode current is greater than the holding current, the device continues to conduct due to positive feedback even if the gate signal is removed. This is because the thyristor is a latching device and it has been latched to the on- state. Latching current – it can be defined as the minimum value of anode current which thyristor must attain during turn-on to maintain the conduction when gate signal is removed. Holding current – it can be defined as the minimum value of anode current below which it must fall for turning - off the thyristor. Latching current is more than holding current. it is usual to take latching current 2 to 3 times greater than holding current. Thyristor Triggering methods The various SCR triggering methods are ▪ Forward Voltage Triggering ▪ Thermal or Temperature Triggering ▪ Radiation or Light triggering ▪ dv/dt Triggering ▪ Gate Triggering Forward Voltage Triggering ▪ In this mode, an additional forward voltage is applied between anode and cathode. ▪ When the anode terminal is positive with respect to cathode (VAK) , Junction J1 and J3 is forward biased and junction J2 is reverse biased. ▪ No current flows due to depletion region in J2 is reverse biased (except leakage current). ▪ As VAK is further increased, at a voltage VBO (Forward Break Over Voltage) the junction J2 undergoes avalanche breakdown and so a current flows and the device tends to turn ON(even when gate is open) ▪ This method is not preferred because during turn on of thyristor, it is associated with large voltage and large current which results in huge power loss and device may be damaged. Thermal or Temperature Triggering ▪ The width of depletion layer of SCR decreases with increase in junction temperature due to the generation of electron-hole pair. ▪ Therefore in SCR when VAR is very near its breakdown voltage, the device is triggered by increasing the junction temperature. ▪ By increasing the junction temperature the reverse biased junction collapses thus the device starts to conduct. ▪ This method is not preferred because temperature changes the characteristics of SCR. Radiation or Light triggering ▪ For light triggered SCRs a special terminal niche is made inside the inner P layer instead of gate terminal. ▪ When light is allowed to strike this terminal, free charge carriers are generated. ▪ When intensity of light becomes more than a normal value, the thyristor starts conducting. ▪ This type of SCRs are called as LASCR dv/dt Triggering ▪ When the device is forward biased, J1 and J3 are forward biased, J2 is reverse biased. ▪ Junction J2 behaves as a capacitor, due to the charges existing across the junction. ▪ If voltage across the device is V, the charge by Q and capacitance by C then, ic =dQ/dt Q=CV ic =d(CV)/dt =C.dV/d+V.dC/dt ………. as dC/dt = 0 ic = C.dV/dt ▪ Therefore when the rate of change of voltage across the device becomes large, the device may turn ON, even if the voltage across the device is small. Gate Triggering ▪ This is most widely used SCR triggering method. ▪ Applying a positive voltage between gate and cathode can Turn ON a forward biased thyristor. ▪ When a positive voltage is applied at the gate terminal, charge carriers are injected in the inner P-layer, thereby reducing the depletion layer thickness.
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