For applications like alarm circuit which need to trigger once and stay ON forever, cannot use transistor. So, for overcome these problems we use Thyristor. Thyristor and Transistor both are electrical switches but the power handling capacity of Thyristors are is far better than transistor. Due to having high rating of Thyristor, given in kilowatts, while of transistor power ranges in watts.
A Thyristor is taken as a closed couple pair of transistors in analysis. The main difference between the transistor and Thyristor is, Transistor need continuous switching supply to remain ON but in case of Thyristor we need to trigger it once only and it remains ON. So, to overcome these problems we use Thyristor. There are some more differences between Thyristor and Transistor are given below in table:.
The basic circuit for obtaining Thyristor V-I characteristics is given below, the anode and cathode of the Thyristor are connected to main supply through the load. The gate and cathode of the Thyristor are fed from a source Es, used to provide gate current from gate to cathode.
As per the characteristic diagram, there are three basic modes of SCR: reverse blocking mode, forward blocking mode, and forward conduction mode. In this mode the cathode is made positive with respect to anode with switch S open. Junction J1 and J3 are reversed biased and J2 is forward biased. When reverse voltage applied across Thyristor should be less than V BR , the device offers a high impedance in the reverse direction. Therefore, Thyristor treated as open switch in the reverse blocking mode.
V BR is the reverse breakdown voltage where the avalanche occurs, if voltage exceeds V BR may cause to Thyristor damage. When anode is made positive with respect to cathode, with gate switch open. Thyristor is said to be forward biased, junction J1 and J3 are forward biased and J2 is reversed biased as you can see in figure. In this mode, a small current flows called forward leakage current, as the forward leakage current is small and not enough to trigger the SCR.
Therefore, SCR is treated as open switch even in forward blocking mode. As the forward voltage is increased with gate circuit remain open, an avalanche occurs at junction J2 and SCR comes into conduction mode. We can turn ON the SCR at any moment by giving a positive gate pulse between gate and cathode or by a forward breakover voltage across anode and cathode of the Thyristor. By applying forward voltage between anode and cathode, with keeping gate circuit open, junction J2 is reverse biased.
As a result, the formation of depletion layer occurs across J2. This type of turn-on is normally avoided. Just like photodiodes, if light photons reaches the junctions of a thyristor, it will produce electron-hole pairs. As we know that the junctions have capacitance. So, if the anode-to-cathode voltage rising rate is sufficiently high enough, it may charge the capacitive junction to turn the thyristor into the conduction state.
The thyristor needs to be protected against fast-rising current spikes. After switching, It requires a minimum time to establish the current uniformly throughout the junctions.
Otherwise, the fast-rising current during switching may damage the junction due to excessive heat and the device will fail eventually. As we know that if we apply a fast rising voltage between the anode and cathode of a thyristor, it may turn ON without the gate input. In such a case, a capacitor is used in parallel which will limit the fast-rising voltage.
To limit the discharging current of the capacitor, a resistor is used with it in series. Such kind of circuit that suppresses a voltage spike is known as Snubber. Using Various Methods. Phase-controlled thyristor also known as a Silicon Controlled Rectifier SCR is turned on by applying the gate current, when it is in forward bias. So, it turns off when the anode current reaches zero. It has two separate gate terminals; one for each thyristor. One of the gate terminals turns on the current in forward direction.
They are generally silicon-controlled rectifier SCR but they have fast switching speed. It uses the resonant inverter for forced commutation. The light photon particles upon hitting the junction produce electron-hole pairs, which triggers the current flow throw the device. LASCR electrically isolate the high power circuit form the light source circuit. It can conduct in both direction and they are used for phase control in AC applications.
So it can be used in either direction. RCT can conduct in reverse direction without any control input. Its made of an SCR with a diode in an antiparallel configuration for the reverse conduction of the reactive load currents.
It is used in applications where reverse blocking is not necessary. However, it has low reverse voltage rating than its forward voltage rating.
Because of the reverses current flow, it allows the RCT to drain its carriers from its junction relatively fast, providing much fast switching speed.
However, it can be turned off by applying negative gate voltage. Hence, a very small amount of reverse current i. This state of the device is known as the reverse blocking mode or off state. Proceeding further the actual operating mode of thyristor arises when an external gate pulse is provided to it. Here sufficient positive voltage is provided to anode and gate with reference to the cathode. In the above figure, we can clearly see that a forward voltage is provided to the gate terminal with respect to the cathode terminal.
Now, in this case, the junction J 0 , J 1 and J 2 all comes in forward biased condition. So, the majority carriers start drifting to the collector region of transistor Q 1. And as we know that collector of Q 1 forms connection with the base of transistor Q 2. This base current drives the transistor Q 2.
Also, the junction is forward biased, so the majority carriers drift to the collector of Q 2. In this way, a large current flows through the device.
Thus, by applying gate trigger pulse, large electric current flows through the thyristor. This state is termed as forward conduction mode. Now, even if we remove the gate voltage then also this cumulative action will take place and the current flows through the device once it reaches the minimum value. This minimum current is known as latching current.
Now the question arises then what must be done in order to off the thyristor once it gets on? So, the answer to the above question is by reducing the anode current up to its lowest value which is called holding current. Thus, a reduction in anode voltage will bring the thyristor again to its forward blocking mode.
Here, the figure clearly shows the 3 regions of operation of the thyristor. As in the case of forward applied voltage initially when a gate trigger pulse is not applied, then the device operates in forward blocking region.
But, as gate trigger pulse is also provided along with forward voltage then it causes a large flow of current through the device. This represents the forward conduction region. At the time when a reverse voltage is provided only reverse leakage current flows through the device.
But, after a certain reverse voltage, the device suffers from avalanche breakdown and this voltage is termed as the reverse breakdown voltage of the device.
0コメント