Turn off and Turn on characteristics
These characteristics are discussed below of thyristor as follows:
Turn ON Switching Characteristics of thyristor
A forward biased thyristor is turned ON by applying a positive gate voltage between the gate and the cathode, as shown in figure(1).
Figure(2) , shows the waveforms of the gate current(IG), anode current(IA) and anode to cathode voltage(VAK). The total switching period being much smaller compared to the cycle time, IA and VAK before and after switching will appear flat.
As shown in figure , there is a transition time “T-off” from forward OFF state to forward ON state. This transition time is called the thyristor turn ON time and can be divided into three separate intervals namely, They are
- Delay time (Td)
- Rise time (Tr)
- Spread time ( Tp)
Delay Time (Td)
It is the time between the instant at which the gate current reaches 90% of its final value and the instant at which the anode current reaches 10% of its final value. It is the time taken by the anode voltage to fall from VAK to 0.9 VAK
Rise Time (Tr)
For a resistive load, “rise time” is the time taken by the anode current to rise from 10% of its final value to 90% of its final value. At the same time, the voltage VAK falls from 90% of its initial value to 10% of its initial value. However, current rise and voltage fall characteristics are strongly influenced by the type of the load. For inductive load the voltage falls faster than the current. While, for a capacitive load, current rises rapidly.
Spread Time ( Tp)
It is the time taken by the anode current to rise from 90% of its final value to 100%. During this time conduction spreads over the entire cross-section of the cathode of the thyristor. The spreading interval depends on the area of the cathode and on the gate structure of the thyristor.
Turn OFF Switching Characteristics of thyristor
- Once the thyristor is ON and its anode current is above the latching current level, the gate losses control. It can be turned OFF only by reducing the anode current below the holding current. The OFF time tq of a thyristor is defined as the time between the instant anode current becomes zero and the instant the thyristor regains forward blocking capability. If forward voltage is applied across the device, during this period the thyristor turns ON again.
- During turn OFF time, excess minority carriers from all the four layers of the thyristor must be removed. Accordingly, tq is divided into two intervals, the reverse recovery time(tRR) and the gate recovery time(tGR), figure shows the variation of the anode current and the anode to cathode voltage with time during turn OFF operation for an applied sinusoidal voltage(VI).
- The anode current becomes zero at time t1 and starts growing in the negative direction with the same DiA / Dt till time t2. This negative current removes excess carriers from the junctions J1 and J3. At time t2 excess carriers densities at these junctions are not sufficient to maintain the reverse current. The value of the anode current at time t2 is called as the reverse recovery current(IRR). The reverse anode current reduces to the level of reverse saturation current. The total charge removed from the junctions between t1 and t3 is called the reverse recovery charge(QRR). Fast decaying reverse current during the interval t2 – t3 coupled with the di / dt limiting inductor may cause a large reverse voltage spike to appear across the device. This voltage must be limited below the VRRM rating of the device. Up to time t2 the voltage across the device(VAK) does not change substantially from its state value. However, after the reverse recovery time, the thyristor regions the reverse blocking capacity and VAK starts following the supply voltage. At the end of the reverse recovery period(trr) trapped charges still exist at the junction J2 which prevents the device from blocking forward voltage just after trr. These trapped charges are removed only by the process of recombination. The time taken for this recombination process to complete between t3 and t4 is called the gate recovery time(tgr). The time interval tq = trr + tgr is called “device turn OFF time” of the thyristor.
- No forward voltage should appear across the device before the time tq in order to avoid its inadvertent turn ON. While designing an thyristor circuit, one must provide a time interval (tc > tq) during which a reverse voltage is applied across the device, where tc is the circuit turn OFF time.
What is Thyristors in Power Electronics ?
Thyristors in Power Electronics are used as power semiconductor devices which are used as on/off switches in power control circuits. A power semiconductor device is a semiconductor device used as a switch or rectifier in power electronics for example in a switch-mode power supply. A thyristor is the most important type of power semiconductor device. They are extensively used in power electronic circuits. They are operated as bi-stable switches from non-conducting to conducting state. Thyristors are high speed switches that can be used to replace electromechanical relays in many circuits as they have no moving parts, no contact arcing, or suffer from corrosion or dirt. But in addition to simply switching large currents “ON” and “OFF”, thyristors can be made to control the mean value of an AC load current without dissipating large amounts of power.
Table of Content
- What is a Thyristor?
- Properties of Thyristors
- Construction of Thyristor
- Working of Thyristor
- V-I characteristics of thyristor
- Types of Thyristors in Power Electronics
- Examples of Thyristors in Power Electronics