Switching Characteristics Of A Thyristor
Switching characteristics of a thyristor:
Case study: Semiconductor circuits
Author: Eze-Odikwa Tochukwu Jed
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The thyristor is subjected to various voltages or currents during its turn-on and turn-off processes. Due to the applied voltage and current variation, we obtain the switching or dynamic or ON off characteristics of the thyristor. For a reliable and economic design of converter circuits incorporating thyristor, the study of switching characteristics of the thyristor is of vital importance.
What is a thyristor?
The thyristor is another semi-conductor device that is used for lossless and fast switching processes in electronic circuits. The semi-conductor device SCR can be turned on using different methods which include:
- forward voltage triggering
- Gate triggering
- dv/dt triggering
- thermal triggering
- radiation triggering
- Light or photo triggering
However Gate triggering is the most commonly used method among all the above listed method. The switching characteristics of the SCR will be based on this method.
During the turn-on and turn-off processes the SCR is subjected to varying currents and voltages. The time variation of current and voltage across the thyristor gives the switching characteristics of the SCR. During turn the SCR passes through the following time:
- Delay time (td)
- Rise time (tr)
- Spread time (tp)
Delay time (td): This is the time taken for
- Gate current to rise from 10% to 90% of its final value.
- Anode voltage falls from its value Va to 0.9Va
- Anode current (Ia) to rise from its initial value forward leakage current to 10% of its final value (Ia).
The (td) is reduced by applying high gate current and high voltage between the anode and the cathode.
Rise time (tr): This is the time taken for
- The anode current to rise from 10% to 90% of its final value
- Forward blocking voltage to fall from 0.9Va to 10%Va
The tr is inversely proportional to the final gate current Ia and its build up rate Ir also depends on the nature of the load circuit. It has been found that RL loads have higher rise time as a result of the slow rise of the anode current. While RC loads show a reverse as a result of high di/dt.
Spread time (ts): This is the time take for
- The anode current to rise from 0.9Ia to its final value of Ia
- Forward blocking voltage to fall from 0.1Va to the on-state voltage drop which is about 1.0v-1.5v.
During this period the conduction spreads over the entire cross sectional area of the cathode of the SCR and hence the name spread time was derived. The spread time depends on the area of the cathode and the structure of the Gate. The turn on time is the sum of td,tr,ts
This is typically between 1 to 4ms
During turn-off of the thyristor: Note that once the SCR begins conducting at full capacity, it can only be turned off by reducing the forward current to a level below the holding current capacity, a process known as commutation. There are also various commutation methods which include class A, class B, class C etc. Note also that if a forward voltage is applied to the SCR immediately after reducing the anode current to zero, the SCR will begin to conduct again even without applying a +ve gate signal. The turn-off time of the SCR is the minimum time between when the anode current is reduced to zero and when the SCR is capable of blocking forward voltage during the turn off time, all excess charges are removed from the 4 layers of the SCR.
The turn off time Ioff = trr+tgr
Ioff = trr+tgr
trr= reverse recovery time
At t1 the anode current is reduced to zero and the anode current builds up in the reverse direction at the same rate until at tz where the reverse anode voltage develops and the reverse recovery current begins to decrease in magnitude. note that the reverse current is as a result of the carriers stored in the 4 layers at t3, junctions J1 and J3 of the SCR are able to block the reverse voltage making the reverse recovery current to almost fall to zero. However at t2, junction J2 still has some charges trapped hence the SCR is not able to block the forward voltage at t3 but the trapped charges at J2 can’t flow to the external circuit.
These trapped charges decay through the mechanism of recombination (made possible by maintaining reverse voltage across the SCR). The decay of these trapped last between t3 and t4 and is known as Gate recovery time tgr. The turn-off time of the SCR are between 3 to 100ms. At any point the turn-off time increase with:
- Increase in forward current
- Increase in di/dt (rate of change of current) at the time of commutation
- Decrease in junction temperature
However decreases with increase in magnitude of reverse voltage. This is because high revrese voltage pulls out the carriers from junctions J1 and J3.
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