❖ The principle of thyristor operation can be explained with the use of its two-transistor model
❖ Two transistor model is obtained by bisecting the two middle layers, along the dotted line, in two separate halves as shown in Figure.
❖ The relation between the collector current and emitter current is shown below
❖ In the off-state of a transistor, collector current IC is related to emitter current IE as
❖ Where α is common base forward current gain and ICBO leakage current of collector-base junction
❖ Let’s for transistor T1 this relation holds
❖ And that for transistor T2
❖ Now, by the analysis of two transistors model we can get anode sncurrent,
❖ From equation (i) and (ii), we get,
❖ If applied gate current is Ig then cathode current will be the summation of anode current and gate current i.e
❖ By substituting this value of Ik in (iii) we get,
❖ From this relation we can assure that with increasing the value of towards unity, corresponding anode current will increase.
❖ The methods of turning-on a thyristor, in fact, are the methods of making α1+α2 to approach unity.
i. Gate Triggering
❖ Now the question is how ( α1+α2 ) increasing?
❖ At the first stage when we apply a gate current Ig, it acts as base current of T2 transistor i.e IB2 = Ig
❖ and emitter current of the T2 transistor IE2=Ik .
❖ Presence of base current will generate collector current as
❖ This IC2 is nothing but base current IB1 of transistor T1, which will cause the flow of collector current,
Since Ib1 = Ic2
anode positive with respect to cathode and with gate current Ig=0, under these conditions, the device is in the forward blocking state.
❖ and hence, α1 increases
❖ Now, new base current of T2 is
❖ which will lead to increase emitter current
❖ and as a result α2 also increases and this further increases
❖ As IB1=IC2, α1 again increases.
❖ This continuous positive feedback ( α1+α2 ) effect towards unity.
ii. Forward-voltage triggering
❖ If the forward anode to cathode voltage is increased, the collector to emitter voltage of both the transistors are also increased
❖ As a result, the leakage current at the middle junction J2 of thyristor increases, which is also the collector current of Q2 as well as Q1.
❖ With increase in collector currents IC1 and IC2 due to avalanche effect, the emitter currents of the two transistors also increase causing α1+α2 to approach unity.
❖ This leads to switching action of the device due to regenerative action. The forward-voltage triggering for turning-on a thyristor may be destructive and should therefore be avoided.
iii. dv/dt triggering
❖ The reversed biased junction J2 behaves like a capacitor because of the space-charge present there. Let the capacitance of this junction be Cj.
❖ charging current across the junction is given by
❖ This charging or displacement current across junction J2 is collector current of Q2 and Q1
❖ Currents IC2, IC1 will induce emitter current in Q2, Q1.
❖ In case rate of rise of anode voltage is large, the emitter currents will be large and as a result, α1+α2 will approach unity leading to eventual switching action of the thyristor.
iv. Temperature triggering
❖ At high temperature, the forward leakage current across junction J2 rises .
❖ This leakage current serves as the collector junction current of the component transistors Q1 and Q2 .
❖ Therefore, an increase in leakage current IC1, IC2 leads to an increase in the emitter currents of Q1, Q2 .
❖ As a result, (α1+α2) approaches unity. Consequently, switching action of thyristor takes place.
v. Light triggering
❖ When light is thrown on silicon, the electron-hole pairs increase.
❖ In the forward-biased thyristor, leakage current across J2 increases which eventually increases α1+α2 to unity as explained before and switching action of thyristor occurs.
❖ As stated before, gate-triggering is the most common method for turning-on a thyristor. Light-triggered thyristors are used in HVDC applications.
Password: Turn-onmethods
pdf made by Dr M Balasubbareddy