When current required by the load is more than the rated current of a single thyristor, SCRs are connected in parallel in a string. For equal sharing of currents, V — I characteristics of SCRs during forward conduction must be identical as far as possible. In Fig. (a) are shown two SCRs in parallel and …
When system voltage is more than the voltage rating of a single thyristor, SCRs are connected in series in a string. These, SCRs should have their V-I characteristics as close as possible. On account of inherent variations in their characteristics, the voltage shared by each SCR may not be equal. For instant consider two SCRs …
❖ Presently, SCRs with voltage and current ratings of 10 kV and 3 kA are available.❖ However, for some industrial applications, the demand for voltage and current ratings is so high that a single SCR cannot fulfil such requirements.❖ In such cases, SCRs are connected in series in order to meet the high voltage demand …
high temperature protection ❖ With the increase in the temperature of the junction, insulation may get failed. So we must take proper measures to limit the temperature rise.Protective measures:❖ We can achieve this by mounting the thyristor on heat sink which is mainly made by high thermal conductivity metals like aluminum (Al), Copper (Cu) etc.❖ …
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Overvoltage Protection ❖ It is the most important protection scheme w. r. t. others as thyristors are very sensitive to over voltages.❖ Maximum time thyristor failures happen due to over-voltage transients.❖ A thyristor may be subjected to internal or external over-voltages.❖ Internal Over-Voltages : After commutation of a thyristor reverse recovery current decays abruptly with …
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❖ A snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel with the thyristor.❖ Strictly speaking, a capacitor C, in parallel with the device is sufficient to prevent unwanted dv/dt triggering of the SCR.❖ When switch S is closed, a sudden voltage appears across the circuit. Capacitor Cs behaves …
di/dt protection The value of di/dt can be maintained below acceptable limit by using a small inductor, called di/dt inductor, in series with the anode circuit.Typical di/dt limit values of SCRs are 20—500 A/μ sec.Local spot heating can also be avoided by ensuring that the conduction spreads to the whole area as rapidly as possible.This …
❖ A forward biased thyristor can be turned on by applying a positive voltage between gate and cathode terminal.❖ But it takes some transition time to go from forward blocking mode to forward conduction mode❖ This transition time is called turn on time of SCR and it can be subdivided into three small intervals as …
❖ 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 …
With anode positive with respect to cathode, a thyristor can be turned on by any one of the following techniques. We know that SCR includes two stable states namely forward conduction and forward blocking. SCR triggering method can be defined as, when the SCR is switching in forward blocking state to forward conduction state which …