Test: Thyristors - 2
20 Questions MCQ Test Power Electronics | Test: Thyristors - 2
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During the turn on time, the voltage across the SCR is going down and the current through it is slowly rising as it is going into the conduction mode.
For an SCR the total turn-on time consists of
i) Delay time
ii) Rise time and
iii) Spread time
During the delay time the
i) Delay time
ii) Rise time and
iii) Spread time
During the delay time the
Initially for a fraction of a microsecond (delay time) after the gate signal is applied the anode current only flows near the gate terminal where the gate current density is maximum, as the gate current takes some time to spread all over the cross section of the device.
The minimum value of anode current below which it must fall to completely turn-off the device is called as the
The device will remain in the conducting state unless the anode current falls below the holding current value.
For an SCR the total turn-on time consists of
i) Delay time
ii) Rise time and
iii) Spread time
During the rise time the
The losses are maximum during the rise time because both Ia & Va are high.
The latching current is the value of current on which the device will remain in the on state even after removal of the gate signal. Whereas, the holding current is the threshold above which the device will work.
For an SCR the total turn-on time consists of
i) Delay time
ii) Rise time and the
iii) Spread time
The spread time interval depends upon
During the spread time the conduction starts spreading all over the SCR cathode cross-section structure, which depends upon the structure of the gate & cathode. Higher the cathode area more is the time required for the charges to spread all over.
For effective turning off of the SCR after the anode current has reached zero value, ______________
To enable the device to regain its reverse blocking capabilities, the stored charges in the junctions of the SCR must be removed.
If the thyristor turn off time is more than the circuit turn off time, the circuit will be turned off and the thyristor will keep conducting, which is not at all desirable.
It is the gate current versus the gate voltage plot and gives the minimum and maximum values of gate parameters.
The area under the curve of the gate characteristics of thyristor gives the
As the gate characteristics is a plot of Ig vs Vg consisting of two curves one for the maximum values & other for the minimum the area between them gives the total average gate power dissipation. (A very important parameter in designing of the triggering circuits).
A tangent drawn from the Y-axis to the Pavg on the gate characteristics gives the value of the
It gives the min gate to source resistance.
Lesser time is required to inject the charges & turn on the device with higher gate pulse magnitude.
The average gate power dissipation for an SCR is 0.5 Watts the voltage applied to the gate is Vg = 10 V. What is the maximum value of current Ig for safe operation?
Vg.Ig = 0.5 W, the power dissipation mustn’t exceed the average power dissipation.
For an SCR, the gate-cathode characteristic has a slop of 130. The gate power dissipation is 0.5 watts. Find Ig
Vg/Ig = 130 .. (given)
Vg.Ig = 0.5 watts .. (given)
use both the given data & find the gate current.
The two transistor model consists of p-n-p and n-p-n transistors, of which the middle n-p layer is common in both the transistors.
Latching current for an SCR is 100 mA, DC source of 200 V is also connected from the SCR to the L load. Compute the minimum width of the gate pulse required to turn on the device. Take L = 0.2 H.
For L load, E = L di/dt
I = E/L t
Therefore, 0.100 = 200t/0.2
T = 100 μsec.
The gate-source voltage is Es = 16 V and the load line has a slope of 128 V/A. Calculate the gate current for an average gate power dissipation of 0.5 W.
Load line is nothing but Rs
Es = 16V
Vg.Ig = 0.5
Rs = 128
We have Es = Ig x Rs + Vg.
From the two transistor (T1 & T2) analogy of SCR, the total anode current of SCR is ___________ in the equivalent circuit.
The sum of both the collector currents of T1 and T2 forms the total anode current of SCR. Refer the model.
Consider the two transistor analogy of SCR, if α1 & if α2 are the common-base current gains of both the transistors then to turn-on the device
To turn on the device sum of both the current gains should approach unity value.
Latching current for an SCR is 100 mA, a dc source of 200 V is also connected to the SCR which is supplying an R-L load. Compute the minimum width of the gate pulse required to turn on the device. Take L = 0.2 H & R = 20 ohm both in series.
E = Ri + L di/dt
Solve the above D.E for I & substitute the above values.
t = 100.503 μsec.
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