1.a) Latching current for an SCR inserted in between a dc voltage source of 200v and the load, is 100mA. Compute the minimum width of gate – pulse current required to turn –on the SCR in case the load consists of
a) L = 0.2H b) R = 20Ω in series with L = 0.2H
c) R = 20 Ω in series with L = 2H.
b) Discuss the conditions which must be satisfied for turning – on SCR with a gate signal. [8+8]
2.a) Which current rating of an SCR is the most important?
b) SCR’s with a rating of 1000v and 200A are available to be used in a string to handle 6kv and 1kA. Calculate the number of series and parallel units required in case derating factor is 0.1 and 0.2. [8+8]
3. Consider a single phase half-wave rectifier with R load purely and the delay angle is 900. Determine
a) Rectification efficiency b) Form factor c) Ripple factor
d) Total utilization factor e) Peak inverse voltage of thyristor. [16]
4. Determine the expression for the following performance factors of a single – phase fully controlled bridge converter.
a) Input displacement factor b) Input current distortion factor.
c) DC voltage ratio d) Input power factor
e) Input harmonic factor f) Voltage ripple factor
g) Active power input h) Reactive power input. [16]
1. (a) What is the importance of Surge current rating of a thyristor, explain in detail.
(b) A thyristor has half-cycle surge current rating of 1000mA for a 50Hz supply.
Calculate its one-cycle surge current rating and I2t rating. [10+6]
2. A single phase fully controlled converter is connected to a load comprised of 2 ohms
resistance and 0.3H inductance. The supply voltage is 230V at 50Hz. Estimate the
average load voltage, average load current and input power factor for a firing angle
of 200. Assume continuous and ripple free load current, draw load voltage waveform.
1. The latching current of a thyristor with d.c.
Calculate the value of minimum width of the gate pulse current when connected
to a pure inductive load of 1H. Compute the effect, if a resistance of 10 ohms is
connected in series with the load. [8+8]
2. Explain the operation of a singe phase half wave converter for R-load with neat
circuit diagram and necessary waveforms. Also derive the output average voltage
and current for α = 300. [12+4]
1. A 200A thyristor operates in parallel with a 300A thyristor. Their ON state voltage
drops are respectively 1.5V and 1.0V. Calculate the value of the resistance to be
inserted in series with each thyristor so that they share a load of 500A in proportion
to their respective current ratings. [16]
2. (a) Compare mid-point converters and bridge type converters and bring out im-
portant features.
(b) Compare discontinuous and continuous current modes of operation of convert-
ers and bring out salient features. [8+8]
1.Explain in detail various voltage ratings and current ratings of a thyristor.
4. Two SCRs are connected back-to-back have a load resistance of 400 ohms and a
supply of 110V ac. If firing angle is 600, find
(a) the rms output voltage
(b) average power.
1.a) Sketch the dynamic characteristics of SCR during turn-ON process. Write down the necessary conditions required to turn-ON SCR.
b) A thyristor is connected in series with an inductance of 0.1 H and a resistance of 20 ohms and a sinusoidal voltage source of 230 V, 50 Hz. The latching current of SCR is 25 mA. Calculate the minimum width of gate pulse required to ensure reliable triggering if it is gated at an angle of 300 in every positive half cycle of supply voltage. [8+8]
2.a) How a thyristor is protected from high values of dv/dt and di/dt. Derive an expression for parameters of the circuit required to protect thyristor from high values of dv/dt .
b) Design a UJT relaxation oscillator circuit for triggering a thyristor. The UJT has the following parameters:
η=0.72, Ip= 60μA , Vv=2.5 V, Iv=4mA, V=15 V, RBB= 5 k ohms
The leakage current with emitter open is 3mA.The triggering frequency is 1kHz and Vg(min) of thyristor is 0.3 V. Also calculate the minimum and maximum values of triggering frequency. [8+8]
3.a) What is meant by the term ‘phase control’? Draw the circuit of a single phase half controlled bridge rectifier with common anode configuration. Explain the sequence of firing of thyristor to obtain output voltage across R-L load without free wheeling diode. Derive an expression for its output voltage.
b) A single phase half controlled bridge rectifier is connected to a source of 115 V, 50 Hz and a load of 20 ohms. Determine the average load current at firing angle of i) 30 deg ii) 60 deg. [8+8]
4.a) Explain the operation of single phase full wave controlled rectifier feeding an inductive load. Consider the effect of source inductance and derive an expression for output voltage in terms of source inductance and firing angle.
b) A single phase fully controlled rectifier is operated from 120 V, 50 Hz supply. It has a load of having R= 0.5 ohm and inductance of 6.5 mH and E=10 V. The firing angle is 60 deg. Calculate the average load current, average and rms values of thyristor current. [8+8]
1.a) Sketch static V-I characteristics of SCR. Explain how SCR could be operated as a switch with the help of the characteristics.
b) For an SCR the gate-cathode characteristics are given by a straight line passing through origin with gradient of 16 V/A. The turn-on time is 4 μsec and gate current required is 500 mA. Given the gate source voltage of 15 V, calculate
i) Gate power dissipation
ii) The resistance to be connected in series with gate.
2.a) Draw the two transistor analogy of SCR and explain why gate loses control once thyristor is fired when anode current is greater than latching value. (8)
b) Derive an expression for dynamic resistance to be connected across each thyristor in string of n number of series connected SCRs. [8+8]
3.a) Derive an expression for average and rms values of output voltage in a single phase half wave controlled rectifier with R-L load.
b) An SCR is connected between a source of 330 Sinωt and a load of 10 ohms and a battery with emf of 165 V. When it fired with a continuous dc signal, calculate
i) Average current ii) power supplied to battery iii) power dissipated in resistor.
[8+8]
4.a) Explain the operation of full wave controlled rectifier with input supply derived from a single phase transformer with centre tapped secondary.
b) Carry out harmonic analysis of input current in a fully controlled rectifier feeding an R-L load without free wheeling diode.
1.a) A BJT is acting as a switch, the forward current gain β is in the range 8 to 40. The load resistance is RC = 11Ω. The dc supply voltage is VCC=200v and the input voltage to the base circuit is VB=10v. Find
a) The value of RB that results in saturation with an over drive factor of 5.
b) The β forced
c) The power loss PT in the transistor.
b) What are the various applications of an IGBT? [8+8]
2.a) Compare a BJT, MOSFET, IGBT with respect to the following:
a) Basel Gate control variable
b) Control characteristic
c) Switching frequency
d) On-stage voltage drop
e) Max voltage rating
f) Max current rating.
b) Write down the advantages and limitations of BJT and MOSFET. [8+8]
3.a) Explain the features of phase angle control.
b) A 230v, 50Hz one pulse SCR controlled converter is triggered at a firing angle of 400 and the load current extinguishes at an angle of 2100. Find the circuit turn off time, average output voltage and the average load current for
a) R = 5Ω, L = 2mH
b) R = 5Ω, L = 2mH, E = 110v. [8+8]
4. Determine the expression for the following performance facts of single-phase fully controlled bridge converter
a) Input displacement factor b) Input power factor
c) DC voltage ratio d) Input current distortion factor
e) Input harmonic factor f) Voltage ripple factor
g) Active power input h) Reactive power input.
4. Explain only the operation of a single phase fully controlled bridge converter for firing angle α for a R load (do not derive any expressions).
Derive the following expressions:
The converter is connected to a 120v, 50Hz supply. The load current Ia is continuous and its ripple content is negligible. The turns ratio of the transformer is unity. Express the input current in a Fourier series and determine the HF of the input current. [16]
