JNTU
JNTU B.Tech III POWER ELECTRONICS Previous Question Paper (November/December - 2017 - R13)
Note: This question paper contains two parts A and B. Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit. Each question carries 10 marks and may have a, b, c as sub questions.
PART - A
Sketch static V-I characteristics of IGBT and mark the region in which the device is operated as a switch.
Answer
The static V-I characteristics of an IGBT are plotted with collector-emitter voltage (VCE) on the x-axis and collector current (IC) on the y-axis for various gate-emitter voltages (VGE). The curve shows a linear ohmic region at very low VCE, followed by a constant current active region where IC is controlled by VGE. The device is operated as a switch in the cut-off region (VGE < VGE(th), where IC ≈ 0) and the saturation region (VGE > VGE(th), where VCE is low and the device is fully ON). In switching applications, the IGBT is driven between these two states to minimize power dissipation.
Define the ratings of SCR (i) Average ON state current (ii) Forward breakover voltage.
Answer
(i) Average ON-state current (IT(AV)) is the maximum average value of the forward current that an SCR can conduct continuously under specified cooling conditions without exceeding the junction temperature limits. It is determined by the thermal capacity of the device and the heat dissipation capability of the heat sink.
(ii) Forward breakover voltage (VBO) is the minimum positive anode-to-cathode voltage at which the SCR switches from the forward blocking state to the forward conduction state, with the gate terminal left open. Exceeding this voltage triggers the device into conduction even without a gate signal, potentially leading to damage if the current is not limited.
What is the effect of connecting free wheeling diode across R-L load in controlled rectifiers?
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Write down general expression for average voltage of p- pulse fully controlled rectifier.
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Draw the output voltage waveform of single phase AC chopper.
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A Class B turn-off circuit commutates an SCR. The load current is constant at 10Amps. Dimension the commutating components L and C. The supply voltage is 100V dc. Turn off time spec of SCR is 20 micro sec.
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Draw the equivalent circuit of a cyclo-conveter.
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What are advantages and disadvatages of cyclo-converter as compared to ac voltage controllers.
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What type of commutation is used in basic series inverter? How frequency of output can be controlled in series inverter ?
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How can a PWM control signal be obtained using a carrier wave and reference wave with the help of diagram.
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PART - B
Draw dynamic characteristics of SCR during turn off. Explain how turn off process can be carried out?
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Explain the two-transistor analogy of SCR.
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What are problems encountered when SCRs are operated in (i) series (ii) parallel. Derive an expression to find the value of resistance to be connected across each thyristor for voltage balancing.
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Design RC firing circuit with following specifications: AC input voltage: 115 V, Thyistor ratings: Vg(min) : 2.5 V , Vg(max) : 5V, Ig(min) : 1 mA , Ig(max) : 5 mA, Load: 15 ohms resistance
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Explain the operation of single phase half controlled bridge rectifier with R-L-E load. Derive the expression for (i) Average output voltage (ii) RMS value of output voltage
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A fully controlled rectifier is used to charge a 115 V battery. The battery is already charged to 60 V. The source voltage of bridge is 230 V at 50 Hz. Find the range of firing angle possible.
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Draw the output voltage waveform of 3-phase fully controlled rectifier for a firing angle of 60 degrees. Indicate firing sequence. Also derive expression for output voltage.
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A 3 phase fully controlled bridge rectifier is operating from a 400V, 50 Hz supply. The load is highly inductive and current constant and continuous. Find the load voltage at firing angle of 45 deg.
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Draw the circuit of class-B commutation circuit. Explain how thyristor is commutated in class-B chopper. What are disadvantages of this commutation circuit?
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A class-A chopper circuit has a load resistance of 100 ohms, capacitance of 10 micro farads and inductance of 10 mH. Find the time for which thyristor will remain in ON state. What will be the turn ON time if the load resistance is decreased to 25 ohms.
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Derive expressions for minimum and maximum values of load current in a step down chopper with R-L-E load.
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A step-up chopper has a source of 250 V (dc) in series with inductance of 0.1 H. If the semiconductor switch is operated with different values of duty ratio, plot output voltage vs duty ratio.
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Distinguish between an ac voltage controller and a cyclo-conveter with respect to operation and control aspects.
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Derive an expression for rms value of output voltage of single phase bridge type ac voltage controller.
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Draw the circuit of single phase voltage controller with antiparallel connection of two thyristors and an R-L load. Explain its working. Sketch load voltage and load current waveforms. Derive an expression for output voltage.
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Explain the operation of single phase bridge inverter with the help of load voltage and load current waveforms for R-L Load.
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A 3-phase bridge inverter is fed from a dc source of 200 V. If the load is star connected of 10 ohms / phase, determine rms value of load current and required current rating of thyrstors.
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Discuss various voltage control techniques employed in inverter circuits.
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