JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III POWER ELECTRONICS Previous Question Paper (August - 2022 - R16)
Answer any five questions
All questions carry equal marks
Main
Explain the construction, principle of operation and static characteristics of a SCR.
Answer
A Silicon Controlled Rectifier (SCR) is a four-layer, three-junction (P-N-P-N) semiconductor device with three terminals: Anode (A), Cathode (K), and Gate (G). It is constructed by alternating P and N layers, creating junctions J1, J2, and J3.
The principle of operation is based on the two-transistor analogy, where the SCR acts as a regenerative feedback system. When the anode is positive with respect to the cathode, junctions J1 and J3 are forward-biased, while J2 is reverse-biased, blocking current. Applying a positive gate current injects carriers into the P-layer near the cathode, lowering the breakdown voltage of J2 and triggering the device into a conducting state (latching). Once triggered, the gate loses control, and the SCR remains ON until the anode current falls below the holding current.
The static V-I characteristics consist of three regions:
- Forward Blocking Region: The device is OFF; a small leakage current flows until the forward breakover voltage is reached.
- Forward Conduction Region: The device is ON, behaving like a closed switch with a low voltage drop.
- Reverse Blocking Region: When the cathode is positive, J1 and J3 are reverse-biased, and the SCR blocks current until the reverse breakdown voltage is exceeded.
Compare the performance characteristics of MOSFET and BJT.
Answer
| Feature | BJT | MOSFET |
|---|---|---|
| Control Parameter | Current controlled | Voltage controlled |
| Switching Speed | Slower (due to storage time) | Very fast |
| Input Impedance | Low | Very high |
| Thermal Stability | Negative temperature coefficient (prone to thermal runaway) | Positive temperature coefficient (easier to parallel) |
| Drive Circuit | Complex | Simple |
MOSFETs are preferred for high-frequency switching applications due to their superior switching speeds and simple gate drive requirements. BJTs are typically limited to lower-frequency applications where their lower conduction losses at high current ratings may be advantageous.
Based on two transistor analogy, explain the turn on mechanism of the SCR. Also derive an expression for the anode current.
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A 100 A SCR is to be used in parallel with a 150 A SCR. The on state voltage drops of the SCRs are 2.1 and 1.75 V, respectively. Calculate the series resistance that should be connected with each SCR, if the two SCRs have to share the total current 250 A in proportion to their ratings.
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Describe the principle of operation of single phase half wave controlled rectifier with RL load and freewheeling diode.
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The full wave controlled bridge rectifier has an AC input of 160 V rms at 50 Hz and a 10 ohm load resistor. The delay angle is 45°. Determine the average current in the load, the power absorbed by the load and the input power factor.
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Explain the operation of a 3-phase full converter with highly inductive load. Sketch the waveforms for output voltage, output current, current through any thyristor and source current. Also, derive the expression for average output voltage.
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Compare half controlled and full controlled bridge rectifiers.
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What are the merits and demerits of AC voltage controllers?
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A single phase full wave AC voltage controller has a resistive load of 5 ohms and an input voltage 230 V, 50 Hz. The firing angle of thyristors T1 and T2 is 120°. Calculate (i) the rms value of load voltage, (ii) input power factor. (iii) Average value of current of thyristor, (iv) rms current of thyristor and (v) load power.
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Explain with waveforms the principle of working of 1-phase to 1-phase step up cycloconverter.
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Describe the modes of operation of Triac with necessary diagrams.
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With the circuit diagram and output voltage waveforms, explain the working of Jones chopper.
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A step down DC chopper has a resistive load of R = 15 ohms and input voltage Edc = 200 V. When the chopper remains ON, its voltage drop is 2.5 V. The chopper frequency is 1 kHz. If the duty cycle is 50 %, determine (i) average output voltage, (ii) RMS output voltage, (iii) Chopper frequency and (iv) effective input resistance of chopper.
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What is pulse width modulation? List the various PWM techniques. How do these differ from each other?
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Design a series inverter circuit for operation in the frequency range 1 to 5 kHz. The load resistance may vary from 25 ohm to 100 ohm. The peak load current is limited to 3 A and the supply voltage is 100 V.
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