JNTU
JNTU B.Tech III POWER ELECTRONICS Previous Question Paper (September - 2021 - R13)
Answer any five questions. All questions carry equal marks.
Main
Give the constructional details of SCR with the help of schematic diagram and circuit symbol.
Answer
A Silicon Controlled Rectifier (SCR) is a four-layer, three-junction, three-terminal semiconductor device. Its construction consists of alternating layers of P-type and N-type material, forming a P-N-P-N structure. The three terminals are the Anode (A), connected to the outer P-layer; the Cathode (K), connected to the outer N-layer; and the Gate (G), connected to the inner P-layer.
The three junctions are J1 (between the first P and first N layer), J2 (between the first N and second P layer), and J3 (between the second P and second N layer). The schematic diagram represents these layers stacked vertically, while the circuit symbol is a diode symbol with an additional terminal called the Gate attached to the P-layer near the Cathode.
Compare Power MOSFET, Power IGBT and Power BJT.
Answer
| Feature | Power BJT | Power MOSFET | Power IGBT |
|---|---|---|---|
| Control | Current controlled | Voltage controlled | Voltage controlled |
| Switching Speed | Low | Very High | Medium |
| Input Impedance | Low | Very High | High |
| On-state Resistance | Low | High | Low |
| Secondary Breakdown | Yes | No | No |
| Device Type | Bipolar | Unipolar | Hybrid (Bipolar+MOS) |
Power BJTs are current-driven and suffer from slow switching and secondary breakdown issues. Power MOSFETs offer extremely fast switching speeds and high input impedance due to their unipolar nature, making them ideal for high-frequency applications, though they have higher on-state resistance at high voltages. Power IGBTs combine the high input impedance of a MOSFET with the low on-state conduction loss of a BJT, making them the preferred choice for high-voltage, high-current applications where moderate switching speeds are acceptable.
Explain the dynamic characteristics of SCR in detailed.
Answer
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Two thyristors having a difference of 10mA in latching current are connected in series in the circuit. Voltages across the devices are 600V and 320V. Calculate the required equalizing resistance and capacitance, if the permissible difference in blocking voltage is 15V and the difference in the recovery charge is 6µC.
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Explain the discontinuous mode of operation of single phase half wave converter with RL load with neat waveforms. Derive the expression for average output voltage.
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A single phase half-wave rectifier is used to supply power to load having impedance 10 ohms from 230V, 50Hz, ac supply at a firing angle of 30 degrees. Calculate i) Average values of output voltage and current ii) RMS values of output voltage and current.
Answer
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Explain the effect of source inductance on Single phase Full Converter.
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What are the advantages of freewheeling diode?
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Explain the operation of Step down Chopper with RLE load with relevant waveforms.
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A step-up chopper has input voltage of 220V and output voltage of 660V. If the conducting time of thyristor-chopper is 100μs, compute the pulse width of output voltage. In case the output-voltage pulse width is halved for constant frequency operation, find the average value of new output voltage?
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Describe the operation of AC Chopper with associated voltage and current waveforms.
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Enumerate the disadvantages of Jones chopper.
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Describe the operating principle of single-phase to single-phase step-up cycloconverter with the help of mid-point and bridge type configuration. Illustrate your answer with appropriate circuit and waveforms.
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Explain in detail about 1800 Conduction Mode of 3 – ϕ VSI with relevant output line and Phase voltages.
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