Question 1

In the single-phase rectifier shown (figure), the average load current is 15 A. Determine the root-mean-square (rms) current in the transformer secondary winding.

Accepted Answer

Introduction / Context: This problem tests understanding of current waveforms in transformer-fed rectifiers. For a typical single-phase full-wave rectifier using a center-tapped secondary, each half of the secondary conducts for half the cycle. Hence, rms current in a half-winding is not the same as the average direct current delivered to the load. Given Data / Assumptions: Average (dc) load current Idc = 15 A. Full-wave rectification with a center-tapped secondary (each half-winding conducts for 50% of the time at the magnitude equal to the load current during its conduction interval). Ideal diodes/thyristors, negligible ripple for current shape reasoning. Concept / Approach: For a center-tapped full-wave rectifier, current in each half of the secondary is a rectangular pulse of amplitude approximately equal to load current during its half-cycle conduction window (duty = 0.5). The rms of a rectangular waveform over a full period is amplitude * √(duty). Step-by-Step Solution: Let I_cond be the conduction current amplitude in a half-winding ≈ Idc = 15 A.Conduction duty in each half: D = 0.5.Irms (per secondary half) = I_cond * √D = 15 * √0.5 = 15 * 0.7071 ≈ 10.61 A. Verification / Alternative check: If you integrate i^2(t) over one full period with i(t) = 15 A for half the time and 0 otherwise, Irms = √( (15^2 * 0.5) ) = 10.61 A, confirming the shortcut formula. Why Other Options Are Wrong: 15 A: equals average current, not rms per half-winding. 14.14 A: equals 15/√(0.5) misconception; wrong application. 7.5 A: half the average, not rms. 12.0 A: arbitrary and not tied to duty-cycle math. Common Pitfalls: Confusing load dc current with rms transformer current. Forgetting each half-winding conducts only half the cycle. Final Answer: 10.61 A

Question 2

In a full-wave converter with mid-point (M–2) connection, each thyristor has a PIV of 400 V. For the same output voltage using a fully controlled bridge converter, what is the required PIV per thyristor?

Accepted Answer

Introduction / Context: Peak inverse voltage (PIV) rating depends on converter topology. For a given dc output, different rectifier configurations impose different maximum reverse voltages on each device. This question compares the mid-point (M–2) full-wave converter and the full bridge (B–2) converter. Given Data / Assumptions: M–2 converter PIV per thyristor = 400 V. Same dc output voltage to be obtained using a fully controlled bridge. Ideal devices and transformer, standard rectifier relationships. Concept / Approach: For a single-phase center-tapped (M–2) converter, PIV per device ≈ 2Vm (peak of half-secondary seen reflected). For a single-phase full bridge (B–2), PIV per device ≈ Vm. Thus, for the same dc output, the B–2 requires roughly half the PIV of M–2 per device. Step-by-Step Solution: Given M–2 PIV = 2Vm = 400 V ⇒ Vm = 200 V.Bridge (B–2) PIV per thyristor ≈ Vm = 200 V.Therefore, use thyristors rated at about 200 V (plus design margin). Verification / Alternative check: Textbook rectifier PIV results: center-tapped diode/thyristor PIV = 2Vm; bridge PIV = Vm. The ratio is consistently 2:1 for equal dc outputs with comparable transformer secondaries. Why Other Options Are Wrong: 400 V and 800 V: correspond to center-tapped values or doubled ratings; unnecessary for the bridge at the same dc output. 2002 V: unrealistic and not topology-derived. 100 V: underestimates required blocking capability. Common Pitfalls: Mistaking line-to-line peak with half-winding peak. Ignoring that bridge shares reverse stress between legs differently than M–2. Final Answer: 200 V

Question 3

Compare average dc output of half-wave versus full-wave (M–2) controlled rectifiers with a resistive load: relation between Vdc1 (half-wave) and Vdc2 (full-wave) for any firing angle α.

Accepted Answer

Introduction / Context: Average output voltage of phase-controlled rectifiers depends on firing angle and whether one or two half-cycles are used. Full-wave topologies deliver pulses twice per cycle, doubling the average (for the same Vm and α) relative to half-wave circuits. Given Data / Assumptions: Resistive load. Half-wave controlled rectifier (single thyristor): Vdc1 formula. Full-wave controlled rectifier (M–2): Vdc2 formula. Concept / Approach: Standard results: For half-wave controlled rectifier, Vdc1 = (Vm / 2π) (1 + cos α). For the full-wave (M–2), Vdc2 = (Vm / π) (1 + cos α). Therefore Vdc1 = 0.5 * Vdc2 for any α in [0, π]. Step-by-Step Solution: Write Vdc1 = (Vm / 2π) (1 + cos α).Write Vdc2 = (Vm / π) (1 + cos α).Divide: Vdc1 / Vdc2 = 1/2 ⇒ Vdc1 = 0.5 Vdc2 (independent of α). Verification / Alternative check: At α = 0, Vdc1 = Vm/π, Vdc2 = 2Vm/π ⇒ ratio 0.5. At α = π/2, both reduce by the same factor, maintaining the 0.5 ratio. Why Other Options Are Wrong: Equality or inequalities contradict the established factor-of-two difference due to pulse repetition rate. Dependence on α cancelling in the ratio is overlooked in the wrong choices. Common Pitfalls: Using diode-rectifier (α = 0 only) values and generalizing incorrectly. Confusing Vm and Vrms when forming averages. Final Answer: Vdc1 = 0.5 Vdc2 for all firing angles

Question 4

Reverse-conducting thyristor (RCT): how many external leads does the device provide?

Accepted Answer

Introduction / Context: A reverse-conducting thyristor (RCT) integrates an anti-parallel diode within the same package as the thyristor. Understanding its terminal count clarifies how it connects in power converters where reverse conduction is needed for commutation or freewheeling paths. Given Data / Assumptions: Device type: reverse-conducting thyristor (thyristor + anti-parallel diode internally). Standard thyristor terminals are anode (A), cathode (K), and gate (G). The freewheel diode is internally bonded anti-parallel to A–K. Concept / Approach: RCTs still require a gate to trigger forward conduction. Therefore, despite including an internal diode, they present the same number of external terminals as a standard SCR: three leads (A, K, G). The internal diode does not create extra external terminals; it shares A and K with the thyristor structure. Step-by-Step Solution: Identify required user connections: gate control, main anode, main cathode.Internal diode is reverse parallel between A and K → no separate diode terminals.Hence, total external leads = 3 (A, K, G). Verification / Alternative check: Manufacturers’ datasheets for RCT devices list three pins. Packages and symbols confirm a gate plus two main terminals. Why Other Options Are Wrong: 2 leads: would correspond to a diode or a two-terminal device; an SCR requires a gate. 1 or 4+ leads: not representative of standard RCT packages. Common Pitfalls: Assuming the internal diode needs separate pins. Confusing RCTs with two-terminal devices like DIACs. Final Answer: 3

Question 5

When using thyristors in series and/or in parallel in power converters, when is current derating required?

Accepted Answer

Introduction / Context: Thyristors are often connected in series (to increase voltage rating) and in parallel (to increase current rating). Practical devices never share perfectly due to parameter tolerances and dynamic effects, so derating ensures safe operation. Given Data / Assumptions: Series strings face static and dynamic voltage imbalance. Parallel groups face static and dynamic current imbalance. Realistic spreads in VBO, tq, gate sensitivity, and thermal coefficients exist. Concept / Approach: Derating is applied to accommodate unequal sharing. In series, snubbers and grading networks help, but residual imbalance remains. In parallel, ballast inductors/resistors assist sharing, but positive temperature coefficients and switching transients still cause unequal currents. Hence derating is advisable in both cases. Step-by-Step Solution: Identify imbalance sources in series: device tolerances, transient dv/dt, recovery non-uniformity.Identify imbalance sources in parallel: on-state voltage spread, thermal runaway risk, di/dt skew.Conclusion: apply current (and power) derating when operating in series or in parallel. Verification / Alternative check: Industry application notes recommend derating plus equalizing components for both series and parallel configurations to maintain reliability under worst-case conditions. Why Other Options Are Wrong: Series only / Parallel only: incomplete—both need safety margins. None: unsafe; ignores real-world mismatches. Common Pitfalls: Assuming ideal matching eliminates sharing issues—manufacturing spreads persist. Neglecting temperature effects on on-state characteristics. Final Answer: series or parallel

Question 6

Assertion–Reason: A transistor needs a continuous base drive for conduction, whereas a thyristor (SCR) requires only a gate pulse because of latching. The reason given is “Transistors find widespread use in power electronics.” Evaluate.

Accepted Answer

Introduction / Context: Understanding control requirements: BJTs and MOSFETs are controlled devices needing continuous drive (base or gate) to remain on, while thyristors are latching devices that remain on after a brief gate pulse until current falls below holding current. This is foundational in power electronics control strategies. Given Data / Assumptions: Assertion (A): Transistor requires continuous base (or sustained gate for MOSFET) drive; SCR needs a trigger pulse only. Reason (R): “Transistors are widely used in power electronics.” We evaluate truthfulness and explanatory linkage. Concept / Approach: Latching behavior of SCRs arises from regenerative feedback inside the pnpn structure; once on, it remains on as long as anode current exceeds the holding current. Transistors lack this latch and require continuous drive. The “widespread use” statement is true but does not explain the latching mechanism or drive requirement difference. Step-by-Step Evaluation: Check A: True—continuous drive vs latching pulse.Check R: True—transistors are indeed widely used.Causality: R does not explain why SCR needs only a pulse; explanation should reference latching/holding current, not popularity. Verification / Alternative check: Standard SCR theory: once turned on, device conducts until current extinction; references emphasize holding and latching currents, unrelated to transistor adoption rate. Why Other Options Are Wrong: “R explains A”: incorrect linkage. “R wrong” or “A wrong”: each statement is factually correct. Common Pitfalls: Confusing MOSFET's gate charge (low static drive power) with zero drive—MOSFET still needs Vgs maintained. Equating market usage with device physics. Final Answer: Both A and R correct but R is not correct explanation of A

Question 7

Converter commutation overlap: identify the dominant cause of overlap between outgoing and incoming devices during line-commutation.

Accepted Answer

Introduction / Context: In line-commutated converters, device current transfers from one valve to the next during commutation. The supply inductance resists instantaneous current change, causing an overlap interval when both devices conduct, reducing output voltage momentarily. Given Data / Assumptions: Single-phase or three-phase line-commutated rectifier/inverter. Ideal devices, practical ac source with finite inductance. Overlap is the finite duration where two valves conduct simultaneously. Concept / Approach: By Faraday’s law, di/dt through an inductance L requires applied voltage. The ac source inductance impedes rapid current transfer, creating commutation overlap. Source resistance mainly produces voltage drop but does not prevent instantaneous current transfer as strongly as inductance. Step-by-Step Reasoning: At firing instant, new device is triggered; current must rise from 0 to load current while old device current falls.Because of source inductance Ls, di/dt is limited, so both devices conduct for a short angle μ.Hence, overlap is fundamentally attributable to source inductance. Verification / Alternative check: Classical commutation analysis yields overlap angle μ ≈ (Xs * Id) / V_phase, where Xs is source reactance (ωLs), directly tying μ to inductance. Why Other Options Are Wrong: Resistance: affects losses, not primarily the overlap mechanism. “Both”: the principal controlling parameter is inductance; resistance has a secondary effect. High firing angle: increases reactive effects but is not the cause. Load capacitance: unrelated in line-commutated overlap. Common Pitfalls: Blaming firing angle rather than supply reactance. Ignoring that overlap reduces average output voltage. Final Answer: source inductance

Question 8

A single-phase semiconverter supplies a highly inductive R–L load and includes a freewheeling diode across the load. Compare the shapes of output voltage and output current waveforms.

Accepted Answer

Introduction / Context: In semiconverters (half-controlled bridges) feeding highly inductive loads, current tends to be smoothed by inductance while voltage remains pulsed. A freewheeling diode provides a path during source non-conduction, further decoupling current shape from the instantaneous source voltage. Given Data / Assumptions: Highly inductive load ⇒ large L, current ripple small. Freewheeling diode (FD) across load. Phase-controlled thyristors determine when source applies positive voltage. Concept / Approach: Output voltage v_o(t) is a chopped sinusoid (or zero during freewheeling), whereas output current i_o(t) is nearly continuous due to the inductor storing energy and because the FD maintains a current path when the converter is not supplying voltage. Thus v_o(t) and i_o(t) are generally dissimilar in shape. Step-by-Step Reasoning: During conduction: v_o(t) ≈ rectified line segment; i_o(t) rises slowly.During freewheeling: v_o(t) ≈ 0 (across load), but i_o(t) continues flowing through FD, decaying slowly.Hence, current is quasi-DC while voltage is pulsed → not similar. Verification / Alternative check: Waveform sketches from standard texts show pulsed v_o with near-continuous i_o for large L and an FD. Why Other Options Are Wrong: “Are similar/identical”: contradict inductive smoothing and freewheeling action. “Only if α = 0”: even at α = 0, v_o is sinusoidal-rectified while i_o is smoothed. “May be similar”: not for highly inductive loads with FD. Common Pitfalls: Assuming resistive-load intuition carries over to inductive cases. Ignoring FD role in maintaining current when source is off. Final Answer: are not similar

Question 9

Operating quadrants of a single-phase semiconverter feeding an R–L–E (RLE) load: identify the correct set of quadrants in the v–i plane.

Accepted Answer

Introduction / Context: Quadrant operation describes signs of voltage and current at the load. A semiconverter (half-controlled bridge) supplies a unidirectional current with positive average voltage—typical of rectification. For an RLE load (with back EMF), the topology still restricts operation primarily to the first quadrant. Given Data / Assumptions: Semiconverter = two thyristors + two diodes (half-controlled bridge). Freewheeling path available ensures current remains positive. RLE load implies possible current continuity but not reversal of polarity in standard operation. Concept / Approach: Semiconverters cannot force negative current through the load; they deliver positive current with positive average load voltage, placing operation in the first quadrant. Regeneration (second or fourth quadrants) requires fully controlled bridges or dedicated inversion modes not available in a basic semiconverter. Step-by-Step Reasoning: Thyristors conduct in forward direction only; diodes also conduct forward only.Load current remains positive due to device polarity and freewheeling behavior.Average load voltage is positive in rectification; thus operation is in quadrant I. Verification / Alternative check: Converter quadrant charts show semiconverters as one-quadrant converters (rectifiers) unlike full bridges which can be two-quadrant or four-quadrant with control schemes. Why Other Options Are Wrong: Quadrants II, III, IV imply negative voltage and/or negative current not supported by basic semiconverter hardware. Mixed quadrants require bidirectional current control absent here. Common Pitfalls: Assuming RLE back EMF enables inversion without proper switching devices. Confusing semiconverter with fully controlled converter. Final Answer: first quadrant only

Question 10

Assertion–Reason: Inverters and choppers employ fast-switching thyristors because these devices have low turn-off time (tq). Evaluate the statements.

Accepted Answer

Introduction / Context: High-frequency switching converters (inverters and choppers) demand devices that can turn on and, crucially, turn off rapidly to minimize switching losses and allow higher operating frequencies. Fast-switching thyristors are engineered for low reverse recovery and short turn-off times. Given Data / Assumptions: Assertion (A): Inverters and choppers use fast-switching SCRs/thyristors. Reason (R): Fast-switching SCRs have low turn-off time tq. Focus is on commutation speed and loss reduction. Concept / Approach: Turn-off time tq determines the minimum interval after current extinction before a thyristor can block forward voltage again. Lower tq enables higher switching frequency and reduces overlap/recovery losses—key requirements in PWM inverters and dc choppers. Step-by-Step Evaluation: Check A: True—fast devices are chosen to meet switching frequency targets.Check R: True—low tq is the defining characteristic of fast thyristors.Causality: The reason directly explains why such thyristors are preferred in high-frequency converters. Verification / Alternative check: Datasheets specify tq and trr (recovery) for inverter-grade SCRs; application notes tie allowable PWM frequency to tq margins. Why Other Options Are Wrong: Stating R does not explain A contradicts the direct link between low tq and converter suitability. Any claim that A or R is wrong conflicts with standard converter design practice. Common Pitfalls: Confusing di/dt ratings with tq; both matter, but tq governs maximum switching frequency. Overlooking that modern designs often prefer IGBTs/MOSFETs for even faster switching. Final Answer: Both A and R are correct and R is correct explanation of A

Question 11

In a bipolar junction transistor (BJT), if the common-base current gain is α = 0.98, determine the corresponding common-emitter current gain β.

Accepted Answer

Introduction / Context: This problem checks fundamental BJT relationships between the common-base current gain (alpha, written as α) and the common-emitter current gain (beta, written as β). Knowing how to convert between α and β is essential for circuit analysis, bias design, and interpreting datasheets. Given Data / Assumptions: Common-base current gain α = 0.98. Standard BJT relations hold for a properly biased transistor in active region. Leakage currents are neglected (typical assumption for basic calculations). Concept / Approach: In BJTs, the two gains are linked by the identities: β = α / (1 − α) and α = β / (1 + β). With α very close to 1, small changes in α lead to large changes in β. We directly apply the conversion formula to compute β from the given α value. Step-by-Step Solution: Start with the standard relation: β = α / (1 − α).Substitute α = 0.98: β = 0.98 / (1 − 0.98).Compute denominator: 1 − 0.98 = 0.02.Evaluate: β = 0.98 / 0.02 = 49.Hence, the common-emitter gain is β = 49. Verification / Alternative check: Use the inverse relation α = β / (1 + β). With β = 49, α = 49 / 50 = 0.98, which matches the given α. This confirms consistency. Why Other Options Are Wrong: 4.9: Off by a factor of 10, likely a decimal slip.0.5 or 0.02: These are much too small for α close to 1.48: Close, but not exact; correct computation yields 49. Common Pitfalls: Confusing α with β directly or using β = 1 − α (incorrect). Always apply β = α / (1 − α). Final Answer: 49

Question 12

Identify the correct set of terminal names for a power BJT (bipolar junction transistor).

Accepted Answer

Introduction / Context: This question tests core semiconductor device knowledge by asking for the correct terminal names of a power BJT. Recognizing device terminals is essential to wiring, biasing, and analyzing circuits in analog electronics and power electronics. Given Data / Assumptions: Device type: BJT (bipolar junction transistor). Power BJT still follows the same terminal naming as a small-signal BJT. We assume standard device nomenclature used in textbooks and datasheets. Concept / Approach: A BJT has three terminals: emitter (E), base (B), and collector (C). In contrast, MOSFETs use source (S), gate (G), and drain (D). JFETs use source, gate, and drain as well. Therefore, the only correct naming for a BJT is E, B, and C. Step-by-Step Solution: Identify device type: BJT → terminals are E, B, C.Cross-check alternatives: Any mention of Source or Drain refers to FETs, not BJTs.Hence the correct set is Emitter, Base, Collector. Verification / Alternative check: Review a BJT symbol: the arrow on the emitter (NPN pointing out, PNP pointing in) confirms the presence of an emitter; the other two terminals are base and collector. Why Other Options Are Wrong: Any option containing Source or Drain corresponds to FET devices, not BJTs. Common Pitfalls: Confusing BJTs with MOSFETs or JFETs due to similar three-terminal appearance in schematics. Final Answer: Emitter, Base, Collector

Question 13

A single-phase full-wave converter (M-2 connection) supplies a highly inductive load. If each thyristor is triggered at firing angle α in its own half-cycle, what is the conduction period of each thyristor?

Accepted Answer

Introduction / Context: Controlled rectifiers with highly inductive loads exhibit nearly continuous current, which alters the conduction interval of the devices. Understanding the interplay between firing angle and load inductance is central to power electronics and converter design. Given Data / Assumptions: Topology: Single-phase full-wave converter (M-2 connection, two devices). Load: Highly inductive (large L makes current continuous or nearly continuous). Each thyristor is fired at angle α in its respective half-cycle. Concept / Approach: With a highly inductive load, current does not fall to zero quickly. In a full-wave controlled rectifier, each thyristor conducts for approximately π radians because the inductor maintains current flow into the next half-cycle until the complementary device naturally commutates the current. Thus, the conduction interval becomes independent of α (for continuous current conditions) and equals π radians per device. Step-by-Step Solution: Assume continuous current due to large L.When a thyristor is triggered at α, the inductor forces current to continue through it until the line voltage reverses and the complementary thyristor can take over.This results in a conduction span of π radians for each device in steady state.Therefore, conduction period per thyristor = π radians. Verification / Alternative check: In the discontinuous current case (small L), the conduction angle could shrink; however, the question specifies a highly inductive load, implying continuous conduction, validating π radians. Why Other Options Are Wrong: π − α, π − 2α: These apply to discontinuous or different conditions, not highly inductive continuous current operation.π + α: Exceeds half-cycle; not correct for standard continuous conduction in a single-phase full-wave rectifier.α: This is merely the delay angle, not the conduction interval. Common Pitfalls: Confusing firing delay angle α with conduction duration; ignoring the role of inductance in sustaining current. Final Answer: π radians

Question 14

A single-phase semi-converter (half-controlled bridge) operates in how many quadrants of the voltage–current plane?

Accepted Answer

Introduction / Context: Semi-converters (half-controlled bridges) are widely used in DC drives and power supplies. Understanding their quadrant operation is crucial for predicting whether they can handle regenerative braking or reverse power flow. Given Data / Assumptions: Converter type: Single-phase semi-converter (half-controlled bridge). Standard operation with passive DC load (e.g., R–L). No active power reversal path is provided. Concept / Approach: A semi-converter has two thyristors and two diodes. Output voltage remains nonnegative for normal operation, and current direction is typically one way due to the diode structure. This means the converter can operate only in the first quadrant (positive voltage, positive current). Two-quadrant operation requires full control of current polarity or voltage polarity that a semi-converter does not inherently provide. Step-by-Step Solution: Examine bridge composition: controlled devices cannot force negative output voltage while maintaining current direction.Diodes prevent reverse power flow, limiting operation to forward motoring quadrant.Hence, the converter operates in one quadrant (the first quadrant). Verification / Alternative check: Full converters (fully controlled bridges) can extend operation to additional quadrants (with appropriate load), confirming the limitation of semi-converters. Why Other Options Are Wrong: Two/Three/Four quadrants: Require additional control paths or device arrangements not present here.Depends on firing angle only: Firing angle changes average output but not inherent quadrant capability. Common Pitfalls: Assuming any controlled rectifier offers regeneration by default; overlooking diode conduction paths. Final Answer: One quadrant

Question 15

For the chopper feeding an R–L–E load (resistive–inductive with back electromotive force) as shown, when does the freewheeling diode conduct?

Accepted Answer

Introduction / Context: Choppers (DC–DC converters) using inductive loads rely on a freewheeling path to maintain current continuity when the main switch turns off. This question evaluates understanding of inductor current behavior and the role of the freewheeling diode in protecting components and smoothing current. Given Data / Assumptions: Load contains an inductor (L) and possibly a back EMF (E) such as a motor. Main switch is a thyristor or transistor that alternately applies and removes source voltage. Diode is placed to provide a path for inductor current during off intervals. Concept / Approach: Inductor current resists abrupt changes. When the main device turns off, the inductor maintains current; without an alternate path, voltage spikes would occur. The freewheeling diode becomes forward-biased and conducts, letting the current circulate through the load and diode, preventing overvoltage and smoothing current decay. Step-by-Step Solution: Main device ON: Source supplies the load; diode is reverse-biased.Main device OFF: Inductor attempts to keep current flowing; diode forward-biases, providing a closed loop.Therefore, the freewheeling diode conducts during the OFF interval of the thyristor. Verification / Alternative check: Observe current waveforms in chopper textbooks: current ripple continues through the diode path in OFF time, confirming conduction then. Why Other Options Are Wrong: When the thyristor is on: Diode is typically reverse-biased by source voltage.Partly on and off: Standard operation is OFF-time conduction.Never conducts / Only at start-up: Incorrect; conduction happens every cycle during OFF-time. Common Pitfalls: Forgetting that inductors demand a path for current continuity; misidentifying diode polarity. Final Answer: When the thyristor is off

Question 16

In power electronics, what does a chopper (DC–DC converter) fundamentally do to the input supply?

Accepted Answer

Introduction / Context: DC–DC converters, commonly called choppers, are ubiquitous in electric drives, renewable energy systems, and battery-powered electronics. Understanding their basic function is foundational before studying control strategies, device selection, and ripple management. Given Data / Assumptions: Input: A steady DC source. Output: A controllable average DC voltage. Switching element: A transistor or thyristor operated with a duty cycle. Concept / Approach: A chopper regulates the average output voltage by rapidly switching the input DC on and off, varying the duty cycle D = TON / T. The result is a variable DC level after suitable filtering, without requiring an intermediate AC conversion stage (for basic buck/boost/buck-boost topologies). Step-by-Step Solution: Consider a simple buck chopper: Vout(avg) ≈ D * Vin.By adjusting D from 0 to 1, the effective DC output varies from 0 to Vin.No intermediate AC line-frequency conversion is required; switching happens at kHz range. Verification / Alternative check: Practical converters may include inductors and capacitors to reduce ripple, but the fundamental action remains direct DC-to-variable-DC conversion via switching. Why Other Options Are Wrong: DC→AC→DC: That describes a different architecture (inverter plus rectifier), not a basic chopper.AC frequency conversion and AC→DC are unrelated to DC–DC choppers.DC→AC only is an inverter function, not a chopper. Common Pitfalls: Confusing choppers with inverters or AC regulators; ignoring the role of duty cycle in setting the average output. Final Answer: Converts constant-voltage DC directly into variable-voltage DC

Question 17

Assertion–Reason in DC choppers: Assertion (A): If the duty cycle of a chopper is low, the current would not become discontinuous. Reason (R): If the load time constant L/R is low, the current may become discontinuous.

Accepted Answer

Introduction / Context: Chopper-fed inductive loads can operate in continuous or discontinuous current mode depending on switching parameters and the load time constant. This assertion–reason item probes how duty cycle and L/R affect current continuity. Given Data / Assumptions: Chopper with R–L load. Duty cycle D can be low or high. Load time constant τ = L / R sets current decay rate during OFF time. Concept / Approach: Low duty cycle implies longer OFF time fraction, giving current more time to decay. A small τ (low L/R) also causes faster decay. Both conditions favor discontinuous current. Hence, the assertion that low duty cycle prevents discontinuity is wrong; the reason that small L/R can cause discontinuity is correct. Step-by-Step Solution: During OFF time, i(t) decays approximately as i * exp(−t/τ).Lower duty cycle → larger OFF interval → more decay → more likely i reaches zero → discontinuous mode.Smaller τ = L/R → faster exponential decay → increased chance of discontinuity.Therefore: Assertion (A) is wrong; Reason (R) is correct. Verification / Alternative check: Simulation or timing diagrams confirm that elongated OFF periods and small τ values produce discontinuous current at moderate loads. Why Other Options Are Wrong: Any option claiming A is correct contradicts chopper current decay physics.Saying R is wrong ignores the well-known impact of τ on decay. Common Pitfalls: Assuming duty cycle only changes average voltage, not current mode; overlooking the exponential nature of inductor current decay. Final Answer: A is wrong but R is correct

Question 18

In a thyristor (SCR), if the applied gate current is increased, what can be said about the anode current?

Accepted Answer

Introduction / Context: Thyristors are latching devices. The gate current influences turn-on behavior (latching current, turn-on time), but the steady anode current is set by the external circuit once the device is conducting. This question assesses that distinction. Given Data / Assumptions: Device: SCR operated in its forward-blocking to forward-conducting transition. Gate current magnitude can be varied. External anode circuit (source and load) determines current once latched. Concept / Approach: Increasing gate current reduces turn-on time and required latching/holding currents, so the device turns on more reliably and possibly earlier in a phase-controlled rectifier. However, the anode current after turn-on is governed by supply voltage, load impedance, and instantaneous waveform phase, not by the gate current (beyond the turn-on moment). Thus, depending on the circuit, average anode current may increase (earlier firing) or remain similar; generically, it may increase or decrease based on timing and load. Step-by-Step Solution: Gate current ↑ → improved triggering, lower latching/holding current thresholds.In phase control, earlier conduction → larger conduction angle → higher average anode current.In a DC chopper-like circuit with fixed ON timing already saturated, change may have little effect.Hence the safe, general statement: anode current may increase or decrease; it is not fixed by gate current alone. Verification / Alternative check: Datasheets indicate gate current affects di/dt limits and turn-on characteristics, not steady-state current for a given load line. Why Other Options Are Wrong: Always increase/decrease/constant: Overly absolute; ignores circuit dependence.Zero after turn-on: Contradicts the nature of conduction in an SCR. Common Pitfalls: Confusing control of a transistor's collector current via base current with an SCR's latching behavior. Final Answer: Anode current may increase or decrease

Question 19

Assertion–Reason about rectifiers: Assertion (A): An ideal rectifier should have a unity harmonic factor. Reason (R): An ideal rectifier should have a unity displacement factor.

Accepted Answer

Introduction / Context: Power quality in rectifiers is described by displacement factor (related to phase angle between fundamental voltage and current) and harmonic factor (ratio of harmonic RMS to fundamental RMS). This assertion–reason problem distinguishes these two metrics for an idealized rectifier current waveform. Given Data / Assumptions: Ideal rectifier: zero device drop, instantaneous switching, purely resistive AC source. Current may be non-sinusoidal (contains harmonics), depending on topology and smoothing. Definitions: power factor = displacement factor * distortion factor (where distortion factor relates to harmonics). Concept / Approach: Unity displacement factor means the fundamental component of current is in phase with the supply voltage (cos φ = 1). Harmonic factor (HF) is not unity for a non-sinusoidal current; in fact, HF = Ih / If1, where Ih is RMS of all harmonics and If1 is RMS of the fundamental. For a perfectly sinusoidal current, harmonic factor would be zero, not one. Thus, stating that an ideal rectifier “should have unity harmonic factor” is incorrect, whereas stating it “should have unity displacement factor” can be valid for an idealized, phase-aligned fundamental. Step-by-Step Solution: Recognize: Ideal rectifier current often contains harmonics → HF ≠ 0 and certainly not necessarily 1.Displacement factor for an ideal, resistive situation is 1 (no phase shift of fundamental).Therefore: A is wrong; R is correct. Verification / Alternative check: For a smoothed DC load drawing near-sinusoidal AC current, HF can be reduced but not inherently forced to “unity.” Conversely, unity displacement factor is achievable with purely resistive source conditions. Why Other Options Are Wrong: Any option claiming A is correct misdefines harmonic factor.Claiming R is wrong ignores correct definition of displacement factor for an idealized phase alignment. Common Pitfalls: Confusing “unity” with “zero” for harmonic factor; mixing up displacement and distortion factors. Final Answer: A is wrong but R is correct

Question 20

A thyristor has a maximum allowable junction temperature of 120 °C in an ambient of 40 °C. If the thermal resistance (junction-to-ambient) is 1.6 °C/W, what is the maximum allowable internal power dissipation?

Accepted Answer

Introduction / Context: Thermal design ensures semiconductor devices operate within safe junction temperatures. The key parameters are junction temperature limit, ambient temperature, and the device's effective thermal resistance to ambient. This question applies the basic thermal equation to find allowable power dissipation. Given Data / Assumptions: Maximum junction temperature Tj,max = 120 °C. Ambient temperature Ta = 40 °C. Thermal resistance Rθ = 1.6 °C/W (junction-to-ambient). Steady-state operation; heat-sinking performance encapsulated in Rθ value. Concept / Approach: Use the thermal relation analogous to Ohm's law: ΔT = P * Rθ, where ΔT = Tj,max − Ta. Solve for P (power dissipation) allowed by the temperature rise limit. Step-by-Step Solution: Compute allowable temperature rise: ΔT = 120 − 40 = 80 °C.Thermal equation: P = ΔT / Rθ.Substitute: P = 80 / 1.6 = 50 W.Hence, the device may dissipate up to 50 W without exceeding 120 °C junction temperature. Verification / Alternative check: If a heat sink reduces Rθ, allowable P would increase. Conversely, higher ambient would reduce allowable P. The computed value aligns with typical datasheet thermal math. Why Other Options Are Wrong: 20 W: Underestimates; corresponds to ΔT = 32 °C at Rθ = 1.6.92 W or 128 W: Would require much lower Rθ or lower ambient to be feasible.62.5 W: Would imply ΔT = 100 °C, exceeding the available 80 °C margin. Common Pitfalls: Mixing up junction and case temperatures; using the wrong temperature difference; forgetting units °C/W. Final Answer: 50 W

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