60 solved GATE EE questions on Power Electronics, drawn from 10 exam years and grouped by year. Every question shows the official answer and a step-by-step solution.
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Identify which devices conduct during each subinterval of the switching cycle before writing voltage and current equations. These selected foundations connect thyristor characteristics, controlled rectifiers, DC–DC converters, and inverters through original exercises. State assumptions about load type (resistive, inductive, or constant-current) and conduction mode explicitly.
Our study notes and original examples support the PYQs below; they are not official exam questions or a replacement for the current syllabus.
A thyristor turns on when a gate pulse is applied while it is forward-biased; it turns off only when the anode current falls below the holding current. Natural commutation relies on the AC source reversing polarity. Forced commutation uses an auxiliary circuit to reverse-bias the thyristor. The firing angle α delays turn-on from the natural conduction point.
Check yourself: What mechanism returns the thyristor current to zero in this circuit?
For a single-phase fully-controlled bridge with a highly inductive load (constant load current), the average output voltage is V_dc = (2V_m/π) cos α. With a resistive load, conduction may become discontinuous for large α. Three-phase bridges have lower ripple and higher power capability; the average voltage formula changes to V_dc = (3V_m,LL/π) cos α for a six-pulse bridge.
Check yourself: Is the load sufficiently inductive to assume continuous and constant current?
A buck (step-down) converter in continuous conduction mode (CCM) gives V_out = DV_s, where D is the duty ratio. A boost (step-up) converter gives V_out = V_s/(1 − D). A buck-boost converter gives V_out = −DV_s/(1 − D). These relations follow from volt-second balance on the inductor over one switching period.
Check yourself: Is the inductor current continuous throughout the switching cycle?
A single-phase full-bridge inverter produces a square-wave or quasi-square-wave AC output from a DC source. The fundamental-frequency RMS component of a square wave of amplitude V_dc is (2√2/π)V_dc. Pulse-width modulation (PWM) shapes the switching pattern to control the output voltage magnitude and reduce lower-order harmonics.
Check yourself: Have you identified which pair of switches conducts during each half-cycle?
A single-phase AC voltage controller uses back-to-back thyristors or a triac to regulate RMS voltage by delaying the firing angle. For a resistive load, the RMS output voltage involves the integral of v² from α to π. Power factor decreases as α increases because the displacement and distortion factors both worsen.
Check yourself: Does your integral account for conduction in both half-cycles?
Original teaching example · not a PYQ
Original mini-example: a single-phase fully-controlled bridge rectifier supplies a highly inductive load (assume constant load current) from a 230 V RMS, 50 Hz AC source. The firing angle is α = 30°. Assuming ideal thyristors, find the average output voltage.
Average output voltage V_dc = 230√6/π V ≈ 179.3 V.
An ideal buck (step-down) DC–DC converter has input voltage V_s = 200 V, duty cycle D = 0.6, and an LC output filter feeding a resistive load R = 10 Ω. Assuming continuous inductor current, find the average output voltage and the average load current.
V_out = 120 V; I_out = 12 A.
In CCM, volt-second balance requires D(V_s − V_out) = (1 − D)V_out, which gives V_out = DV_s = 0.6 × 200 = 120 V. The average inductor current equals the load current: I_out = V_out/R = 120/10 = 12 A.
Supplemental reading, not an official GATE reading list or an endorsement of these notes.
Free supplemental lecture notes and problem sets covering rectifiers, DC–DC converters, inverters, and PWM techniques for power conversion.
This page shows 60 recent questions from the released archive, newest first. For older questions and complete papers, browse all GATE EE papers. Questions can carry more than one subject tag; counts are not marks weightage.
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