Power Electronics And Drives : Practice Questions and Answers

When the switch is ON in a Buck Boost converter, it creates a direct path from the input source to the inductor, which stores energy. The diode is reverse-biased during this phase (OFF state), blocking current flow to the output. When the switch turns OFF, the inductor discharges through the now forward-biased diode to supply the load.

An IGBT is constructed from four semiconductor layers — P+, N−, P, and N+ — forming a PNPN structure. These layers enable high voltage blocking in the N− drift region and efficient current conduction between collector and emitter when the gate is triggered.

THD measures harmonic distortion relative to the fundamental frequency. A very low THD means the output waveform closely resembles a pure sine wave, ensuring loads such as motors and transformers operate efficiently, minimizing EMI, and reducing thermal stress on components.

A single-phase fully controlled rectifier uses four thyristors in a full-bridge configuration to convert AC input into controlled DC output. The firing angle α determines the output voltage level, ranging from maximum positive DC at α = 0° down to zero at α = 180°.

A Thyristor (SCR) is a four-layer PNPN semiconductor device with three terminals: Anode, Cathode, and Gate. It acts as a bistable switch, remaining OFF until a gate trigger turns it ON, after which it conducts until current drops below the holding level. Inductors, transistors, and transformers do not match this description.

A single-phase semiconverter uses both diodes and thyristors, giving it advantages of reduced reactive input, lower cost (fewer thyristors than a full converter), and improved power factor. However, the lower average DC output voltage for the same firing angle compared to a fully controlled converter is a limitation, not an advantage.

A buck converter is a step-down DC-DC converter that reduces a higher DC input voltage to a lower DC output voltage. The switching and filtering action of the inductor, diode, and capacitor produces a stable DC output, not AC or any other waveform.

The average output voltage of a single-phase half-wave controlled rectifier is (V_{o(avg)} = \frac{V_m}{2\pi}(1 + \cos\alpha)). With (V_m = 400) V and (\alpha = 60°): (V_{o(avg)} = \frac{400}{2\pi}(1 + \cos 60°) = \frac{400}{2\pi} \times 1.5 = \frac{300}{\pi}).

In a single-phase PWM inverter, the output voltage waveform is symmetrical around the time axis, ensuring consistent positive and negative cycles. Due to this symmetry, odd harmonics cancel out and are absent in the PWM waveform, resulting in an output that closely approximates a pure sine wave.

The three terminals of an IGBT are gate, collector, and emitter — not drain, source, and gate. Drain and source are MOSFET terminals. The IGBT is a bipolar device (not unipolar), is unidirectional, and is fully controlled via its gate voltage.

A TUF close to unity is desirable as it indicates the transformer is being used efficiently. A higher pulse number reduces DC ripple and harmonics, improving smoothing and commutation. Statement b is incorrect — the PIV to no-load DC output voltage ratio should be kept low, not high, to prevent diode breakdown.