(SEM VI) THEORY EXAMINATION 2024-25 POWER ELECTRONICS

BEE603 – POWER ELECTRONICS

Section-Wise Solved Answers (2024–25)

SECTION A

Attempt all questions in brief (2 × 7 = 14 marks)

(a) Explain Power MOSFET

A Power MOSFET is a voltage-controlled semiconductor switching device widely used in power electronic circuits. It operates by applying voltage to the gate terminal, which controls the flow of current between drain and source. Power MOSFETs offer high switching speed, high input impedance, and low switching losses, making them suitable for high-frequency applications such as SMPS, inverters, and choppers.

(b) Define latching current and holding current

Latching current is the minimum anode current required to keep a thyristor in ON state immediately after it is triggered and the gate signal is removed. Holding current is the minimum anode current below which the thyristor turns OFF. Holding current is always less than latching current.

(c) Difference between freewheeling diode and feedback diode

A freewheeling diode provides an alternate path for inductive load current when the main switch is OFF, preventing sudden voltage spikes. A feedback diode is used to return energy back to the source. Freewheeling diodes protect switches, while feedback diodes improve efficiency through energy recovery.

(d) Control strategies for varying duty cycle in choppers

Duty cycle in choppers is controlled using time ratio control techniques. These include constant ON-time control, constant OFF-time control, and pulse-width modulation (PWM). By varying the ratio of ON time to total switching period, the output voltage can be smoothly controlled.

(e) Difference between AC voltage controller and cycloconverter

An AC voltage controller changes the RMS value of output voltage without changing frequency, whereas a cycloconverter directly converts AC power at one frequency to another AC power at a lower frequency. AC voltage controllers are simpler, while cycloconverters are used for variable-speed AC drives.

(f) Preferred conduction mode in three-phase VSI

The 180-degree conduction mode is preferred in three-phase VSI because it provides better utilization of DC bus voltage, reduced harmonic distortion, and smoother output voltage waveforms compared to 120-degree conduction mode.

(g) Methods of voltage control in single-phase inverter

Voltage control in single-phase inverters is achieved using PWM techniques such as single-pulse PWM, multiple-pulse PWM, and sinusoidal PWM. These methods control output voltage by varying pulse width or modulation index.

SECTION B

Attempt any three (7 × 3 = 21 marks)

(a) V-I characteristics and triggering of thyristor

The V-I characteristics of a thyristor show three regions: forward blocking, forward conduction, and reverse blocking. Initially, the thyristor blocks forward voltage until gate triggering is applied. Once triggered, it conducts heavily with low voltage drop.
Triggering methods include gate triggering, forward voltage triggering, dv/dt triggering, and thermal triggering. Gate triggering is the most common and reliable method.

(b) Buck-boost converter duty ratio calculation

For a buck-boost converter operating in continuous conduction mode, the output voltage is given by:

Vo=D1−DVDCV_o = \frac{D}{1-D} V_{DC}Vo​=1−DD​VDC​

Given that output voltage magnitude is constant at 48 V and input varies from 32 V to 72 V, duty ratio adjusts accordingly to maintain constant output. By substituting minimum and maximum input voltages, the corresponding range of duty cycle is obtained.

(c) Operation of Current Source Inverter (CSI)

A Current Source Inverter uses a constant DC current source as input. It employs large inductance at input to maintain constant current. CSI produces controlled AC output current. It is mainly used in high-power motor drives. The inverter ensures current commutation between switching devices using capacitors.

(d) Integral cycle control in AC voltage controller

Integral cycle control controls output voltage by turning ON the AC supply for a fixed number of cycles and turning it OFF for a fixed number of cycles. RMS output voltage depends on the ratio of ON cycles to total cycles.
For a given load and supply voltage, RMS output voltage and input power factor can be calculated using standard expressions.

(e) Comparison of half-wave and full-wave rectifiers

A half-wave rectifier uses one diode and converts only one half cycle of AC input, resulting in low efficiency and high ripple. A full-wave rectifier uses both half cycles, giving higher efficiency, smoother output, and better transformer utilization. Output voltage expressions are derived considering load type (R, RL, RLE).

SECTION C

Q3. Attempt any one

(a) Thyristor protection and series-parallel operation

Thyristor protection includes dv/dt protection using RC snubber circuits, di/dt protection using inductors, and over-voltage protection using MOVs.
When thyristors are connected in series, voltage sharing problems arise, and equalizing circuits are required. In parallel operation, current sharing issues occur, requiring balancing resistors or inductors.

(b) Commutation and its types

Commutation is the process of turning OFF a conducting thyristor. Types include natural commutation and forced commutation.
In forced commutation, external circuits force current to zero. Class A (load commutation), Class B (resonant commutation), and Class C (auxiliary commutation) are commonly used methods.