Questions
7–9 questions per paper
Difficulty
Medium-Hard
Importance
Core — high scoring potential
Overview
Power Electronics is a critical subject for electrical engineering PSU exams, focusing on the static conversion and control of electric power using semiconductor devices. It bridges the gap between power systems and electronics, requiring a strong understanding of switching characteristics, rectification circuits, and inversion topologies to excel in competitive assessments.
Thyristors: SCR, DIAC, and TRIAC
Thyristors are the workhorses of power conversion, functioning as high-speed switches in controlled rectifiers and inverters. Mastering the latching current, holding current, and turn-on methods is essential for solving gate-control logic questions.
- SCR turns off when anode current falls below holding current (Ih).
- Latching current (Il) is always greater than holding current (Ih).
- dV/dt protection is provided by a snubber circuit.
- TRIAC is a bidirectional device suitable for AC power control.
- DIAC is a bidirectional trigger diode used to switch TRIACs.
Controlled Rectifiers
Controlled rectifiers use SCRs to vary the output voltage by adjusting the firing angle alpha. These circuits are frequently tested via questions on ripple factor, harmonic content, and average output voltage calculations.
- Average voltage for Single Phase Half-Wave: Vm/2pi(1+cos alpha).
- Full-wave bridge rectifier: Vdc = 2Vm/pi(cos alpha).
- Freewheeling diode improves power factor and eliminates negative voltage segments.
- Performance parameters include Form Factor and Ripple Factor.
- Discontinuous conduction occurs at large firing angles in inductive loads.
DC-DC Converters (Choppers)
Choppers are used to transform fixed DC to variable DC levels, critical for DC motor control applications. PSU exams often require calculation of duty cycle and output voltage for both Buck and Boost topologies.
- Buck converter output: Vo = D * Vin.
- Boost converter output: Vo = Vin / (1 - D).
- Buck-Boost converter output: Vo = Vin * D / (1 - D).
- Duty cycle D = Ton / T.
- Operation in CCM vs DCM depends on the load current and inductor size.
Inverters (VSI and CSI)
Inverters convert DC to AC, and their classification depends on the nature of the source and the commutation technique used. Expect questions regarding the Fourier series of output waveforms and specific switching configurations.
- VSI (Voltage Source Inverter) has a stiff DC voltage source input.
- CSI (Current Source Inverter) utilizes a large series inductor.
- 180-degree mode of operation is standard for three-phase VSIs.
- Pulse Width Modulation (PWM) reduces low-order harmonics.
- McMurray and Bedford circuits are classical forced commutation techniques.
Formula Sheet
Average DC Voltage (1-Ph Half Wave): Vm/2pi(1+cos alpha)
Average DC Voltage (1-Ph Full Bridge): 2Vm/pi(cos alpha)
Buck Converter: Vo = D*Vin
Boost Converter: Vo = Vin/(1-D)
Buck-Boost Converter: Vo = Vin*D/(1-D)
RMS Voltage (Full Wave): Vm/sqrt(2)
Form Factor: Vrms/Vdc
Ripple Factor: sqrt((Vrms/Vdc)^2 - 1)
Exam Tip
Memorize the relationship between duty cycle (D) and output voltage for all three chopper types (Buck, Boost, Buck-Boost), as these are the highest frequency numerical questions in PSU exams.
Common Mistakes
- Confusing the formulas for half-wave vs full-wave controlled rectifiers in the presence of freewheeling diodes.
- Neglecting the impact of source inductance on the commutation overlap angle in rectifier circuits.
- Forgetting the difference between latching and holding current, leading to errors in SCR turn-on and turn-off logic.
More Revision Notes
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