Controlled Rectifier - Power Electronics - Past Exam Paper, Exams of Power Electronics

Download Controlled Rectifier - Power Electronics - Past Exam Paper and more Exams Power Electronics in PDF only on Docsity! Semester I Examinations 2008 / 2009 Exam Code(s) 4BN Exam(s) B.E. Degree (Electronic Engineering) Module Code(s) EE411 Module(s) Power Electronics Paper No. I External Examiner(s) Professor G. Irwin Internal Examiner(s) Professor G. Ó Laighin Professor W.G. Hurley Instructions: Answer 3 questions from 4. All questions will be marked equally (20 marks). Duration 2 hrs No. of Pages 3 Department(s) Electronic Engineering Requirements None Q1. Explain the following terms: distortion factor, displacement factor, power factor. [4 marks] In a half-wave phase controlled rectifier with a free wheeling diode (shown in Figure 1) and a highly inductive load, the source is 240Vrms at 50 Hz and the load resistance is 8 Ω, assuming the firing angle α=0, (a) Sketch the output voltage, the source current and the current through the freewheeling diode; [4 marks] (b) Calculate the power absorbed by the load; [4 marks] (c) Calculate the power factor as seen by the source. [4 marks] (d) How would the operation of the circuit change if the free wheeling diode were replaced by an SCR fired  radians after the rectifying SCR is fired. Derive an expression for the average load voltage in this case. [4 marks] DVsin t Id Figure 1 Q2. Draw the circuit and associated waveforms for a buck converter and explain its operation. [8 marks] Design a Voltage Regulator Module (VRM) for local power regulation of a microprocessor in a mobile phone. The VRM is powered from a 3.3 V source and uses a buck converter. The microprocessor specifications call for a 1.85 V supplying 60 mA. The switching frequency is 500 kHz. Neglect the parasitic effects of the controlled MOSFET switch and diode. (a) Find the duty cycle of the controlled switch needed to output 1.85V; [4 marks] (b) Choose an inductor that limits the current ripple to +/- 10%; [4 marks] (c) Calculate the value of the duty cycle that yields maximum output current ripple. [4 marks] Q3. Describe three advantages of using a transformer in a switched mode power supply. [4 marks] Draw the circuit and associated waveforms for a flyback converter and explain its operation. [8 marks] A flyback converter operates at 50 kHz, 100V and 1000 W with an output voltage of 500 V and with a turns ratio of 1:4. Calculate (a) the value of the output capacitance so that the output ripple is 10 V; [4 marks] (b) the minimum value of primary inductance to ensure continuous flux. [4 marks]