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EE123 Power ElectronicsDepartment of Electrical EngineeringUniversity of California – Riverside

Instructor: Roman Chomko

Final ExamEE 123

 Winter 2013

FinalEXAMINATION

Print Your Name:  ______________________________________ 

Date:  March 19, 2013 (Tuesday) 

Time: 7.00 p.m. - 10.00 p.m. Location:  MSE 113

Special Notes: 

1.  This is an OPEN BOOK , OPEN NOTES test

2.  Notes sharing is strictly prohibited

3.  Paper and other supplies WILL NOT be provided4.  Have calculators available

5.  Cell phones and other communication gadgets must be off

Objectives:

To assess understanding of electrically non-isolated DC-DC converters(buck, boost, buck-boost) and isolated DC-DC converters (flyback andforward) operating in CCM or DCM modes; DC-AC inverters.

Material:

Primary *:   Lectures 10-16, Homeworks  4, 5, Lab Manuals 5 - 7  

* Nothing beyond Lecture Notes and Lab Manuals is required

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Design Problem #1  (20 points)

Design a buck converter that has an output of 12 V from an input of 18 V. The outputpower is 10W. The output voltage ripple must be no more than 100 mV peak-to-peak.

1)  Specify the duty ratio, switching frequency, and inductor and capacitor values.Design for continuous inductor current  (CCM). Assume ideal components. Providethe schematic of the buck converter;

2)  At what load resistance the converter thus designed will operate at the boundarybetween CCM and DCM (discontinuous current) modes?

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Design Problem #2 (20 points)

Design a DC-DC converter to produce a regulated -15 V output from a source that varies

from 12 V to 18 V. What is the range of required duty cycles? The converter must be

designed for the continuous inductor current (CCM). The load is a 15 Ω  resistor. Provide

the schematic of this converter and specify its type.

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Design Problem #3 (20 points)

Design a flyback converter for an input of 24 V and an output of 40 W at 40 V. Specify thetransformer turns ratio and magnetizing inductance, switching frequency, and capacitor to

limit the ripple to less than 0.5 %. Provide the schematic of the flyback converter used.

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Problem #4 (20 points)

Figure P4 

The forward converter has parameters V S  = 100V, N 1 /  N 2 = 5, N 1 /  N 3 = 1, Lm = 300 mH, L x =

50 mH, R = 5 Ω, C  = 100 µF, D = 0.4, and the switching frequency of f  = 50 kHz.

a)  Determine the output voltage, the maximum and minimum currents in  L x  and theoutput voltage ripple.

b)  Is the magnetizing current I  Lm is reset to zero during each switching cycle with theparameters given?.

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Problem #5 (20 points)

Figure P5 

The square-wave inverter in Figure P5 has a DC input of 150 V and supplies a series  RL 

load with R = 20 Ω and L = 40 mH.

a)  Determine an expression for steady-state load current;

b)  Sketch the output load voltage and current.