Download EO323_04-05 - University of Brighton

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SCHOOL OF ENGINEERING
MODULAR HONOURS DEGREE COURSE
LEVEL 3
SEMESTER 2
2004/2005
POWER ELECTRONICS & ADVANCED ANALOGUE
ELECTRONICS TECHNOLOGY
Examiners: Dr. B. Baha & Dr. S. Singh
Attempt THREE questions only.
Answer the question in Section A
Time allowed: 2 hours
Total number of questions = 4.
and two questions from Section B
All questions carry equal marks.
The figures in brackets indicate the relative weightings of parts of a question.
Special requirements: None.
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Section A
1) The circuit in Figure Q1 is made up of three distinct stages linked in a feedback
manner. A differential amplifier using operational amplifier A1, a voltage inverter,
INV and an integrator using operational amplifier A2. The circuit is initially in a
stable state with the input voltage Vin equal to zero and no voltage across the
capacitor Vc. However whenever an input voltage is applied to the circuit the
voltage Vd is eventually driven to zero as is Vo and the capacitor voltage Vc
eventually is equal to the input voltage Vin.
a) Describe and analyse the behavior of the differential amplifier stage. Show
mathematically that Vo = -Vd, and that it is independent on the input voltage
Vin.
(12)
b) Derive a relationship between the voltage Vo and the rate of change of the
feedback voltage
dVfb
making reference to the resistor current Ir and capacitor
dt
voltage Vc in the explanation of your derivation.
(8)
c) If the input voltage Vin is a positive step input, sketch a diagram of the input
voltage, the feedback voltage Vfb and the difference voltage Vd against time on
a single graph plot.
QUESTION 1 CONTINUED ON PAGE 2/6
(5)
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Vc
C
Ir
r
INV
A2
R
R
VA1
Vd
V+
Vfb
R
Vo
R
Vin
0V
Vin input voltage
Vo output voltage
Vd difference voltage
Vfb feedback voltage
R, r resistors
A1,A2 op amps
C capacitor
Vc capacitor voltage
INV Inverter
V- op amp A1 inverting input voltage
V+ op amp A1 non-inverting input voltage
Ir current in resistor r.
Figure Q1
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Section B
2)
a) Explain the main functions of semiconductor devices within any power
electronic systems and identify other essential components for such systems.
(3)
b) For a step-up dc-dc converter, the following parameters are given:
L = 1.8 mH, Vin = 50 V, Vo = 120 V, R = 20, C = 147 F and the
switching frequency is 15 kHz.
i) Calculate the values of the currents flowing in the inductor (IL), capacitor
(Ic) and input and out current ( Iin, Io).
(12)
ii) Sketch the current waveform associated with the filter inductor L and also
sketch the current through the output capacitor C.
(6)
iii) Estimate the peak-peak value of the ripple voltage across the capacitor during
steady state operation.
(4)
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3)
a) What are the main differences between Bipolar-junction Transistor and Metal-Oxide
Semiconductor-Field-Effect-Transistors (MOSFETs)? Suggest suitable power
electronics applications for each of these devices.
(6)
b) An input voltage of 15 V is connected to the circuit shown in Figure Q3. Assume
the circuit is in the steady-state condition and the value of L =5 H and output
power is Pload = 250 W are given.
i) Calculate the average output voltage Vo(t) when the on time of the switch is 6s
and the switching frequency is aproximately167 kHz.
(4)
ii) Assume that C has a large value so that Vo(t) = Vo. Calculate the load current
and the r.m.s value of the capacitor current ic.
(8)
iii) Plot the voltage across the inductor (vL) and the current through the inductor (iL)
as indicated in Figure Q3.
(4)
iv) Explain the differences between the analysis and practical results that you will
obtain from this power electronic circuit.
(3)
L
Vo
iL
+ vL
S
_
iC
iR
+
Vs
D
-
Figure Q3
C
R
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4)
a) Why is it desirable to increase the switching frequency of dc-dc converters? Discuss the
idea behind resonant and quasi-resonant converters (QRCs), which may be used to
increase the switching frequency of power supplies. Discuss the main disadvantages of
resonant and QRCs.
(4)
b) The control circuit is an essential part of any switched mode power supply. Identify two
types of control method and briefly explain multi-loop control method for switched mode
power supplies.
(10)
c) With the aid of a suitable diagram, show that the small-signal gain of a pulse-widthmodulation (PWM) controller for a switched mode power supply, is given by

d

vc
=
(8)
1
Vp


Where d and v c are small perturbations to the steady state values of the duty cycle D
and control voltage Vc respectively.
d) What are the advantages and disadvantages of higher and lower bandwidth for the
control loop of a SMPS and what is the optimum limit of bandwidth for such
a controller?
(3)