Seat No.: Enrolment
GUJARAT TECHNOLOGICAL UNIVERSITY
ME SEMESTER-III EXAMINATION WINTER 2018
Subject Code: 2730709 Date: 19-Nov-2018
Subject Name: Modeling and Analysis of Power Converters
Time: 02:30 PM To 05:00 PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
Q.1

Discuss the effect of negative feedback on the network transfer functions.
07

A buck-boost converter has inductor's resistance RL. All other losses can be ignored. Derive an expression for the non-ideal voltage conversion ratio V/Vg.
07
Q.2

With an example of Buck-Boost converter, explain the process of obtaining a Canonical model for a dc-dc converter.
07

A Boost converter with an input source of 20V is controlled in an open loop with a duty cycle of 0.4 to feed a resistive load of 5Ω. If the internal resistance of the inductor RL=0.01Ω and on-state resistance of the switch Ron=0.01Ω, determine the efficiency of the converter. Also determine the output voltage. Ignore all other losses.
07
OR

Using the principle of circuit averaging or average switched network, obtain the DC and small signal ac averaged circuit model of the Buck converter operating in CCM.
07
Q.3

For a Buck converter having following parameters obtain the magnitude Bode plot for open loop line to output transfer function.
Vg 100V; Vo 40V; L 0.4mH; C 100μF; R=2Ω.
07

Obtain the phase bode plot for the line-to-output transfer function of the converter mentioned in the previous question Q.3(a).
07
OR
Q.3

Discuss the method of graphical construction of impedance or transfer functions and hence, obtain the control to output transfer function for the following circuit.
07

Discuss the significance of Phase Lag type compensators with relevant equations and waveforms in improving the performance of the converters.
07
Q.4

An unregulated dc input voltage varies over the range 35V to 70V. A buck converter reduces this voltage to 28 a feedback loop varies the duty
07
cycle as necessary such that the converter output voltage is always equal to
28 V. The load power varies over the range 10W to 1000W The element
values for the converters are C=470μF and switching frequency
fs =75kHz. Losses may be ignored.
Over what range of input voltage and load current does the converter
operate in CCM?
Determine the maximum and minimum values of the steady-state
transistor duty cycle.
Derive the control-to-output transfer function for a duty-cycle controlled boost
converter operating in CCM.
07
OR
Q.4 Obtain the averaged large-signal equivalent circuit for a duty-controlled
Boost converter operating in DCM. Also derive the equation for voltage
gain M for DCM operation.
07
Q.4 Discuss the significance of an artificial ramp for current-programmed
control of a converter. Also, derive the necessary condition that is required
to ensure the stability.
07
Q.5 For the figure shown below, obtain the transfer function using
Middlebrook's extra element theorem. Treat C as an extra element.
07
Obtain the following functional block diagram of a Buck converter
operating with a current-programmed control and show that
L
D T
F S
g 2
2


L
D T
F S
v 2
2
and
a S
m M T
F
1

07
OR
Q.5 With the addition of an input filter the control to output transfer function of
the converter is expressed as
07


Z Z
Z s Z s
G s G s
O D
O N
vd new vd old


1
1

Derive the expression for and for the Buck converter.
Compare the current programmed mode controlled DC-DC converter with
that of duty cycle controlled converter on various aspects.
07