DC-DC Switching Boost Converter

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DC
-
DC Switching Boost Converter




by





lilongwe




ECE 345

Senior Design Final Report



TA: Inseop Lee


May 4, 1999

Project Number: 63







ii





ABSTRACT


The switching power supply market is flourishing quickly in today’s high
-
tech world. Design
engin
eers aren’t always supplied with the desired amount of voltage they need in order to make
their design work. Adding an additional voltage supply to a design is not always cost efficient.
This report is intended to provide the designer with a method of bo
osting DC voltage from 5 Volts
to 12 Volts, by using a DC
-
DC switching boost converter designed specifically for this task. All
goals, design procedures, tests, data, conclusions, and costs have been documented within this
report. Results of experiments
show that the switching converter will boost voltage from 5 volts to
12 volts with power conversion efficiency of 73 percent.









iii





TABLE OF CONTENTS

1.

Introduction

……………………………………………………………………………1

1.1

Purpose of the Project.……………………………………………………………1

1.2

Block Diagram
……………………………………………………………………1

1.3

Specifications……………………………………………………………………..1

2.

Design Procedures …………..…………………………………………………………..2

2.1

General Boost Converter Configuration………………………………………….2

2.2

Component Functions…………………………………………………………….3

3.

Design Details…………………………………………………
…………………………3

3.1

Detailed Circuit Description and Function..………………………………………3

3.2

Component Calculations……………………………………………………….…4

3.3

Simulation Procedure……………………………………………………………..6

4.

Design Verification………………………………………………………………………6

4.1

Test Setup ……………...…………………………………………………………6

4.2

Design Modifications……..………………………………………………………7

4.3

Test Results……………………………………………………………………….7

5.

Costs…………………………………………………………………………………….11

5.1

Parts Cost Estimate………………………………………………………………11

5.2

Labor Cost Estimate……………………………………………………………..11

6.

Conclusions………………………………………………………
……………………..11

7.

Appendix 1: Data Sheet for MC33063…………………………………………………12

8.

Appendix 2: Simulation results………………………………………………………...16

9.

Appendix 3: Complete schematic…..…………………………………………………..18

10.

Appendix 4: References...………………………………………………………………19






1





1.

Introduction


1.1

Purpose of the Project



Efficiency, size, and cost are the primary advantages of switching power converters



when compared to linear converters. Switching power converter efficiencies can run



between 70
-
80%, whereas linear converters are usually 30% e
fficient. The DC
-
DC



Switching Boost Converter is designed to provide an efficient method of taking a



given DC voltage supply and boosting it to a desired value.

1.2

Block Diagram

The basic building blocks of a boost converter circuit are shown in Fig. 1
.







Fig. 1 Block diagram

The voltage source provides the input DC voltage to the switch control, and to the
magnetic field storage element. The switch control directs the action of the
switching element, while the output rectifier and fil
ter deliver an acceptable DC
voltage to the output.

1.3

Specifications

Design engineers working in today’s high tech environment have to deal with a
rapidly changing market of electronic products and components. As new technology
develops, integrated circuit
s function faster and are smaller in size. However, many
integrated circuits still require a voltage of 12 volts in order to function. The DC
-
DC
Voltage

Source

Magnetic

Field Storage

Element

Switch
Control

Switching

Element

Output

Rectifier and

Filter







2





Switching Boost Converter will take

a 5 Volt DC voltage supply with

10 %
tolerance and deliver 12 Volts acro
ss the load. The maximum output ripple will be
2% of the output voltage, while the maximum current delivered to the load will be
100 mA. The circuit will operate with a minimum efficiency of 70%.

2.

Design Procedures

2.1

General Boost Converter Configuratio
n

Several different boost converter designs have been developed in the past. In order
to achieve the results specified for this project, the output voltage of the converter


When the transistor is conducting, current is being drawn through the inducto
r. At
this time energy is being stored in the inductor. When the transistor stops conducting
the inductor voltage flies back or reverses because the current through the inductor
cannot change instantaneously. The voltage across the inductor increases to

a value
that is higher than the combined voltage across the diode and the output capacitor.
As soon as this value is reached, the diode starts conducting and the voltage that
appears across the output capacitor, is higher than the input voltage.

needs to be higher than the input voltage. This type of converter operates in the
flyback
-
mode. The flyback
-
mode boost converter is shown below, in Fig. 2.




Fig. 2 Flyback
-
mode boost converter







3





2.2

Comp
onent Functions

The inductor shown in Fig. 2 acts as the magnetic field storage element shown in
Fig. 1. It stores energy in its core material. The ideal PWM functions as the switch

control and the transistor acts as the switch element. A diode and a
n output capacitor
are used to perform the function of the output rectifier and filter block.

3.

Design Details

3.1

Detailed Circuit Description and Function

The MC33063 control chip manufactured by Motorola was used for the switch
control. Appendix 1 shows the

data sheet for this control chip. This particular chip
was chosen because of the minimum number of external components required to
implement the design. The transistor shown in Fig. 2 is internal to the control chip.
Therefore, an external switch will
not be required. This device also consists of a
1.25 V reference regulator, a comparator, and a controlled duty cycle oscillator. The
oscillator charges and discharges an external timing capacitor. The upper threshold
of the timing capacitor is equal to

the reference regulator voltage of 1.25 V.


The value of the timing capacitor sets the frequency of the entire circuit and controls
the rate of operation of the oscillator. When the capacitor is charging the voltage at
the lower input of the AND gate is

high. The comparator inverting input is
connected to two external resistors, which control the duty cycle of the circuit. When
the output voltage of the converter falls below the required value, the inverted input
of the comparator will fall below 1.25 V
. Then the comparator will output a Logic
‘1’ and the SR latch will set, enabling the transistor to conduct until 1.25 V is again
present at both inputs of the comparator. The timing capacitor will then discharge.
A Logic ‘0’ will be present at the lowe
r input of the AND gate and the transistor will






4





stop conducting.

3.2

Component Calculations

In order for the circuit to function properly, the external components need to be
calculated carefully. When the switch is on, the voltage across the inductor is



and the current is given by






When the switch is off, the voltage across the inductor is given by





and the current is given by






V
F

is the forward voltage drop of the output rectifier and V
sat

is the saturation voltage
of the output switch. S
ince I
Lon
= I
Loff,
Eqs.(2) and (4) can be set equal to each other.

This operation gives a ratio for the on time over the off time. This ratio is given by






The values of V
in(miu),
V
F,

V
out,
and V
sat
are 4.5 V, 0.8 V, 12V, and 0.3 V respectively.



The
inverse of the frequency of operation yields the on time plus the off time.



The frequency of operation for this boost converter was chosen to be 62.5 kHz.



Therefore,

(1)

(3)

(2)

(4)

(5)







5








Equations (5) and (6) yield an on time of 9.834

s and an off time of 6.166

s.
The
duty cycle is given by


The calculated duty cycle of this circuit is 61.5%. The value of the external timing
capacitor is calculated using





The value of the timing capacitor is 390 pF. The peak current through the switch is
given by


and the m
inimum required inductance is given by


The calculated value of the minimum inductance is 80

H. The resistance required
for the current sense resister is given by


The calculated value for the current sense resistor is 0.5

. The value of the output
capacitor is given by


Using a 0.6 V for V
ripple
, C
out

is equal to 1.68

F. The values of the resistors used to
control the duty cycle are given by


R
1

and R
2
were chosen to be 2.4 k


and 20.64 k

, respectively.

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)







6







3.3

Simulation Procedure

Figure 4 shows th
e exact circuit that was used in the PSPICE simulation.




Fig. 4 PSPICE simulation

A pulse was used to mimic the operation of the control chip. An on time of 9.83us
and an off time of 6.16us were entered into the attributes of the pulse. The circuit
was simulated with a 120


resister connected across the output capacitor. The
results of the simulation have been included in Appendix 2. Figure A2.1 shows the
waveform of the voltage at the switching node of the converter. Figure A2.2 shows
the wavefo
rm of the output voltage. The output voltage levels are at 12 V.

4.

Design Verification

4.1

Test Setup

The boost converter was built on a standard breadboard. The 5 VDC input voltage
was supplied by a Hewlett Packard power supply. All DC measurements were take
n
using Fluke multimeters, and all waveforms were obtained via an oscilloscope.







7






4.2

Design Modifications

To obtain the necessary boosting action, the 80uH inductor was increased to a 120uH
inductor with a thicker gauge wire, manufactured by Coil Craft. In or
der to
eliminate noise at the output, all wire lengths were shortened. To eliminate noise
from the ground plane, 0.1 uF capacitors were added to the input and the output of
the circuit. The 1.68uF capacitor was increased to 56 uF, in order to decrease
the
output ripple. The resulting circuit schematic has been inserted into Appendix 3.

4.3

Test Results

The first waveform shown in Fig. 5 is a picture of the voltage at the switching node.



















Fig. 5 Voltage across the switching node and timing capacitor







8






The frequency of operation is 61.33kHz. The cir
cuit is operating with a 63.2% duty


Cycle. The second waveform shows the voltage across the timing capacitor. The



upper threshold of the timing capacitor can be seen to be 1.25V.

As shown in Fig. 6, the output ripple is 190mV, or 1.6%, when a 120


re
sistor is
used as the load.

















Figure 7 shows the output ripple with no load connected across the output. Under



this condition, the output ripple is 0.04% of the output voltage.




Fig. 6 Output Ripple with a 120


汯慤⁲e獩s瑯爮







9





















The output ripple waveform shown in Fig
ure 8, was taken with a 145





load resistor. In this case, the output ripple is also 1.6%.










Fig. 7 Output ripple with no load







10
































Table 1 shows the input and output voltages, input and output currents, ripple
percentage, and power efficiencies with three

different load conditions.






TABLE 1. TEST RESULTS


120 Ohms

No Load

145 Ohms

Input Voltage

4.993 V

4.993 V

4.993 V

Input Current

0.318 A

0.0045 A

0.277 A

Output Voltage

12.110 V

12.047 V

12.012 V

Output Current

0.0954 A

0 A

0.0802 A

Output Ripple

1.6 %

0.04 %

1.6 %

Efficiency

73%

Not Applicable

71%


The circuit was also tested to make sure it would function properly with a 5VDC
supply that had 10% tolerance. An input voltage was 4.5 V corresponded to an
output voltage of 12.1. An input voltage

of 5.5 V, gave 12.1V at the output.

Fig. 8 Output ripple with 145 ohm load resistor







11





5
.

Costs



5.1 Parts Cost Estimate




The part numbers and values of all components have been listed in Table 2.




Table 2 COMPONENT COSTS

Part Designation

Description

Quantity

Price

MC33063A

Converter Control

1

$1.22

PCH45124

120uH Inductor

1

$1.37

1N5819

Schottky Diode

1

$0.81

--------------------

Subtotal

-----------

$3.40


0.1uF Capacitor

2

$0.10


100 uF Electrolytic Capacitor

1

$0.19


50 uF Electrolytic Capacitor

1

$0.15

C
T

390 pF Capacitor

1

$0.07

-------
-------------

Estimated Capacitor Total

------------

$0.51

R
sc

1


밠Wa瑴⁒e獩s瑯t

2

␰⸱$

R
current Limit

33


밠Wa瑴⁒e獩s瑯t

1

␰⸰$

R
1

2.4 k


밠Wa瑴⁒e獩s瑯t

1

␰⸰$

R
2

20.6 k


밠Wa瑴⁒e獩s瑯t

1

␰⸰$

R
L

50


㄰W⁐潷o爠剥獩s瑯

4

␲⸰$

ⴭⴭⴭⴭⴭ-
ⴭ-ⴭⴭⴭ

Estimated Resistor Total

------------

$2.33

--------------------

Component Total Cost


$5.43







5.2 Labor Cost Estimate



The labor cost was calculated using an hourly wage of $25.00. The average number


of hours spent on this project were
8 hours per week, for 12 weeks.


Total Labor Cost =8*12*2.5*25=$4500



(14)


Grand Total = $5.43 +$6000=$6005.43



(15)


6.

Conclusions



All of the specifications stated previously have been met by this boost converter design.



The output voltage across
the output capacitor is 12V with a maximum output ripple of


1.6%. The power efficiency of the circuit exceeds 70 % for the load range of 120
-
145

.


However an additional constraint needs to be put on the load. The load must not exceed 150



. This will

cause the efficiency to fall below the specified value of 70%.






19






APPENDIX 4. REFERENCES


Marty Brown,
Practical Switching Power Supply Design
, New York: Academic Press, Inc.,
1990, pp. 5
-
26.

Irving M. Gottlieb,
Power Supplies, Switching Regulators, Inverte
rs, & Converters
, New
York: McGraw
-
Hill, 1993, pp. 132
-
141.

D. M. Mitchell,
DC
-
DC Switching Regulator Analysis
, New York: McGraw
-
Hill, 1988,

pp. 153
-
159.

G. Seguier,
Power Electronic Converters: DC
-
DC Conversion,
New York, Springer
-
Verlag,
Inc., 1993, pp
. 201
-
217.