Custom Power Supply Interface for Teaching
MAWv"
C ircuit D esig n
S-T, AM~ rE
H
by
JUL 15 2014
Ruben E. Madrigal
IB R
.LIBRAR[ES
Submitted to the Department of Electrical Engineering and Computer
Science
in partial fulfillment of the requirements for the degree of
Master of Engineering in Electrical Engineering and Computer Science
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February 2014
@ Massachusetts Institute of Technology 2014. All rights reserved.
Signature redacted
redacted
Signature
................
.. ........................
Certified by.......
.
A uthor ...... .......................................
Department of Electrical Engineering and Computer Science
February 1, 2014
Dennis M. Freeman
Professor of Electrical Engineering
Thesis Supervisor
Signature redacted
............
Professor Albert R. Meyer
Chairman, Department Committee on Graduate Theses
Accepted by ..........
2
Custom Power Supply Interface for Teaching Circuit Design
by
Ruben E. Madrigal
Submitted to the Department of Electrical Engineering and Computer Science
on February 1, 2014, in partial fulfillment of the
requirements for the degree of
Master of Engineering in Electrical Engineering and Computer Science
Abstract
This thesis discusses the design and implementation of a custom power supply interface for the Pioneer mobile robot used in MIT's 6.01 course, "Introduction to
Electrical Engineering and Computer Science." The interface is a printed circuit
board that provides bipolar voltage rails of +7VDC and -7VDC, expanding on its
predecessor, which only provides a unipolar voltage rail of +1OVDC. The board is
mounted internally to the robot and can power the student breadboard circuits via
the bipolar voltage rails. This redesigned power supply interface will help the course
staff teach students about circuit design in a much simpler context and allow students to focus more on engineering different circuits rather than spending time on
tangential problems.
Thesis Supervisor: Dennis M. Freeman
Title: Professor of Electrical Engineering
3
4
Acknowledgments
Dedicado a mi familia - sin ustedes no soy nada.
It is impossible to adequately express my gratitude to those who sacrificed much
to bring me where I am today, to those who always gave me their undying love, and
to those who encouraged me to stay strong, have faith, and believe in myself. Please
know that you are always in my heart and are the reason I am able to keep moving
forward.
Likewise, it is no small feat to appropriately thank those who help me carry out
this project.
I would like to thank Professor Dennis Freeman, for his wonderful guidance, indispensable support, and unwavering belief in my abilities. He will probably never
understand how much I appreciated our interactions. I would have shattered without
him.
I would also like to thank the 6.01 staff for the opportunity to be part of such a
wonderful course. Specifically, I would like to thank Adam Hartz, for not only helping
me with this thesis and with many 6.01 aspects, but for also becoming my friend. I
admire him tremendously.
In addition, thank you to Scott Page for always willing to help me think through
the many issues associated with my project with humor and excitement. The board
would not have worked as quickly as it did without his help in soldering it together.
I hope that I am able to give as much as he gives to others.
Thank you to Janice Balzer, our group administrator, for her warm demeanor and
help in obtaining the materials for this project.
And to the countless many, who offered their sincere wishes, words of encouragement, companionship, and prayers. Thank you all.
5
6
Contents
1.1
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
1.2
Motivation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
1.3
Thesis Organization. . . . . . . . . . . . . . . . . . . . . . . . . . .
17
.
Prior Design: 10 Connector
. . . . . . . .
21
2.2
Power Supply Interface Design . . . . . . .
23
2.2.1
Positive Voltage Branch Parameters
24
2.2.2
Negative Voltage Branch Parameters
25
2.2.3
Reducing Ripple
25
.
26
.
.
.
2.1
.
19
. . . . . . . . . .
Layout Considerations and Construction
Evaluation
29
Load Regulation
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
3.2
Line Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
3.3
Signal Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
.
.
.
3.1
Discussion
. . . . . . . . . . . . . . . . . . . . . . . .
35
4.1.1
Load Regulation
. . . . . . . . . . . . . . . . . . . . . . . .
35
4.1.2
Line Regulation
. . . . . . . . . . . . . . . . . . . . . . . .
37
4.1.3
Signal Quality
. . . . . . . . . . . . . . . . . . . . . . . .
37
.
.
Electrical Specifications.
.
4.1
35
.
4
.
Design
2.3
3
.
13
.
2
Introduction
.
1
7
5
4.2
Circuit Design Complexity . . . . . . . . . . . . . . . . . . . . . . . .
38
4.3
Physical Constraints and Ease of Construction . . . . . . . . . . . . .
41
4.4
Pedagogy
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
45
Conclusion
5.1
Thesis Summary
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
5.2
Suggestions for Future Designs . . . . . . . . . . . . . . . . . . . . . .
46
A Schematics
47
B Layout
51
8
1-1
Pioneer Mobile Robot
1-2
Motor Representation with Leads M+ and M-.
2-1
10 connector circuit . . . . . . . . . . . . . . . . .
. . . . . . . . . .
22
2-2
10 connector's +10VDC IV Characteristic.....
. . . . . . . . . .
22
2-3
Power Supply Interface Schematic . . . . . . . . .
. . . . . . . . . .
23
2-4
Bottom Layer of Layout
. . . . . . . . . . . . . .
. . . . . . . . . .
28
2-5
Top Layer of Layout
. . . . . . . . . . . . . . . .
. . . . . . . . . .
28
3-1
Comparing IV Characteristics of all Power Supplies . . . . . . . . . .
30
3-2
Fourier Transform of Magnitude 7VDC . . . . . .
. . . . . . . . . .
33
3-3
Fourier Transform of Magnitude 8VDC . . . . . .
. . . . . . . . . .
33
4-1
Maximum Output Current ..............
. . . . . . . . . .
36
4-2
DC-DC Buck Converter
. . . . . . . . . . . . . .
. . . . . . . . . .
38
4-3
Timing Diagram for Buck and Indirect Converters
. . . . . . . . . .
38
4-4
.
List of Figures
Charge Pump Design . . . . . . . . . . . . . . . .
. . . . . . . . . .
39
4-5
Timing Diagram for Switched Capacitor Design
.
. . . . . . . . . .
39
4-6
Buck-boost (Indirect) Converter . . . . . . . . . .
. . . . . . . . . .
40
4-7
Flyback Converter
. . . . . . . . . .
40
4-8
Circuitry for Motor Rotation using +10VDC
. . . . . . . . . .
42
4-9
Circuitry for Motor Rotation using
+7VDC, -7VDC . . . . . . . . . .
43
A-1
TPS7A4700 by Texas Instruments . . . . . . . . .
47
A-2
TPS7A3301 by Texas Instruments . . . . . . . . .
48
.
. . . . . . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
16
.
. . . . . . . . . . . . . . . . .
.
.
. . .
.
9
14
TPS84250 by Texas Instruments . . . . . . . . . . . . . . . . . . . . .
49
A-4 TPS84259 by Texas Instruments . . . . . . . . . . . . . . . . . . . . .
50
. . . . . . . . . . . . . . . . . . . . . . . . .
51
. . . . . . . . . . . . . . . . . . . . . . . . . . .
52
B-3 Top Layer of Layout Without Planes . . . . . . . . . . . . . . . . . .
52
A-3
B-1
Bottom Layer of Layout
B-2 Top Layer of Layout
10
List of Tables
2.1
Output Filter Solutions . . . . . . . . . . . . . . . . . . . . . . . . . .
26
A.1
Component values used for the TPS7A4700.
. . . . . . . . . . . . . .
47
A.2
Component values used for the TPS7A3301.
. . . . . . . . . . . . . .
48
A.3
Component values used for the TPS84250.
. . . . . . . . . . . . . . .
49
A.4 Component values used for the TPS84259
. . . . . . . . . . . . . . .
50
11
12
Chapter 1
Introduction
This thesis discusses the design and implementation of a custom power supply interface for the Pioneer mobile robot used in MIT's "Introduction to Electrical Engineering and Computer Science" course. The interface is a printed circuit board that
provides bipolar voltage rails of
+7VDC and -7VDC, unlike its predecessor, which
only provides a unipolar voltage rail of +10VDC. Throughout this thesis, design decisions are considered within the following constraints: physical features, electrical
specifications, circuit complexity and pedagogical goals.
1.1
Background
Learning about circuits and understanding them well can be an overwhelming challenge. Students are quickly inundated with unfamiliar electrical devices such as resistors and operational amplifiers, along with fundamental concepts such as Ohm's
Law, Thevenin Equivalents, Kirchhoff's Current and Voltage laws. In the "Introduction to Electrical Engineering and Computer Science" course (MIT 6.01), there
is a substantial hands-on, hardware-based module where students develop intuition
about these new ideas by building breadboard circuits in a laboratory setting. By
using operational amplifiers, wires, and resistors, students are able to enhance the
capabilities of a Pioneer mobile robot (Figure 1-1) and analyze the added behaviors.
One of the primary goals of 6.01 is to teach fundamental design principles of
13
Figure 1-1: Pioneer mobile robot, with head and control circuitry designed on a
breadboard. The head consists of a Lego motor that drives a small printed circuit
board that has two small photodiodes attached. The breadboard circuit has two
category-5 cables connected: one for getting power from the mobile robot, while the
other controls the head.
modularity and abstraction. The course focuses heavily on modeling problems and
building engineering systems from the ground up by introducing the hierarchical ideas
of primitives, combinations, abstractions, and patterns (PCAP). Students apply these
concepts by building complex systems around a two-wheeled robot that allows it to
interact with and attempt to control its external environment.
When first encountered by students, the robot comes equipped with only eight
front sonars and an internal microcontroller that allows the students to program a
brain for the robot. The early labs in 6.01 mainly use the robot sonars as its only
interface to sense the external environment. In the circuit module, students extend
the robot's capabilities by adding a head (Figure 1-1) with light-sensitive "eyes",
giving the robot a new dimension to sense its environment.
The head consists of two photosensitive diodes or "eyes", attached on top of a Lego
motor to allow the head to turn bidirectionally. The head is mechanically mounted
on the Pioneer robot and interfaces with the internal microprocessor via a category-5
cable. When an infrared light is within the robot's sensitivity range, the photodiodes
14
should detect the light and turn the head in the direction of the light.
Students
construct circuitry (Figure 1-1) to power the head and control which direction it
should turn, depending on the location of the light. The circuit module culminates
with the students constructing a "pet robot", which is a Pioneer robot enhanced with
light-sensors such that it can follow a beam of infrared light, much like an animal pet
would follow its owner with a treat.
Since the robot must remain mobile, even when it is extended to include new
sensing capabilities, it is important that students' circuits draw their power from the
robot. A safe and portable solution had to be designed, especially since it would be
dangerous for the students to connect their circuits directly to the robot's internal
12V lead-acid battery. A printed circuit board was designed to serve as an IO connector, or bridge, between the student circuits and the robot's battery and internal
microcontroller. The 10 connector offers a unipolar +1OVDC power supply and has
a current limit of about one amp. The current limiter was included to protect the
components during operation or from shorts. More importantly, it would protect the
students and board from any misconnections in their circuits.
1.2
Motivation
In the context of PCAP, the circuit lab exercises encourage understanding of primitives such as resistors, of combinations via wires that create resistive networks, of
the operational amplifier abstraction, and of patterns such as voltage and current
dividers.
A custom-designed IO connector card mounted internally to the robot powers the
student breadboard circuits. The card provides a unipolar voltage rail of +1OVDC
and is able to support a maximum transient output current of about one amp. Since
the card only provides
+1OVDC, students must create their own mid-rail voltage of
+5VDC in order to evoke the correct robotic head motion. For example, the head
must turn to the left if the left photodiode senses more light and to the right if the
right photodiode senses more light. In order to accomplish this behavior, the correct
15
voltage must be applied across the photodiodes and motor leads, M+ and M- (Figure
1-2).
The voltage across the motor leads dictates which direction the motor will
rotate:
Using the 10 connector's
VM+ - VM- > OV; rotate right
(1.1)
VM+ - VM_ < OV; rotate left
(1.2)
+1OVDC supply, the only way to satisfy these equations
and obtain bidirectional control is to divide the power supply rail in half, or
Applying the
+5VDC.
+5VDC to the M- lead simplifies equations 1.1 and 1.2:
VM+ > 5V ; rotate right
(1.3)
VM+< 5V; rotate left
(1.4)
Historically, students struggle to come to this conclusion and do not always understand why they must design a robust buffered
+5VDC reference.
Replacing the
unipolar power supply with the bipolar power supply interface, all the
+5VDC bi-
ases shift to ground (OV), making it is much simpler to think about the equations as
follows:
(1.5)
VM+ > OV ; rotate right
VM+ < OV
M+
(1.6)
rotate left
M-
VM.
VM+
Figure 1-2: Motor representation with leads M+ and M-. The voltage across the
leads dictate which direction the motor will rotate.
16
A redesigned power supply interface that offers a bipolar voltage of magnitude
7VDC would allow students to think less about creating a mid-rail voltage and instead
focus on engineering different circuits to solve the problem at hand. For a student
just beginning to learn about circuits, comparing voltages to zero is a much easier
concept to grasp than comparing it to
+5VDC. This thesis describes the design and
impact of a bipolar power supply for teaching circuit design in 6.01.
1.3
Thesis Organization
The remainder of this thesis is organized as follows: Chapter 2 discusses the design
and implementation of a custom designed power supply interface for the 6.01 pioneer
robot, while Chapter 3 presents the performance of the board. The performance results are discussed in depth in Chapter 4. Chapter 5 provides concluding thoughts
along with some guidance for future designs. Finally, the Appendix illustrates the
module circuit connections and summarizes the components needed in order to construct a power supply interface. Both schematics and layout are included.
17
18
Chapter 2
Design
In this chapter, the final design of the power supply interface is presented. A brief
explanation of the prior design, know as the 10 connector, and the implementation
of its current limiter is also included. Before continuing, it is important to discuss
the goals and specifications that are sought after in designing a new power supply
circuit board. The goals and specifications can be grouped under four categories:
physical constraints and ease of construction, electrical specifications, circuit design
complexity, and pedagogy.
1. Physical Constraints and Ease of Construction
The completed board must be able to fit inside the robot for the sake of mobility,
so its size must be constrained. For this reason, using inductors, transformers,
or large heat sinks must be avoided when possible.
The course registration can reach up to 500 students, which means that there
are an abundant number of robots in use. Since one board must ultimately be
constructed for each robot, it is important that the design of choice be easy to
build. It would help to use a minimum number of components.
2. Electrical Specifications
The students design their circuits using resistors, wires, and operational amplifiers (Fairchild L272A). The L272A op amps require a voltage difference between
19
their positive (VCC) and negative (VEE) supply voltage pins of 4 to 28VDC.
Maximizing this difference also maximizes the performance of the op amp, especially its dynamic range. Thus, the op amps will work with bipolar voltage
rails of magnitude 2VDC to 14VDC. The output voltage magnitude should fall
within a more standard range of 5VDC to 12VDC.
An important aspect of creating a DC power supply is its signal quality. Most
power supply designs use switching converters that place a small voltage ripple
on top of the DC signal. Even though operational amplifiers are designed to
have high power supply rejection (PSRR), their performance still relies on quiet
power supplies.
To eliminate the possibility of unwanted problems due to a
noisy power supply, it is important that the output voltage of the board have
negligible ripple.
Current capability is also an issue to consider. In order to turn the Lego motor
that drives the head, a transient current of about 1A is required.
The new
board must be capable of delivering this amount of current while at the same
time be immune to accidental connections, such as shorting the power supplies
rails together. The circuit must include a way to significantly limit the current
once the demand goes over 1A for the safety of the students.
3. Circuit Design Complexity
In design problems, there is often an endless array of solutions with many tradeoffs to consider. The course employs a large staff with a wide variety of backgrounds in computer science and electrical engineering, so anyone should be
capable of following the design, or at the very least, of helping to build and
maintain the board with minimal obstacles. With this in mind, it is important
that circuit complexity be minimized. For example, a design requiring switches
with complex timing schemes would require additional support circuitry such
as microcontrollers or oscillators.
20
4. Pedagogy
The purpose of creating a new power supply interface is to help teach students
circuit design in a simpler context. By eliminating the need to build a buffered
+5VDC mid-rail voltage, students do not have to deal with the extra 5V shift
that must be accounted for in their circuit design and analysis.
Aside from these constraints, it is also important to note where the power originates
for the new power supply interface. The Pioneer robot derives its power from three
seven ampere-hour, 12 volt direct current lead acid batteries. The batteries will also
serve to power the board. As the batteries drain, the robot's internal circuits will
continue to function until the voltage drops below the programmed shutdown mode
of
+11VDC. The microcontroller will automatically shut down in order prevent data
loss of any system corruption due to low batteries.
This lower limit of the input
voltage will help place a bound on range of input voltages to consider when designing
the power supply interface.
2.1
Prior Design: 10 Connector
The IO Connector was the first successful printed circuit board design that addressed
the issues stated in Chapter 1 and the goals mentioned above. The connector included
a custom power supply that offered an output voltage supply of only +1OVDC, capable of delivering up to one amp of current. The power supply design consisted of a
Fairchild KA334 operational amplifier, a Fairchild TIP31 NPN power transistor (able
to deliver up to 3A of current), and a diode, all connected in a topology similar to a
linear voltage regulator configuration (Figure 2-1).
Initially, the operational amplifier pegs the voltage at node A near 1OV, and at low
currents,
VOUT
is also kept at 1OV. The diode is off at low currents, but as the current
demand increases towards 0.5A, the voltage at node A increases and the diode starts
to exponentially turn on. As higher currents are drawn from
VOUT,
more current is
conducted through the diode. This causes the voltage at nodes A and B to drop, due
to the operational amplifier trying to keep its inputs equal. Since the voltage at node
21
A drops, VOUT must also drop since it is linked to A via the 1.2Q resistor. This is
how the intrinsic, though non-intuitive, current limiter works. Figure 2-2 illustrates
the relationship between output voltage and output current of this design.
+12VDC
RIN
B
VIN
A
1.2
hJ
PPII
Vour
Figure 2-1: Core circuit configuration of the IO connector.
IV Characteristic of +1OVDC
10
9
0
.5
6
5
0
0.1
0.2
0.3
0.4
0.6
Output Current [A]
Figure 2-2: Characterizing the output voltage versus output current of the 10 Connector's
+1OVDC supply rail.
22
2.2
Power Supply Interface Design
The final design of the interface (Figure 2-3), centers around two power modules that
offer a complete solution in a small, low profile Quad Flat No-leads (QFN) package
[4]. The 12V battery in the robot feeds into both these modules to create a positive
and negative 8V power rail.
The positive 8V is created by the TPS84250, which
combines a 2.5A DC-DC buck converter with an internal inductor.
Similarly, the
TPS84259 creates the negative 8V rail using a 2A DC-DC buck converter with an
internal inductor.
The TPS84250 and TPS84259 chips contain inductors, which eliminates the effort
and space needed to include an external inductor on the board. Both modules are
9mm by 11mm packages, which only consume 1.6% of the 96.5mm by 63.5mm printed
circuit board. Since the modules have a small footprint and require a minimal number
of passive components, they do not dissipate much heat, foregoing the need to use
external heat sinks.
+12V
CN
RET
COUT
UVLO
RTl
STSEL
VLo
NR
PGND
AGND
R
C
-8VDC
CoUT
UVLO
RvvL2
d
+7VDC
b
OUT
GNDI ,-
8PV
1P6V
3P2V
TPS7A3301
VIN
-7IVDC
VouT
-
CVIN
CIN
CoUt
TPS842S9
+12V
VOUT
e
VADJb
SSrTR
CT
TS7A4700
WDC
SVIN
VOUT
RvLo,
.1
CW2
+
TPS8250
VIN
U
k_VOUr
ENT
FBT
fRTCU
RT
MTEL
VADJ
NRISS
Figure 2-3: Schematic of the power supply interface consists of two DC-DC power
converter modules, followed by a set of low dropout regulators. The
is a direct connection from the batteries.
23
+12VDC input
Following the DC-DC power converter modules are a set of low dropout regulators
(LDO) that regulate the output voltages at +7V and -7V and are able to supply up to
one amp of current. The purpose of these linear regulators is to filter out the output
voltage ripple that is inherent to all DC-DC switching converters so that the board
can present stable and clean rails to the student boards.
2.2.1
Positive Voltage Branch Parameters
The TPS84250 is a power module designed by Texas Instruments and, depending on
its input voltage, is capable of providing output voltages from +7V up to
+50V, while
providing up to 2.5A of current. According to the datasheets [2], the three primary
parameters that must be selected when designing with the TPS84250 are the output
voltage, undervoltage lockout (UVLO) threshold, and the switching frequency.
The output voltage was chosen to maximize the power supply rails given the
input constraints imposed by the robot's batteries and the allowable minimum input
voltage supported by this module. The TPS84250 supports a minimum input voltage
of 7V or (Vat + 3V), whichever is largest. Since the battery must be kept above the
robot's shutdown voltage of 11V, the minimum input voltage was chosen to be 11V.
+8VDC. Physically, a resistor Rser
In other words, the output voltage used was
sets the output voltage by completing an internal voltage divider that feeds back a
sensed portion of the output voltage [2]. By setting RSET to 90.9kQ, the output
voltage is fixed at +8VDC. The Undervoltage Lockout (UVLO) threshold determines
the input and output voltage threshold where the module activates and deactivates
power conversion. It is recommended to set the UVLO threshold to approximately
80-85% of the minimum input voltage. To set the UVLO threshold, two resistors are
chosen by the following equations:
RuvLo1
=
- VOFF) )(2.1)
2.9- 10-3
(VON
1.25
RUVLO2
=
-
(ON-1.25)
1.25
RUvLO1
24
+
-
(0.9 . 10-3)
(kQ)
(2.2)
Raising the switching frequency significantly improves various performance aspects of the module. It reduces the output voltage ripple, increases the efficiency of
conversion and improves the transient response of the module. A switching frequency
of 600kHz was used.
2.2.2
Negative Voltage Branch Parameters
The TPS84259 is also a power module designed by Texas Instruments [1], capable
of providing output voltages from -3VDC to -17VDC. Unlike the TPS84250, the
negative power module is only capable of providing up to 2A of current, but like
the TPS84250, the same three primary parameters apply when designing with the
TPS84259: output voltage, UVLO threshold and switching frequency.
For purposes of symmetry, an output voltage of -8VDC was chosen. Similarly, by
completing an internal voltage divider with RSET equal to 90.9kQ, the output voltage
was fixed at -8VDC. For the UVLO threshold, the two resistors were chosen by the
following equations:
0.5
RUVLol = 2.9. 0-3
ft
RUVLO2
=
oN-.25)
(kQ)
1.25
(2.3)
(kQ)
(2.4)
+ (0.9 - 10-3)
RuvLO1
For the switching frequency, the TPS84250 only offers two frequencies, 500kHz
for input voltages above +18V and 800kHz for input voltages below
+18V. Since the
battery voltage falls under 18V, a switching frequency of 800kHz was used.
2.2.3
Reducing Ripple
DC-DC converters employ switches to create a periodic signal whose average voltage
can be modulated and a filter to ideally remove all frequencies except the average
voltage, commonly known as DC. Due to the dynamic switches and the unfeasibility
of building an ideal filter, DC-DC buck converters always have a small ripple riding
25
on top of the average voltage. There are various approaches to minimizing this output
voltage ripple. To mitigate this effect, an additional capacitor or a more extensive
filter can be placed at the output of the converter. Another solution would be to
follow the power module with a low dropout voltage regulator. Table 2.1 below, was
adapted from Texas Instruments' Application Report SLVA549 [31 and it summarizes
the peak-to-peak ripple that should be expected given a certain output filter solution.
Table 2.1: Output Filter Solutions
Output Filter
VP
21 mV
2 x 47 pF ceramic capacitors
12 mV
4 x 47 pF ceramic capacitors
2 x 47 piF ceramic caps + -x filter 2.9 mV
3 mV
2 x 47 pIF ceramic caps + LDOs
To minimize the peak-to-peak voltage ripple, ultralow-noise linear voltage regulators (TPS7A4700, TPS7A3301) were placed after the power modules. The voltage
regulators not only deliver clean voltage rails to the student circuits, but would also
help regulate the output voltage. They are also capable of sourcing up to one amp of
current, which is enough to support the transient current required by the Lego motor.
The output voltage ripple is also dependent on the value of the output capacitor, its equivalent series resistance (ESR) and the switching frequency of the power
modules. To maximize performance, capacitors with low ESR were used when appropriate.
2.3
Layout Considerations and Construction
The circuit board was laid out using the commercial program ExpressPCB on a
96.5mm by 63.5mm board, the maximum size that fits inside the Pioneer robot.
Apart from the size constraint, another significant factor considered when laying out
the modules was the thermal stress of the design.
The converters are packaged in Quad Flat No-leads (QFN) modules, designed to
26
dissipate heat into the PCB and not require any additional heat sinks. The copper on
the board must be able to withstand the thermal dissipation put off by the modules
under normal operation. Also, as the battery powering the board discharges, the input
voltage declines, creating a larger difference between the input and output voltages.
This voltage difference generates more heat that must be removed efficiently. Planes
large enough that connect various circuit nodes must be used to conduct heat way
from the components and into the environment. There is a trade-off to how large
the planes can be, though, since the melting ease of the solder will also be affected.
When initially placing the components on the printed circuit board, sufficient heat
must be used to melt the solder to attach the components. If the planes are too large,
heat will leak away from the desired solder joint area and so not enough heat will
gather locally. Low heat results in cold solder joints, where the components are not
well attached to the board and are easily broken, causing massive board failure.
The size of the particular components determines how easily they can be handled
and soldered on the printed circuit board. Even though a smaller component package
could be used to save more board space, such as 0402 (1mm x 0.5mm), soldering it
on the board can be more difficult.
Since the power supply board must be able to sustain an output up to 1A, PCB
traces that carry this significant amount of current must be wider than signal traces.
Current-carrying traces were sized to be 1.27mm, enough to carry 2A. The wider
traces can be noted in Figure 2-5, which shows the final layout of the bottom layer.
Connecting the battery to VIN is one example of a wider trace. Such traces, along
with the size of the planes, can be seen in Figure 2-6, which shows the final layout of
the top layer of the fabricated card.
27
5
4
kGNDGyro
I
00
00
00
00
nFE]
E]
i:;
LO+
1
6
19[
000
1I1
11
0
OVIN
00
00
00
47uFE
19 47uF--
9
L
YOUT
00
EJ E]47uF~
3
0
4.7uF
Y
2SE kI
47uFan
11
1LJ
Ja....____
1
0
0
1
__j
C,~E.'
3
6Tilt
4
TPs3301
6.01 Pioneer I
Connector
December 17, 213
Figure 2-4: Bottom layer of the layout for the power supply interface board.
5
4
3
Gyro
C
00
00
00
00
00
00
00
00
00
00
,,qhI
TPS?P4780
OIN
il
I~
VO+
*1
. 7uF
O
TPS8429
TPS7A338I
6.61 Pioneer
0
Connector
December 17, 2013
Tilt
Figure 2-5: Top layer of the layout for the power supply interface board.
28
Chapter 3
Evaluation
Measurements were gathered from a series of tests designed to characterize the custom
power supply interface.
The tests included analyzing the content of the DC signal
by taking its Fourier Transform using a Tektronix TDS3034B oscilloscope to show its
frequency spectrum, along with tests to determine performance parameters such as
line and load regulation.
3.1
Load Regulation
In general, the voltage of a circuit node often decreases as it delivers an increasing
amount of current to its attached load.
This characteristic is expected from most
circuit topologies that focus mainly on signal conditioning, such as signal amplification
or filtering.
For DC-DC converters, though, the voltage supplied by the converter
must ideally remain constant at its desired level, independent of its load. Since this
is not physically possible, the performance parameter that quantifies the converter's
ability to maintain a constant output voltage under varying load conditions is known
as load regulation and is defined as:
Load Regulation =
AVat
A I.t
29
(3.1)
Once the power supply rails cannot sustain the current demanded by the load, the
voltage will start to drop.
In other words, the load through its current demand
regulates the output voltage. Ideally, load regulation should approach zero for the
board as this means that the output voltage stays constant regardless of load.
The power supply interface card was tested for load regulation by connecting a
variety of high-wattage resistive loads across the output leads of the card. The output
voltage was then measured with a Tektronix TDS3034B oscilloscope. For comparison,
the
+10VDC 10 connector had its load regulation characterized in the same setup as
well. Figure 3-1 plots the output voltage against the output current of
+7VDC and
+10VDC rails, along with the magnitude of the -7VDC rail.
IV Characteristic of +7VDC, -7VDC, and +10VDC
10
--
+10VE DC
+7VD
-7VDC
9
-~~ -...
...---
6L
5
0.-
0
0.2
0. 4
0.6
0.8
1.0
Output Current [A]
Figure 3-1: Comparing IV Characteristics of +10VDC, +7VDC, and the magnitude
of -7VDC.
Each supply rail manages to stay fixed at its desired output voltage for low currents, but start to fall at higher current demands. The +10VDC supply on the original
card drops an entire volt, down to
+9VDC, when the output current reaches 0.5A
and continues to drop exponentially. Similarly, the +7V supply rail on the new card
exponentially drops at higher current demands, but stays fixed at or near
30
+7V at
higher current demands compared to the
performs better than the
+10V supply rail. The -7V supply rail also
+10V rail but falls short when compared to the +7V rail
since the output voltage dives significantly when the output current reaches 0.7A. It
is important to note that none of the designs managed to reach IA as stated in the
electrical specification in Chapter 2.
3.2
Line Regulation
Another performance parameter determined was line regulation, which measures the
sensitivity of the output voltage to variations in the power supply's input voltage. For
example, the input voltage starts to fall as the battery drains throughout the robot's
operation. For a constant output current, line regulation is defined as follows:
Line Regulation =
AVw1
AVin
(3.2)
Like load regulation, this specification should ideally also approach zero for the power
supply interface board. Line regulation was determined by connecting the board to a
varying power supply, while measuring the output voltage. For small output currents
(below 0.5A), the board kept the output voltage at +7VDC and -7VDC down to an
input voltage of 10.5V, well below the 11V battery voltage that would cause the robot
to go into shutdown mode.
3.3
Signal Quality
One of the main goals of this thesis was to create a power supply interface that could
deliver clean, fixed bipolar DC voltage levels that could power student circuits. The
quality of the DC signal provided by the board is better understood by measuring
the frequency content of the supply rails. The expectation is that most of the signal
energy resides at DC (zero frequency) while other frequencies are attenuated. This
expectation translates to the time domain as steady, constant voltages with negligible
ripple.
31
Both the +8VDC and -8VDC are the voltage outputs of the DC-DC converters,
TPS84250 and TPS84259, while the +7VDC and -7VDC are the outputs of the linear
regulators, TPS7A4700 and TPS7A3301. Using a Tektronix TDS3034B oscilloscope,
the FFT was taken of the supply voltages using a preprogrammed Hamming windowing function. The signal energy was measured from 0Hz to 2.5MHz for the following
voltages: +7VDC, -7VDC, +8VDC, and -8VDC (Figure 3-3, Figure 3-4).
From the spectrum shown in Figure 3-3, there is significant energy at DC (OHz)
while the majority of higher frequencies are suppressed below -40dBV or 10mV. The
+7V rail is actually at 7.08V (17dBV) and the -7V is almost symmetrical with a
magnitude of 7.04V (16.85dBV).
To determine if the linear regulators helped in decreasing the ripple on the DC
voltage coming out of the DC-DC converters, the frequency content of the
+8VDC
and -8VDC was also measured. Again, most of the signal energy was present at DC
(OHz) while the higher frequencies were suppressed below -40dBV. The
measured at 8.2V and the -8VDC rail was measured at -8.1V.
32
+8V rail was
FFT of +7VDC
10
...
..
..
.........
-0
(U
-10
.-
-20
01-30
-40
FFT of -7VDC
10
-
20
01020
C
-M30-
4050
____mi
0
500000
iiIi
1000000
1500000
2000000
2500000
Frequency [Hz]
Figure 3-2: Comparing Fourier Transform of
+7VDC and -7VDC.
FFT of +8VDC
10
>
0
-10
-20
-30
2-40
-S0
FFT of -8VDC
20
10
-10
-20
30
-40
-50
0
500000
1500000
1000000
2000000
2500000
Frequency [Hz]
Figure 3-3: Comparing the Fourier Transform of +8VDC and -8VDC.
33
34
Chapter 4
Discussion
In the measurements performed on the power supply interface, the general observation is that the board successfully presents the appropriate bipolar DC voltages for
most current demands; however, the board does fall short of providing one amp of
output current. In this chapter, the results of the board performance presented in
Chapter 3 are discussed in depth and in the context of the goals stated in Chapter 2:
physical constraints, ease of construction, electrical specifications, and circuit design
complexity.
4.1
Electrical Specifications
To summarize, the electrical specifications set constraints on both the desired output
voltage and current.
The magnitude of the output voltage was to be within the
standard range of 5VDC to 12VDC, while the output current had to support and be
limited to IA.
4.1.1
Load Regulation
The power supply interface was designed to deliver up to one amp of current without
the output voltage falling significantly from its desired level. Once the current demand
surpasses one amp, the voltage must drop quickly for the safety reasons mentioned in
35
Chapter 1. From Figure 3-1, none of the supply rails, including the previous
design, managed to output IA. The
+1OVDC
+10VDC rail begins to exponentially decrease
after 0.5A, while the -7VDC pushes the limit to 0.65A before falling abruptly. The
+7VDC rail performed the best, keeping the output voltage at 7V up to a current of
0.9A.
To understand the reason behind the current limitation, Figure 4-1 illustrates the
maximum output current (or safe operating current) possible for both DC-DC converter modules, TPS84259 and TPS84250. Extrapolating for
VIN =
12V and
VOUT =
8VDC, the maximum output current hovers around IA. When the low dropout linear
regulators (TPS7A4700, TPS7A3301) in the next stage need to source an output of
IA, the required current input to the regulators would have to be larger than IA,
which is not possible.
Safe Output Current for Different Magnitudes of Vout
Vout = 3.3V
Vout = 5V
Vout = 9V
*
Vout = 12V
Vout = 15V
- - - - --
2.5-
8
8
10
14
2
Input
. .......
.
.
2.0
-
--------- -
2.0-.
16
18
20
Voltage [V]
Figure 4-1: Maximum Output Current of the TPS84259 and TPS84250 power modules, for different magnitudes of output voltages. Plot adapted from SOT chart given
on TI website.
Even though the new power supply interface could not sustain a fixed voltage for
the specified current range, it did surpass the previous design's current capacity. The
36
10 connector's
+10VDC supply was capable of driving the Lego motor without a
problem, and the new power supply interface managed to do so as well.
4.1.2
Line Regulation
The board was able to keep the output voltage fixed at the desired levels, even
when the input voltage dropped down to 10.5VDC. As mentioned in Chapter 2, the
robot will enter the shutdown mode when the batteries drain below the programmed
input voltage of +11VDC. Since the board can sustain the output voltage well below
the programmed shutdown voltage, the robot will be able to provide the proper
functionality until it is forced to switch off.
4.1.3
Signal Quality
Another aspect of the electrical specification that was put forth in the design phase
was to create a clean DC signal with negligible ripple. Having significant ripple on top
of a signal can cause unwanted effects in circuits. The spectrum shown in Figure 3-3,
indicates that there is significant signal energy at DC (OHz) while other frequencies
are suppressed below -40dBV. Mapping these measurements over to the time domain
gives a ripple of less than 10mV, superimposed on top of the desired dc-signal for all
supply rails. The expected ripple given in Table 1 of Chapter 2 was approximately
3mV, which follows well with the measurements.
Interestingly, the signal frequency content out of the converters was similar to that
out of the low dropout regulators. The purpose of including linear regulators was to
help decrease the ripple further; however, the measurements indicate that the amount
of ripple suppression was negligible. The extra capacitors placed at the outputs of
the power modules may be the reason. Since the power modules are rated to source
up to about 2A (depending on input and output voltages of the power modules), the
low dropout regulators may not be necessary to include in the design.
37
4.2
Circuit Design Complexity
Producing a positive output voltage on a printed circuit board is a straightforward
endeavor using common DC-DC converter topologies; however, creating a stable negative output voltage may require more complexity and design expertise. Some discrete
circuit design approaches that solve this problem include using a buck converter (Figure 4-3), a switched-capacitor based design (Figure 4-4), an indirect DC-DC converter
(Figure 4-6), and a flyback converter (Figure 4-7).
VW
T
C1
qs(t)
R,
C2
Vour
------0
Figure 4-2: DC-DC Buck Converter
(t)
t
DT
T+DT
T
2T
Figure 4-3: Timing diagram for both the Buck and Indirect Converter, where T is
the period and D represents the duty cycle, or the fraction of T that the switch is on.
The switched-capacitor design requires a charge pump stage, along with a controller that helps regulate the output voltage. For example, a simple voltage inverter
(Figure 4-4) can be used to create a negative voltage (V., = -Vi,).
Despite only
using two capacitors, it is clear that the switches must follow a strict timing scheme
(Figure 4-5) in order to not lose charge stored. To implement the timing scheme,
extra support circuitry, such as a microcontroller, would be required.
38
q1 (t)
2)-
q 1(t)
q2 (t)
Figure 4-4: Charge Pump voltage inverter design, where VOUT
= -VIN
q1(t)
DT
T
T+DT
2T
q2(t)
I
I--
DT
T
-----t
T+DT
2T
Figure 4-5: Timing diagram for Switched Capacitor Design, where T is the period
and D represents the duty cycle, or the fraction of T that the switches are on.
Another way to create the negative power supply would be to build a buck-boost
(indirect) DC-DC converter (Figure 4-6), where an inductor is used as an intermediate
energy transfer agent between the input and output ports. The desired output voltage
can then be controlled based on the duty cycle represented by D in the following
equation:
-
= -
D
1- D
Vin
(4.1)
The drawback to using this design is primarily that it requires the use of an inductor.
Inductors can take up valuable space on the board and are often leaky.
39
VIN
721
T)J
qs(t)
I
+
TCe
_
R,
VOUT
Figure 4-6: Buck-Boost (Indirect) Converter that follows the timing in Figure 4-3.
In a similar vein, the flyback converter (Figure 4-7) uses a transformer in addition
to an inductor. The transformer adds another design degree via isolation and the
number of windings (NI: input-side number of windings, N2 : output-side number
of windings) for maximum flexibility for a desired output voltage as shown by the
following equation:
Vo
Vin
1
D
N2(4.2)
- D
N1
Unfortunately, the design still uses an external inductor, while the transformer is
large and suffers from leakage as well.
+ou
C1 =i
VIN
R 1i
Isn-
Figure 4-7: The Flyback Converter utilizes a transformer.
Fortunately, as on-chip designs have scaled tremendously over the years, there has
been an increase in the number of available DC-DC converting power modules that
offer all the above designs in a compact and convenient package [4]. For example,
the TPSO400 is a design based on a charge pump inverts the input voltage without
40
the need of using an inductor and is capable of delivering a maximum output current
of 60mA. The TPS5430 is a buck-boost circuit that creates a negative voltage with
the use of an external inductor, while the TPS54060 is also a buck-boost circuit but
coupled with a transformer to create a bipolar rail design.
Taking advantage of the power modules and low dropout voltage regulator available, a simple two-stage design for each of the positive and negative voltage branch
was chosen to minimize the circuit design complexity. Each branch was composed
of a power module to convert the 12V battery to a desired output voltage, followed
by a linear regulator to regulate the output, keeping the DC signal quiet and clean
(minimizing ripple).
4.3
Physical Constraints and Ease of Construction
The completed power supply interface was laid out on a 96.5mm by 63.5mm printed
circuit board, which met the goal of fitting inside the robot. Since the robot must
remain mobile and power the student circuits simultaneously, it was vital that the
power supply interface fit well without hindering the other circuitry internal to the
robot. The use of IC circuits such as the power modules and low dropout regulators
helped eliminate the need for external inductors, transformers, or large heat sinks.
A small number of external resistors and capacitors were necessary to set the IC
modules.
There was a trade-off between the size of the board and its components with
the ease of constructing the final power supply interface.
Some planes and traces
on the PCB were not strategically sized or placed. In pursuit of saving space, some
components were placed close to the power modules, making it difficult to solder
between components. During the testing phase, a power module failed. Removing
this component without accidentally removing the surrounding components proved
to be difficult.
41
4.4
Pedagogy
As mentioned in Chapter 1, students often struggle to understand why it is necessary
to create a buffered
+5VDC reference for the bidirectional controller of the head. In
order to obtain the correct head behavior using the
students to connect a buffered
+1OVDC supply, it is vital for
+5VDC to the M- lead as shown in Figure 4-8. To
help students experiment with engineering design, the focus should be on developing a
variety of circuits that connects the right (VR) and left (VL) photodiode "eye" voltages
to the M+ lead in a way that results in the correct behavior.
+1OV
VRR
VR
RR2M
VL
R
Vx
+
R
R2
Vx
Figure 4-8: Under the 10 connector's unipolar +1OVDC, the circuitry for motor
rotation requires a buffered
+5VDC mid-rail voltage.
Replacing the unipolar power supply with the new bipolar power supply interface,
all the +5VDC nodes are shifted to OV, as shown in Figure 4-9. The entire circuit
that was connected to the M- lead is eliminated, along with simplifying equations 1.1
and 1.2 as expected:
VM+ > OV; rotate right
(4.3)
VM+ < OV; rotate left
(4.4)
With the +5VDC bias confusion out of the way, students will have much more time
to experiment with different circuits and build better intuition about design.
42
R2
RW
VR
M+
VL
M
R,
R2
Figure 4-9: Under the board, a bipolar 7VDC supply eliminates the need for a buffered
+5VDC mid-rail voltage by setting the nodes previously using the +5VDC voltage
to OV. Students can then focus on designing the circuitry that connects to the M+
lead of the motor.
43
44
Chapter 5
Conclusion
5.1
Thesis Summary
This thesis discussed the design and implementation of a custom power supply interface for the Pioneer mobile robot. The purpose of the interface was to provide a
bipolar power supply of magnitude 7VDC, allowing students learning electronics to
focus more on circuit design rather than on tangential problems.
The custom power supply interface successfully delivered a bipolar voltage supply
of
+7VDC and -7VDC. Even though it was not able to regulate this voltage when
the current demand neared lA, it was still able to provide quiet power supplies for
a larger range of currents than the currently used IO connector. The module-based
design turned out to be highly suitable for our goals.
Unfortunately, more thorough testing was needed before fully determining if this
power supply interface could replace the 10 connector.
Since the purpose of this
thesis was to explore the additional feature of bipolar voltage supplies, this design
iteration of the board did not include the DAC circuitry that allows communication
between the robot and a laptop. Such circuitry would allow for long-term dynamic
tests. For example, running the board such that the head would continuously move
right and left would help make sure that the board could sustain the necessary output
current for long periods of time.
45
5.2
Suggestions for Future Designs
Even though much of the core design has been designed and tested, there is still
further work to be done. As mentioned above, combining the design of this thesis
with the already working DAC circuitry from the current 10 connector would help run
long-term dynamic tests to make sure the board can sustain loads for hours without
overheating or breaking down.
The power modules include current limiters inside so when an overcurrent condition is encountered, the output current is limited and the output voltage is reduced.
To help protect the power supply interface and students further, including an explicit
fuse at the board outputs would be a welcome addition.
Finally, in the next iteration of the board, the layout can be improved by smaller
connections between the passive components and large planes. The planes were used
to help dissipate heat from the power modules, but they do not thermally help the
passive components very much. By using thinner traces from the components to the
planes, soldering the board would be easier.
By adhering to these suggestions, the next iteration of the power interface supply
will be one step closer to replacing the current 10 connector and students will be able
to benefit greatly by having access to a bipolar voltage supply.
46
Appendix A
Schematics
+8VDC
TPS7A4700
01
VIN
+7VDC
VOUT
SENSE
CIN-1EN
NR
OP8V
CU
GND
1P6V
3P2V
Figure A-1: TPS7A4700 by Texas Instruments
Table A.1: Component values used for the TPS7A4700.
CIN
4.7 pF
COUT
47 pF
CNR
47 MF
47
TPS7A3301
-8VDC
VIN
CNT
-7VDC
VOUT
CFF
EN
FB
NR/SS
C4
GND
Figure A-2: TPS7A3301 by Texas Instruments
Table A.2: Component values used for the TPS7A3301.
CIN
10 yF
COUT
47 pF
CFF
10 nF
CNR
1 yF
R1
910 kQ
R2
183 kQ
48
COU7
+12V
TPS84250
0
CN
UV OIVIN
+8VDC
VOUT
R~
OUT2
VADJ 1
UVLO
Css
/LK[
STSEL
COUT1
RRT
AGND
PGND
Figure A-3: TPS84250 by Texas Instruments
Table A.3: Component values used for the TPS84250.
CIN
4.7 pF
COUT1
47 pF
COUT2
47 pF
Css
10 nF
RuVLO1
174 kQ
RuVLo2
22 kQ
RSET
90.9 kQ
549 kQ
49
TPS84259
+12V
CINj
-8VDC
VOUT
VIN
VOUTPT
RuvLoi
UVLO
Couri
AVOUT
RT
RUVL02
STSEL
VADJ
GND
Figure A-4: TPS84259 by Texas Instruments
Table A.4: Component values used for the TPS84259
CIN
4.7 yF
COUT1
47 pIF
COUT2
47 piF
RuvLol
174 kQ
RuVL02
24.3 kQ
RSET
90.9 kQ
50
CoUr 2
Appendix B
Layout
5
MMMMM
174k
22LC
1
GD
UnFE
19
Gyro
9
47 F
:
60
"+
1
u
GLJUIII
1
IU
1V
your
C
3
M47uF
a fl29
0OQ
00
00
00
00
00
00
00
00
00
4
5
0
IN
VOUT
47uFL
]E
fl
29
E 3
4.7uF
2JEE
47uF
19E
J9b
InF
I
0.
I
(U
ri1
LU
C
II
A
GHlD
5
4 LQ
I
TPs7A3301
3
6.01 Pioneer ID Connector
December 17, 2013
Tilt
0
Figure B-1: Bottom layer of the layout for the power supply interface board.
51
5
3
4
0Gyro
VO+
VI+
00
00
00
00
TPS7A470
00
00
00
00
CYIN
900
00
00
*F
-97uF
alf
K
i0
611110
TPS84259
____
e
TPS7A3301
6.01 Pioneer I Connector
December 17, 2013
0
Figure B-2: Top layer of the layout for the power supply interface board.
5
VIN
174kc
M
22k
rO129M
Uyr
Y
GND
4.7uF
M
1rF[: 3 ENE
-@
3
4
El474
j
47 F
250
L
I
y0+
0
TPSa19
3.
I+
00e
TPs7A4700
V)
TPS84250
--
y0-
*VIN
VOUT
.. 0
2,
611411
1
NI
47uF
4.7uF
3ekE
n~e
I
EnEEIrFA
I--111 NW
0.
*
:R~
] N0 4 7 uF r
3.lil
--s
TPS84259
GD
5
3
4
TPS7A3301
6.01 Pioneer 10 Connector
December 17, 2013
Tilt
Figure B-3: Top layer of the layout for the power supply interface board with planes
removed to show specific chip layout.
52
Bibliography
[1] Texas Instruments. 4.5- V to 40- V Input, 15- W, Negative Output, Integrated Power
Solution. TPS84259 datasheet, August 2012 [Revised June 2013].
[2] Texas Instruments. 7- V to 50- V Input, 2.5-A Step-Down, Integrated Power Solu-
tion. TPS84250 datasheet, August 2012 [Revised June 2013].
[3] Texas Instruments. Reducing Output Ripple and Noise with the TPS84259.
TPS84259 Application Report, November 2012.
[4] Rich Nowakowski. Techniques For Implementing a Positive and Negative Output
Voltage for Industrial and Medical Equipment. Available at http: //ww. ti. com/
lit/ml/szzn001/szzn001.pdf.
53