SuPER System DC-DC Buck Converter

By

Thaddeus Guno Kunal Shah Koosh Shah

Senior Project

ELECTRICAL ENGINEERING DEPARTMENT

California Polytechnic State University

San Luis Obispo

Spring 2007

ii

Table of Contents

Section Page

Acknowledgements…………………………………………………………..iii

Abstract……………………………………………………………………….iv

List of Tables and Figures……………………………………………………..v

I. Introduction…………………………………………………………………...1

II. Background…………………………………………………………………..3

III. Requirements………………………………………………………………..9

IV. Design……………………………………………………………………...13

V. Construction/Assembly...…………………………………………….……..22

VI. Testing……………………………………………………………..............25

VII. Conclusions and Recommendations……………………………………....34

VIII. Bibliography……………………………………………………………...39

Appendices

A. Parts List, Vendors, Cost …………………………………………………...40

B Simulation Source Code..……………………………………….…………...42

C. Analysis of Senior Project Design...………………………………………..43

iii

Acknowledgements

As a team of three, we would like to give our special thanks to Dr. Harris for

all his support and encouragement throughout our senior project journey. We would

also like to thank all the members of the SuPER team, especially Tyler, for all their

knowledge and assistance. Lastly, we would like to thank our family and friends for

their support through our college career at California Polytechnic State University,

San Luis Obispo.

iv

Abstract

This senior project focuses on the DC-DC converter of the SuPER project.

The DC-DC converter that this senior project focuses on is the Buck Converter, a

simple step down non-isolated DC-DC converter. The DC-DC converter is the heart

of the SuPER project because it provides a physical interface between the solar cell

and the battery and loads. This senior project report will discuss the requirements,

design, and testing of the DC-DC converter. In addition, it will also focus on the

testing and understanding of the high side driver chip used in the DC-DC converter

circuit. Lastly, since the DC-DC Converter was not completed, this report will

include recommendations to the future generation that works on the DC-DC

converter.

v

List of Tables and Figures

Tables Page

Table I: DC-DC Buck Converter Specifications [5]………………………...…...11

Table II: MOSFET Test Results…………………………….……………….…..26

Table III: HSD Testing Data……………………………………………..……....28

Table IV: Data for DC Testing……………………………………………….….30

Table V: Test Results with Inverter and High-side Driver…………………....…33

Table VI: Parts, Vendor, and Cost……………………………………………….40

Figures Page

Figure 1: Simple Buck Converter…………………………………………………3

Figure 2: Simple Buck Converter with SW1 ON…………………………………4

Figure 3: Simple Buck Converter with SW1 OFF…………….……………..........4

Figure 4: Plot Defining Duty Cycle…………………………………………….....5

Figure 5: A Piece of the Simulink Schematic of the Buck Converter…………….8

Figure 6: Transient Response at the Output……………………………………...14

Figure 7: Steady State Output………………………………………………........15

Figure 8: Switching Cycle of MOSFET…………………………………………15

Figure 9: Schematic of the Buck Converter Printed Circuit Board……………...17

Figure 10: First PCB Layout……………………………………………………..20

Figure 11: Second PCB Layout………………………………………………….21

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Figure 12: Top View of the High-side Driver Chip……………………………...25

Figure 13: Basic HSD and MOSFET Test……………………………….…........26

Figure 14: HSD Testing Circuit………………………………………….……....28

Figure 15: DC Testing Circuit for HSD………………………………………....30

Figure 16: Circuit Layout of MC14007UB…………………………………...…31

Figure 17: CMOS Inverter Test Circuit………………………………………….31

Figure 18: Input/Output of a Working CMOS Inverter with 5VCC……….…......32

1

I. Introduction

The Sustainable Power for Electric Resources (SuPER) project is originally

proposed by Dr. James Harris. The SuPER project’s main goal is to provide families

in third-world countries a low-cost, sustainable source of electrical power for a span

of 20 years [1]. The project is to design a prototype using today’s technologies while

assuming the possible future technological advancement according to a Moore’s Law

model. The prototype will consist of 4 main subsystems: a solar photovoltaic source,

power control, energy storage (battery), and a standardized DC electrical power

output interface [1].

The SuPER project chose DC, not AC to power the load simply because the

solar photovoltaic source has a DC output and all the basic appliances, like the ones

used in the SuPER project, can be DC based. The many benefits of having a DC load

have mostly to do with power efficiency. Light emitting diodes (LEDs) (today, a 3W

white LED has the same lumens as a standard 60W light bulb), many basic

refrigeration devices, and small electrical motors can be used to pump water for

drinking or irrigation, all could be run by a DC power supply. The SuPER project is

designed as a family unit therefore eliminating DC power transmission line losses.

This report will discuss the DC-DC Buck Converter needed to transfer power

from the solar photovoltaic source to the battery and the rest of the system. The

background section will give information on how a Buck Converter works in theory.

2

The requirements will show the scope of our portion of the project and how it

changed multiple times during the duration of the two quarters. It will also present

the specifications for the Buck Converter. Design will present the simulation design

of the Buck Converter and the PCB layout with the use of PCB123. Construction and

assembly will discuss all the errors we made with the PCB board and the problems

that came up when trying to test the high-side driver. Testing will present our testing

procedures and results. There will also be three appendices at the end: Appendix A

presents a list of all the parts, vendors and prices we used in for the buck; Appendix B

presents the simulation source code for our simulation of the simple Buck Converter;

and Appendix C will present the team’s senior project analysis.

3

II. Background

In this background section of this report, we present a detailed discussion of

how the DC-DC Buck Converter operates as well as present former efforts to design a

Buck Converter for the SuPER System. The DC-DC Buck Converter is a circuit

which converts a source of direct current from one voltage to another. It is essentially

a step-down converter in the sense that it bucks the voltage level (the output voltage

is less than the input, hence the name Buck Converter) [2]. The Buck Converter is

comprised of three main components: the MOSFET switch, a diode, an inductor –

which is the main source of energy storage – and a capacitor. A schematic of a

simple Buck Converter with its main components can be seen in Figure 1. A

schematic of a simple Buck Converter in the on and off state can be seen in Figures 2

and 3, respectively.

Figure 1: Simple Buck Converter

4

Figure 2: Simple Buck Converter with SW1 ON

Figure 3: Simple Buck Converter with SW1 OFF

The Buck Converter is easy to understand if we look at the two main states of

operation: SW1 ON and SW1 OFF. When SW1 is ON, the inductor delivers current

to the load. With a voltage (Vin – Vo) across L1, current rises linearly. The rise is

determined by diL/dt = (Vin - Vo) / L1. C1 smoothes out L1’s current changes into a

stable voltage at Vo. Also, C1 can be big enough such that Vo does not change

significantly during one switching cycle [3]. D1 as we can see is left out since it is

reversed biased.

5

When SW1 is OFF, the inductor maintains current to the load. As L1’s

magnetic field collapses, current falls linearly through L1. The fall is again

determined by the voltage across L1 and its inductance: diL/dt = -Vo / L1. Although

L1’s current direction is the same, the voltage across L1 is negative as seen in Figure

3. This allows L1 to maintain current flow by reversing its voltage when the applied

voltage is removed. Also, D1 turns on when the left end of L1 swings negative,

providing a path for L1’s current to flow [3].

With all the components given, the means to control the output voltage would

be to use a Pulse-Width Modulation (PWM) signal to drive SW1. Since Vo/Vin = D,

where D, duty cycle = Ton/Ts, the desired output voltage can be obtained by

adjusting the duty cycle D. The definition of Ton and Ts can be seen in Figure 4,

where fs, switching frequency, is equal to 1/Ts and Ts is the switching period.

Figure 4: Plot Defining Duty Cycle

Typically using a high frequency range will bring about two benefits: 1. as

frequency goes up, parts usually get smaller, lighter and cheaper – which is what we

need for our portable PCB design; and 2. the delay from input to output created by the

switching time Ts becomes smaller [4]. All this was taken into account and

implemented in the design of a Buck Converter for the SuPER System.

6

The DC-DC Buck Converter is the heart of the whole SuPER system. Two

full years of testing and integration of the buck converter has been done. However,

there has been no success in implementing a successful Buck Converter. The first

Buck Converter used in the SuPER system was the MX60, a high-capacity DC-DC

converter manufactured by Outback Power Systems [5]. Before fall of 2006, two

previous efforts for constructing a functional converter by Eric Phillips and by Robert

Johnson did not succeed. Starting fall of 2006, seniors Robert Casanova and Joe

Shein began new efforts to develop the converter. They worked closely with

Professor Taufik, but ultimately the converter was not successfully integrated with the

SuPER System. The students working on the Buck Converter tried to gain experience

from the past students in order to make their converter work successfully. However,

each group did not succeed in getting a Buck Converter to work with the SuPER

System.

This previous work led to the winter of 2007 development effort by Koosh

Shah, Kunal Shah, and Thaddeus Guno. We tried simulating a design that was

already simulated with the full system design using the Simulink system model. The

simulations we ran with PSpice led to the layout of a high-current PCB. According to

the SuPER System design, there will be a high frequency PWM signal input from a

microcontroller that basically controls the duty cycle of the MOSFET switch on the

Buck Converter. The 500 kHz PWM signal for this converter is sourced by a

microcontroller, and some of the difficulties with this approach involve proper

marriage of this signal to the MOSFET switch [5]. The previous experience led to the

7

realization that a high-side driver chip would be necessary to help turn on the

MOSFET switch in the Buck Converter. Not only that, the high-side driver would

also be needed to keep the MOSFET in the linear region rather than going into the

cut-off or the saturation regions. The implementation and testing of the high-side

driver is what a majority of our testing will focus on.

Another problem fixed by Tyler Sheffield has to do with interfacing the

current source to the DC-DC converter in Simulink. As a result of the SuPER

simulation system, it is necessary to include some circuit element in parallel with the

current source. Various options were explored, including resistors and controlled

voltage sources, but a capacitor actually provides the optimum system behavior [5].

An optimal value of the capacitor was found by experimentation to be at about 10uF.

The Simulink model includes this extra capacitor at the input and is something we

took into account when building the layout for our Buck Converter. A part of the

schematic of the Buck Converter taken from the Simulink model with the extra

capacitor included can be seen in Figure 5.

8

Figure 5: A Piece of the Simulink Schematic of the Buck Converter

9

III. Requirements

The requirements for our senior project seemed to change a few times

due to a number of problems encountered during our last quarter of the senior project

journey. Initially, Thaddeus Guno and Koosh Shah were going to build a Cuk

Converter due to its better characteristics and results when compared to the Buck

Converter. After building this converter, preliminary tests would be taken to see if

the converter works. Next, we would integrate the converter with the SuPER System

as a whole. Kunal Shah was going to build up on the past Buck Converter designed

by Robert and Joe. Improvements should be made in order that the past Buck

Converter, which supposedly should handle 150 watts, could be modified and

improved in such a way that it handles up to 400 watts when integrated with the

whole SuPER System.

As more time passed, Robert and John were not able to build a successful

Buck Converter to be worked upon. For this reason, our senior project requirements

changed again in which we were counted on just being able to integrate a Buck

Converter with the SuPER System. The testing that went along with the Buck

Converter and the high-side driver included created more problems. The PCB turned

out to be erroneous and could not be fixed right away. Testing had to be done using a

breadboard with only the high-side driver chip soldered onto the PCB board. Again a

number of problems were encountered with just the high-side driver itself. We could

10

not get the high-side driver to operate properly which once again changed our

requirements as a whole. The whole senior project is clearly a tremendous learning

experience in which we are required to learn everything that we can possibly learn

with the time available. This knowledge can be conveyed to the future generations in

the recommendations section and used wisely to hopefully produce a complete,

integrated Buck Converter. The following section will emphasize in detail the

specifications for the Buck Converter.

Specifications:

The DC-DC Buck Converter provides a physical interface between the solar

cell and the battery and the loads. The converter’s input impedance is the impedance

seen by the PV panel terminals, and is controlled by the converter’s switching duty

cycle [5]. By adjusting the duty cycle and monitoring the converter’s output, a simple

“Perturb and Observe” algorithm for maximum power point tracking can be

implemented [8]. The specifications of the Buck Converter are unchanged since the

start of the SuPER System since a working Buck has yet to be produced.

The Buck Converter needs to be able to handle a wide range of input voltages,

ranging from approximately 24V to 40V, depending on the solar irradiance and the

converter’s own duty cycle. The input current should never exceed more than 4.75A.

The efficiency of the converter should be approximately 80% with a step down in no

more than 150 watts of power. When the battery is charging, the output voltage is

expected to vary from 11.5V to 14V depending on the battery level. In real time, the

converter should be able to output approximately 12V to the battery. Overall, the

11

output voltage ripple should be extremely small – around 50mV. Assuming that the

converter has perfect efficiency, and using the formula for power P = V*I, the

maximum output current should be approximately 13A. Another requirement

specific to building our own Buck Converter on a PCB should make sure all the

terminal blocks or connectors are rated at 30A. The specifications for the Buck

Converter can be seen in tabular form in Table II.

Table I: DC-DC Buck Converter Specifications [5]

The switching frequency used for the current PWM signal is approximately

500 kHz. This rate is defined by the capabilities of the PIC 18F4320 [5]. This

frequency must definitely be taken into account when picking a MOSFET. The

MOSFET should be one that is a high frequency MOSFET which can also handle

high power. As a result we picked a power MOSFET that could be applicable in high

frequency circuits.

For reference, a schematic of the Buck Converter can be seen in Figure 5.

The specifications of all the parts used in our Buck Converter design can be seen in

Appendix A. The energy storage elements (an inductor and capacitor) are key

parameters for the converter and are picked based on the design considerations. The

12

value of the inductor is 0.93mH, and is calculated using the following equation for the

desired maximum output current [5]:

The value of the capacitor at the output is calculated to be 3uF using the equation

which gives the least peak to peak output voltage ripple (ΔVo):

The value of the capacitor at the input was chosen to be 10uF and determined by the

SuPER System simulation. The capacitor is included in our final layout since it also

helps stabilize the change in input voltage.

When picking the type of diode to use, we spoke to Dr. Taufik who mentioned

using a Schottky diode for the Buck. A Schottky diode has very low forward voltage

drop as well as very fast switching action. It is typical to use in a switch-mode power

supply application (Buck Converter). The use of a Schottky diode with low forward

voltage drops also leads to increased efficiency [9].

13

IV. Design

A. Simulation

The first part of the senior project consisted of studying the simulation file of

a simple Buck Converter. The template PSpice .cir file of the Buck Converter was

given to us by Dr. Harris. The .cir file is a template of a Buck Converter in source

code. Since most of our PSpice simulations in our Electrical Engineering courses

were done in schematic form, we had to put in extra effort when working with the

source code. We had to research and find out about the syntax while troubleshooting

with the simulation itself. After some time and study, we came out with the

following simulation results.

Simulation Results:

Our first simulation was initiating the transient analysis at the output. The

source code of our template Buck Converter can be seen in Appendix B. The plot in

Figure 6 shows the output response at the output capacitor with hardly any overshoot

within the initial part of the response. The plot is analyzed with a duty cycle of

approximately 30% in which the output should stabilize to 12V (what we want);

however, according to the plot, the output stabilizes to 11.6V which is extremely

close to the theoretical. The plot as a whole shows how much time it takes to run

from a transient state to a steady state, also known as the settling time. According to

14

the plot, the settling time is approximately 50us, which is relatively fast. As a result,

this plot demonstrates a good transient response at the output.

Time

0s 10us 20us 30us 40us 50us 60us 70us 80us 90us 100usV1(c1)

6V

8V

10V

12VVoltage

Figure 6: Transient Response at the Output

The plot of the steady state output at the output capacitor can be seen in

Figure 7. The output is approximately 11.6V which is around the target with a duty

cycle of 30%. This plot also shows that the output voltage ripple is extremely small;

calculated to be about 17.2mV. This is definitely in the range of our specifications.

15

Figure 7: Steady State Output

This next plot, in Figure 8 shows the switching cycle of the MOSFET. Set to

a duty cycle of 30%, the graph definitely conforms to this parameter. According to

our simulation results, the simulated Buck would certainly be a good converter to use

for the SuPER System. The only disadvantage was that we could not simulate the

Buck Converter with the high-side driver that we used for our layout.

Time

10us 12us 14us 16us 18us 20usV1(vsw)

0V

0.5V

1.0VVoltage

Figure 8: Switching Cycle of MOSFET

16

B. Schematic

After the simulation was complete, we had to build the circuit using the

software PCB123 Schematic. Using the same values for all the components, the

schematic was created smoothly. The next step was to assign each component a

package name. This took a long time because we were beginners at using PCB123

Schematic. Another reason why this process took long was because we had to go

through the data sheets for each different component to try and find a somewhat

similar package name in the PCB123 libraries. This task was not easy but we pulled

through it eventually. In addition to this, a challenging part of this process was

actually editing a part for the high side driver chip since we could not find the

package name for it in PCB123. To approach this problem, we went through different

parts in the PCB123 Schematic library to find a part similar to our high side driver

chip. As we found a similar part, we used it and modified it accordingly. Then using

the package name library we were able to find a similar footprint that matched the

data sheet of our high side driver chip. Overall, with the help of PCB123 support and

going by Cal Poly’s motto of ‘learn by doing,’ we were able to complete the

schematic. See Figure 13 to see the Printed Circuit Board Schematic.

17

Figure 13: Schematic of the Buck Converter Printed Circuit Board

C. Layout

The next step was to convert the DC-DC converter schematic from the

PCB123 Schematic software to the layout. The software used for the layout was

PCB123 Layout V2. The first thing that the convert button led us to was to edit the

board configuration. The board configuration consists of the board size, the number

of layers required for the board, the options of using solder mask and silkscreen, the

material thickness, the finished copper weight, and the quantity of boards wanted for

purchase. The board size we first started out with was 5in by 3in. After the layout was

complete, we changed the board size to 2.9in by 2.625in. The reason for this is that

the smaller you keep your board, the lower the price you pay for the PCB boards. The

18

number of layers we used was two and we selected the option of using solder mask

and silk screen. As for the material thickness, we kept the default value of 0.062”.

The finished copper weight we used was 1-oz and the quantity of boards we chose

was two. We were not able to use the 2.5-oz weight because we received too many

unfixable errors. The only solution was to use the 1-oz copper weight. For future

generations, we recommend trying to use the 2.5-oz copper weight since it can carry

more current. However, it should be noted that the price increases with increasing the

copper weight. For PCB123 Layout, the minimum number of boards you must

purchase is two, so this illustrates why we bought two boards.

As this completed the board configuration, we manually arranged the

components on the board. Then we ran the ‘Autoroute’ function which automatically

routes all the connections in the net list. Next, using the PCB Trace Width Calculator

website found on Kaha’s Wiki site we were able to calculate the trace widths for the

ground and input traces since the currents reach high limits at these traces [6]. The

trace widths that we calculated came out to be 139mils after inserting an input current

of 20A, temperature rise of 50 deg C, and an ambient temperature of 35 deg C as

suggested by Kaha Sariashvili. Changing the trace widths for your ground trace and

input traces can be done by going to the edit panel on the left side of the PCB123

Layout screen, clicking on Object Hierarchy, and by changing the properties for the

Net ‘Ground’ trace and the Net input trace. After completing this task, we went on to

changing the hole sizes for the components that needed holes. This was done by

looking at the data sheets for each component that needed holes and changed them

19

accordingly. It turned out that this task would not work by just selecting the pin of the

component and simply changing the hole size in the properties menu. This wouldn’t

work because after applying the changes and actually rechecking each component, the

hole sizes were not changed but back to the default value of ‘None’. So, after

contacting PCB123 support, we were told that we actually have to edit the footprint

of the component and then change the hole sizes and save it. This automatically asked

us to update the component changes and we simply clicked ‘Yes’.

Next we ran the design checks by using the function ‘Run DRC’. This brought

upon us various types of errors and warnings. The warnings that came about were the

silkscreen over copper warnings. These were negligible so we just ignored them. The

errors however could not be ignored and had to be fixed. The three types of errors we

received were the Track-To-Track spacing violation, Track-To-Pad spacing violation,

and Pad-To-Pad spacing violation. These errors mainly occurred because of the

increase in the trace widths. The Track-To-Track errors were the easiest ones to fix

but took the longest to fix because the large trace widths interfered in the routing

leading us to rearrange the routes and make vias (connections between layers). The

Track-To-Pad and Pad-To-Pad spacing violations were the harder ones to fix because

after contacting PCB123, we were told to change the Pad Widths. This was too much

trouble and did not work out at all. However, after receiving feedback from the

SuPER meetings, we were finally able to fix these errors by simply tapering the trace

widths near the pins of the components. After completing the layout with no errors,

20

we realized we had assigned the wrong diode part in the Schematic. See Figure 14 to

see our first Printed Circuit Board Layout.

Figure 14: First PCB Layout

This led us to edit the schematic with the correct diode. The diode was

actually a Schottky Barrier Rectifier. It had two anodes and two cathodes. Since this

was a different configuration, the schematic definitely had to be modified. After

completing this task we had to once again find the correct package name. Then we

had to convert this file into the layout again and went through the whole routing

21

process once again. Finally this task was complete, and the board was ordered. See

Figure 15 for the second Printed Circuit Board Layout.

Figure 15: Second PCB Layout

Refer to the following website for a complete tutorial on the PCB123

Schematic and Layout: http://www.pcb123.com/pcb123tutorial.php. In addition, for

brand new users, the PCB123 website has a downloadable PDF version of the ‘Brand

New Users Guide.’ These tutorials and guides are highly recommended for use by

brand new users of PCB123.

22

V. Construction/Assembly

When we received all our parts and the boards as well, we started out by

trying to see if all the components fit correctly on the board. We realized that this was

a big mistake on our part. We should have rather tested this out by making a one-to-

one printout of the PCB Layout and tested to see if all the components would fit

correctly on paper. So when trying to fit a few of the components on the board, we

realized that there were a few errors. For instance, the terminal blocks would not even

fit onto the board because the spacing of the holes was not correct. The holes were

spaced too closely together. In addition, one of the capacitors fit perfectly, while the

other capacitor did not. The holes were actually too small for the leads of this

capacitor to even fit through. We also came upon a problem with the diode. The three

pin diode holes were not spaced correctly as well. On the other hand, the rest of the

components fit perfectly fine.

Coming upon these errors, we decided to test the high side driver. When

trying to test this chip, we realized that we could not get it to work. This turned out to

be a big problem because in the beginning we were told by Professor Taufik that we

did not need to use the low-side driver part of the chip since the chip was actually a

high side/low side driver. After contacting Texas Instruments, we were told that both

the high side and low side drivers were to be used. This was a big problem because

this told us that we had to make quite a few changes on the PCB Schematic. So in

23

turn, we started editing the schematic. While doing this we realized that there would

be no point in editing the schematic until we got the high side driver chip tested and

working properly to turn the MOSFET on.

In order to test the high side driver chip we used both of the ordered printed

circuit boards and directly soldered the chip where it belonged on the board to test it.

We used long thin wires found in the senior project lab to solder wires to each of the

eight pins. During our testing process however, we kept coming across the problem of

re-soldering the wires back to the pins. The reason for this was because while testing

the chip the pins would get loose and eventually fall off. This resulted in re-soldering

the wires that needed to be re-soldered. We used up both of the boards that we

ordered from PCB123 for testing the high side driver chip but then eventually came to

a conclusion that the whole re-soldering business was really inefficient. This is where

we found out about the SOIC to DIP adapter. The SOIC to DIP adapter was found at

www.ironwoodelectronics.com. You basically take a surface-mount component and

solder it directly on to this adapter. Then you physically take the adapter and stick it

straight into a bread board. This solved the whole problem of repetitive re-soldering

of the wires to each pin. This slowed down our testing process because the order was

supposed to be done by next day air, but Ironwood Electronics is a “Manufacture to

Order” company [7]. They do not stock parts on the shelf due to a high volume of the

different parts that they sell. This basically slowed down the whole process of testing

by more than a week.

24

We received the SOIC-DIP adapter a week after it was shipped. Due to this,

we lost 2 weeks time for testing the high side driver. When we received the adapter,

we were easily able to solder the high side driver onto the adapter. The bottom pins of

the adapter stuck perfectly into the breadboard. This definitely fixed the problem of

re-soldering wires to the chip. This also ended the construction/assembly portion of

our senior project.

25

VI. Testing

In order to make sure the whole buck converter subsystem fully work, we

tested each individual section: high-side driver, inverter, and synchronous buck,

separately. After we finished testing and making sure all the sections were working

the way we expected, we wanted to slowly combine everything. From just the chip,

we include an inverting circuit, then the MOSFETs needed for the synchronous Buck

Converter, and finally the rest of the buck converter.

A. High-Side Driver

Figure 12: Top View of the High-side Driver Chip

Our original design had us using only the high-side portion of the driver to

power a basic Buck Converter. The pins associated with the high-side driver chip can

be seen in Figure 12. After a couple of initial tests and consulting with Texas

Instruments, we learned that we needed to also use the low-side driver to pull down

the circuit and actually turn-off the switch of the Buck converter. The following

26

pages show the tests we ran in chronological order, in an attempt to understand the

high-side driver.

We tried making sure the high-side driver worked the way we expected using

the simple circuit in Figure 13. The idea is the HO (high output) pin would act as a

PWM signal for the MOSFET. When it went high output would equal 0V; when it

went low output would equal 20V.

HO

Output

R1

10k

2

1

M1

20V

Figure 13: Basic HSD and MOSFET Test Circuit

The output resulted in always having a low voltage, 0.437V; in other words,

always tied to ground. At first we thought the MOSFET we were using was acting

up, so we disconnected the HSD and just put two different DC values for the gate

node of the MOSFET to test the MOSFET by itself. Table I shows the test results for

testing the MOSFET.

Table II: MOSFET Test Results

Vg Output 0V 20V 5V 0.437V

27

From that test we concluded that it was not the MOSFET that was acting up

but it was the HSD. It was then that we realized that we did not know how the HSD

chip truly worked. In order to fix this, we spent the next day studying the ucc27201

data sheet and calling Texas Instrument’s Product Information Centers. We learned

that we needed both high-side and low-side drivers in the chip, and the reason why it

was always being pulled down in the earlier test was because the HO pin does not

swing from VCC to ground. The HO pin’s range is from VHS-0.3 to VHB+0.3 [2].

Since HO only operates in a high condition it does not turn off the MOSFET chip.

Next we created the circuit in Figure 14, which is very similar to the sample

circuit in the ucc2701 data sheet. As shown in Table II below, we started with no

pulse signal at either high or low inputs, then alternated with one input having the

signal and the other still tied to ground, and finally we let both inputs have the same

pulse signal. All the while we recorded what happens to the output voltage across

R1. We expect the output to be pulled close to 20V when we put HI to 5V and keep

LI at 0V; and when HI is 0V and LI is 5V we expect the output to be 0V. When both

signals are off, what we guessed would happen to the output voltage was correct,

equaling 0.433V, as shown in Table II. However, no matter what we did with the

input signals we couldn’t get the output to change. We learned later while looking at

the data sheet a little more that the high and low input signals of the chip, pins 5 and 6

respectively, needed to have equal but inverting signals; so both HO and LO will

have alternating high and lows.

28

VDD

HO

Q1

MBREAKN

Q2MBREAKN

20V

C1

0.1uF1

2

HB

LOR1

1k

2

1

12V

C2

0.33uF1

2

HS

Figure 14: HSD Testing Circuit

Table III: HSD Testing Data

Trial VDD HI LI HS/output HB VSS 1 12V 0V 0V 0.433 11.46V GND 2 12V 5V 0V 0.428 11.52V GND 3 12V 0V 5V 0.435 11.48V GND

During the next testing session we included a CMOS inverter with the circuit,

but we were still recording the same results. Some time through the test we realized

that the DC voltage source we were using had become current driven and would

immediately reach the current limit we had set to protect our circuit. With the lab

closing and time running out we decided that there has to be something wrong with

our chip since the source would always become current driven. We decided to solder

a new chip to the second PCB board the following day and hoped that this would fix

the problem.

29

We soldered the new chip to the second board, started testing, but the wires

we soldered to board kept coming off and needed re-soldering. After a few more

trials of de-soldering and re-soldering, the old problem of the voltage supply came up

again. As stated in the assembly section, we learned that some how we had shorted

our chip, and in order to continue with the testing we need a better way of accessing

the pins without having to keep re-solder loose wires. A couple of ways we decided

we can fix this are: having a better board - maybe a surface mount or a generic PCB

board; or having a SOIC – DIP adapter. We were not able to find a better board, but

we were able to find an adapter. However there was a delay in the shipping when we

ordered the adapter, which in turn delayed our testing.

Once the adapter came in the mail, we set up the testing circuit shown in

Figure 15 whereas Table III shows our resulting data. For the first 3 trials we used

the inputs VDD, HI, and LI and recorded the value for every other pin. For trials 4

and 5, we included HS as an input and tied 20V to it. On our 7th trial we started to

run into problems. Earlier, we would have made HI equal 0V and LI equal 5V,

however when we made LI high in the 6th trial, the DC power source we were using

for 12V started to reach the current limit and would not draw the amount of voltage

we wanted as soon as we turn the power supply on. At first this would only happen

when LI was tied high. It would not allow us to even recreate the data from trial 3

and 4. After playing around with the circuit a little more, the same problem started to

occur every single time. No matter what we did the power source would always reach

its current limit instantly.

30

Figure 15: DC Testing Circuit for HSD

Table IV: Data for DC Testing

Trial VDD HI LI HS HB HO LO VSS 1 12V 0V 0V 10.88V 11.16V 10.64V 0.430V GND 2 12V 5V 0V 10.89V 11.17V 10.66V 0.430V GND 3 12V 0V 5V 11.15V 11.16V 10.71V 0.430V GND 4 12V 5V 5V 11.17V 11.17V 10.73V 0.420V GND 5 12V 0V 0V 20V 19.50V 19.60V 0.420V GND 6 12V 5V 0V 20V 19.57V 19.60V 0.450V GND

B. Inverter

After talking to the Texas Instrument technician, it became apparent

that we needed both high-side and low-side drivers used in the chip. In order to do

this the LI and HI inputs need to be opposites of each other. For example when LI is

high, HI would be low and vice versa; we decided to use an inverting circuit as a

hardware solution. We used a CMOS chip - MC14007UB - as seen in Figure 16 from

one of our previous labs to produce an inverting signal.

31

Figure 16: Circuit Layout of MC14007UB

Our test for the inverter consists of a basic CMOS circuit built using the

MC14007UB chip. We used pins 7, 10, 11, 12, and 14 as shown in the Figure 16

above. At first we used 5VCC to power both the chip and create the high rail of the

inverter, and a 5VPP, 500 KHz square wave as an input signal. Figure 17 shows the

simple CMOS inverter test circuit that we built. Figure 18 shows both the input and

output signals for the test.

R1

10k

2

1

FG Output

M1

M2

5V

Figure 17: CMOS Inverter Test Circuit

32

Output

FG Input

Figure 18: Input/Output of a Working CMOS Inverter with 5VCC

After some consideration and integrating the inverter with the high-side

driver, we realized that we should use a 12VDC rail to actually power the chip and the

high rail for the high-side driver. However the PWM signal we are using will only be

a 5VPP square wave. The output had a hard time switching when the inverting pulse

signal was applied to HI and LI. The output waveform would stay high, 12V, when

the input pulse was low, but when the pulse went high the output would only reach a

fraction of the signal. With the equipment we used in the later test, we were not able

to capture an image. But Table IV shows the values we observed on the oscilloscope.

We were expecting a switching (pulse) signal at the output HS, but could not figure

out why the high-side driver chip (which is a black box to us) would give us the

33

unexpected output. Possible corrections to this problem will be discussed in the

conclusion and recommendation section.

Table V: Test Results with Inverter and High-side Driver

VCC FG Output 12V 0V 12V 12V 5V 8V

C. Buck converter

Due to time constraints and lacking various parts we were not able to test the

buck converter. We spent most of our time testing the high side driver to fully

understand how the chip worked.

34

VII. Conclusions and Recommendations

A. Problems and Estimated Completion Time

Since we were not able to get a completed DC-DC converter for our senior

project, there were a few problems that we wanted to address. The problems include

the following: figuring out how to work the high side driver, figuring out why the

power source becomes current driven and getting an inverting circuit for the high side

driver.

The first and foremost problem is figuring out how the high side driver works.

After plenty of testing and consulting, we were still unable to get the results that we

were expecting. The results we were expecting was a pulse width signal at pin HS as

seen in Figure 9. We did contact a Texas Instrument’s technical support

representative and were supposed to get a response within a day or two about why we

were not getting the results we expected. We never received a response from Texas

Instruments so we were basically on our own. In order to solve this problem, we feel

that Texas Instruments needs to be contacted again. Any type of input from Texas

Instruments would be useful in figuring out why our results are incorrect. When

estimating the amount of time that it would take to fix this problem, we came up with

an estimate of 72 hours. We estimated this much time because at least 1-2 days have

to be considered for waiting time to hear back from Texas Instruments. That leaves a

35

minimum of 24 hours for re-testing the circuit according to the input given from

Texas Instruments and getting expected results.

The next problem deals with the power source becoming current driven. In

other words, when testing the high side driver, the circuit would indicate zero voltage

because there was probably a short between the two nodes of the source. As a result, a

new testing method was devised in which we ordered a SOIC to DIP adapter to help

facilitate our testing with such a small chip. It turned out that even with this adapter

we received the same results of the power source becoming current driven. After this

we came to conclude that there has to be something happening internally in the chip

itself. So in order to even understand and fix this problem, we would once again

recommend contacting a Texas Instruments representative and getting feedback on

why such a thing is happening. As for an estimate of how long it would take to fix

this problem in terms of hours, we estimate 72 hours again. This again would give 1-2

days for waiting for a response and 24 hours minimum to re-test the circuit.

The next problem is getting an inverting circuit for the high side driver chip.

We need an inverting circuit because we need a high side and low side input from the

single PWM signal we are receiving from the system. We used a CMOS inverter but

that caused the same problem as mentioned in the paragraph above. In order to fix

this problem, a new inverting circuit needs to be used. Perhaps using an op-amp or

another chip that inverts signals are a couple solution possibilities. As for time needed

to fix this problem, we estimate that it would take approximately 15-20 hours to come

up with an inverting circuit.

36

After all these problems are fixed, the PCB Schematic and Layout are the next

task. The estimated time we feel it would take to complete this task is 10 hours at

most. As soon as the board is completed, you would need to allow at minimum 5 days

for the board to arrive. Then the low power testing needs to be done in order to see if

the circuit works well before moving on to high power testing. We estimate a total of

15 hours maximum for low power testing. The next thing that needs to be done is the

high power testing. This needs to be done because the board needs to be fully tested

before integrating it into the SuPER System. To finish this task we estimate it would

take no longer than 10 hours. This leads us to the next and last task that needs to be

done which is integrating the board into the SuPER system. Keep in mind that system

integration is not simply just hooking up the circuit into the system. Therefore,

system integration would likely take approximately 25-30 hours.

To summarize the minimum total calendar days and person-hours that it

would take to complete this project, it added up to 5 days plus 229 hours. To simplify,

this turns out to be a minimum of 15 full work days (24 hours in a day). It needs to be

noted however that problems are always bound to arise and these numbers are all just

an estimate. It could likely take longer than these hours that we predicted but we feel

it is definitely possible to complete it within the numbers that we chose.

B. Conclusion

Overall, our senior project with the SuPER team has been a tremendous

learning experience. Although we were not successful in building a working Buck

Converter, we learned a lot, first being time management. We realized in order to be

37

successful in any project one must learn how to manage his time properly. If we had

perhaps managed our time in a more efficient manner, we might have accomplished

more than we actually did.

Our requirements for our senior project also kept changing throughout the two

quarters. Since Robert and John were not able to finish building a working Buck

Converter, we could not modify this converter to up the power and build a Cuk

Converter as well. Instead, we were forced to build our own Buck Converter that

would hopefully fit in with the SuPER System. These changes made it hard to focus

on one thing until the end.

Another useful technique we learned had to do with the soldering of the parts.

We learned how to solder accurately and efficiently with extremely small parts. We

also learned how we should be double checking our work for the PCB layout. Buying

boards is not cheap, and so it should be taken into utmost account that the final layout

be as close to perfect as possible. Since our board came with errors, we took it as a

learning experience and moved on with figuring out different ways to test the

converter. Never giving up is a key virtue that goes along with our learning

objectives in the senior project journey.

A few times during our low power testing of the high-side driver, we ran into

a problem where the power source became current driven. In this case, the circuit

would indicate zero voltage because there was probably a short between the two

nodes of the source. As a result, a new testing method was devised in which we had

to order a SOIC to DIP chip to help facilitate our testing with such a small chip.

38

All in all, working on a senior project made us realize that we have to use our

knowledge from all our past Electrical Engineering courses and labs in order to

troubleshoot and complete a successful project. We had to constantly come up with

different test ideas as well as talk with different vendors about certain parts that do

not work as expected. We also realized that when ordering parts or a board, problems

will always arise because things never occur as expected. You never know when

parts will be shipped out – it might take a week even though they say one day.

Problems should always be expected and ready to be handled. Even though we could

not make a successful Buck Converter, we learned a lot through simulation, testing,

time management, and mainly learning about the high-side driver chip which gave us

plenty of problems.

C. Recommendations

When starting the senior project it is essential to research, plan and fully

understand what the project is about. Read over Tyler’s thesis, or at least the

introduction and background, in order to get a good sense of what’s going on with the

whole system. Make a schedule early which has a time table and goal of what you

want to finish each week throughout the two quarters. This will help with time

management, even if you do not follow the schedule exactly; it makes you keep in

mind what you still have to do. The suggestions above will help reduce a lot of time

wasted in the early stages of senior project and less stressful situations during late

stages.

39

VIII. Bibliography

[1] Harris, James G. White Paper for Sustainable Power for Electrical

Resources- SuPER. July 15, 2005. <http://www.ee.calpoly.edu/~jharris/research/super_project/white_paper_susper.pdf>.

[2] Dr. Taufik. “Introduction to Power Electronics,” California Polytechnic State University, San Luis Obispo, 2006.

[3] “SMPS Basics: The Buck Converter,” eCircuit Center, 2005,

<http://www.ecircuitcenter.com/Circuits/smps_buck/smps_buck.htm>. [4] “Buck Converter,” Wikipedia: The Free Encyclopedia, 14 February

2007, Site founded by Jimmy Wales, <http://en.wikipedia.org/wiki/Buck_converter>.

[5] Sheffield, Tyler. “Cal Poly SuPER System Simulink Model and Status and Control System,” Master’s thesis, California Polytechnic State University, 2007.

[6] The CircuitCalculator.com Blog. 2006. The CircuitCalculator.com Blog. 17 Apr. 2007 <http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/>.

[7] Litchy, Lori. “Order on Ironwood Electronics On-Line Catalog.” E-mail to Kunal Shah. 25 May. 2007.

[8] Tal, Eran. “SuPER System Prototype Design and Implementation,”

Master’s thesis, California Polytechnic State University, 2006. [9] “Schottky diode,” Wikipedia: The Free Encyclopedia, 14 February

2007, Site founded by Jimmy Wales, <http://en.wikipedia.org/wiki/Schottky_diode>.

40

Appendix A: Parts, Vendors and Cost

Table VI: Parts, Vendor, and Cost

Price Part Per Unit Quantity Total Vendor Notes

PCB board $121.16 1 $121.16 Sunstone Used a sponsorship*

50V 10uF Capacitor $0.27 1 $0.27 Mouser 20A 45V Schottky Diode $0.89 1 $0.89 Mouser 25A 0.95uH Power Inductor $2.59 1 $2.59 Mouser SOIC to DIP adapter $13.00 1 $13.00

Ironwood Electronics

50V 3uF Capacitor $2.02 1 $2.02 Mouser

Terminal Blocks $1.26 2 $2.52 Allied Electronics

100V 140A Bought 10 for a NMOSFET $3.72 2 $7.44 Digikey reduced unit price 120V 3A, High Freq. HS/LS driver $ - 1 $ -

Texas Instrument Free Sample

Total: $149.89 During testing we used components from previous Electrical Engineering labs. Table

VI shows a table of the parts, their vendors, and the costs.

* Sponsorship:

Since the PCB boards through PCB123 or Sunstone Circuits were expensive

and required of us to buy 2 boards minimum, we decided to see if we could receive a

sponsorship to help the SuPER team with the expenses. Sunstone representatives told

us that students working on a project for a university are eligible to receive a

sponsorship if accepted through an application process.

41

Sunstone Circuits would donate PCB boards only to non-profit organizations

or educational institutions up to a limit of $350 per academic year. In return, Cal Poly

was required to recognize Sunstone contributions by adding their link to our SuPER

website, Sunstone logos to our project, and/or any other applicable recognition of

sponsorship. With the help of Dr. Harris, we applied for the sponsorship through

Sunstone Circuit’s website at http://www.sunstone.com/sponsorships.aspx.

The application required a project type, a project website, a project name and

a project description. After sending a request for sponsorship, we received an e-mail

from Sunstone Circuits a week later stating that our request had been approved and

Sunstone Circuits decided to give the SuPER team a sponsorship credit of $350.

42

Appendix B: Simulation Source Code The following is the source code of our template Buck Converter. It is the source code we used to obtain our simulation results in the Design section. *Dc-Dc Buck Converter Transient Response bucktran.cir is 1 0 sin(0 4 0.5 0.001) ;sin(vo va freq td) with 0 offset, 4a amplitude, 0.5Hz (2 sec period) is 1 0 pulse(0 4 0 .1u .1u 10) rg 1 0 10 ;resistor for current source vs 1 0 40 ;40v dc source s1 1 2 6 0 smod ;voltage controlled switch with model smod vsw 6 0 pulse(0 1 0 .01u .01u 0.6u 2u) ;pulse(v1 v2 td tr tf pw per) with 30% duty cycle at 500khz rvs 6 0 1meg ;resistor for voltage controlled switch source d1 0 2 dmod l1 2 3 700u ;700u inductor rl 3 4 1 ;resistor for tran analysis c1 4 0 30n ;0.03u capacitor rb 4 5 100 ;100ohm resistor for battery vb 5 0 dc 12 ;12v battery voltage rld 4 0 100 ;100 ohm load .model smod vswitch(ron=1m roff=1g von=1 voff=0) ; model for switch .model dmod d .tran .01 .1 0 .2u .probe .end

43

Appendix C: Analysis of Senior Project Design

Project Title: SuPER System DC-DC Buck Converter

Students’ Names: Thaddeus Guno, Kunal Shah, Koosh Shah

Students’ Signatures:

Advisor’s Name: Dr. James Harris Advisor’s Initials:

Date: June 15, 2007

Summary of Functional Requirements

Our senior project focuses on the implementation of the DC-DC Buck

Converter and its integration with the whole SuPER System. The Buck Converter

provides a physical interface between the solar cell and the battery and loads. It

should be able to handle a wide range of input voltages, ranging from approximately

24V to 40V, and be able to output a voltage ranging from 11.5V to 14V depending on

the battery level. This output voltage along with the power from the battery will

suffice to power the rest of the SuPER System as well as the DC loads.

Primary Constraints

During design and testing, the main constraint we faced was figuring out how

to implement the high-side driver. The high-side driver chip and the erroneous PCB

board made it hard to move on with our project and implement a working DC-DC

Converter. The lack of understanding on how the chip works and consulting Texas

Instrument professionals about it made a big impact on our project. It delayed us

44

from moving forward and was a significant challenge we were not able to accomplish

due to time constraints. Buying new parts to help us out with testing was also another

constraint. Since parts do not ship exactly on time and may take more than a week to

arrive postponed a lot of our testing time. As a result, working efficiently and

allowing more time for parts to come in is a great recommendation for future students

working on their senior projects.

Economic

The original estimated cost for this design was about $100.00. Being a general

estimation, we were not far off from our original estimate. Our total at the end of our

senior project came out to be $149.89. Since we had mistakes in our board, this would

call for a new board. So, since we were not able to complete our DC-DC converter,

the cost for us would have been much higher. However, thinking about it, if our board

was clean, with no mistakes, our general estimate would have been legitimate. We

have to note that mistakes and problems are always bound to occur, therefore possibly

increasing the price of a project. Refer to Appendix A for details on the parts and

costs. As for our estimated development time, we predicted the project to be finished

in about 150 hours. Our actual estimated development time came out to be around

100 hours. Since our DC-DC converter was not complete, we also calculated the

amount of time left in order to finish this project. The calculated calendar days and

person-hours can be seen in the Conclusion section. To summarize, we calculated a

total of 15 (24 hours in a day) full days left to complete the project.

45

Environmental

Our senior project is powered by a photovoltaic source and therefore it should

have negligible effect on the environment.

Manufacturability

Our senior project can be manufactured in the sense that all the components

and the PCB board can be bought from various vendors and put together to build a

Cal Poly made Buck Converter. It would not be cost effective to manufacture all the

components used to make the Buck Converter. However, the PCB board can be

manufactured in the IME laboratory and will cost less than to purchase.

Sustainability

The Buck Converter itself should have a long life cycle; however, the High

Side Driver chip we used does not. After a couple of tests, we were constantly

running into problems with the chip. With this in consideration, it would be highly

beneficial to actually come up with a design that replaces the chip. A couple issues

arise when changing the design of the buck. Among those are the rise and fall time of

the signal that actually goes into the gate of the MOSFET(s) of the Buck Converter.

That time constant needs to be faster than the switching frequency of the PWM

signal. This would ensure the MOSFET(s) will be done switching when the PWM

switches.

Ethical

There are no ethical issues relating to the design of our circuit. We attribute

any help we received to the people who helped us.

46

Health and Safety

There seem to be no health and safety concerns associated with this design.

Social and Political

With the use of the Texas Instrument’s High Side Driver chip, Texas

Instruments would want some recognition with the SuPER project since they supplied

the chip for free as a “free sample.” However, the SuPER team would like the system

to be entirely built from passive parts by Cal Poly students, so we would need to

change the design anyway.

Also, since we received a sponsorship from Sunstone Circuits, in return, Cal

Poly was required to recognize Sunstone contributions by adding their link to our

SuPER website, Sunstone logos to our project, and/or any other applicable

recognition of sponsorship.

Development

A new tool that we used for this design was PCB123 Schematic and Layout.

We learned how to use this software independently and had a good learning

experience using it. Since we were brand new to this software, we used a lot of help

from the visual tutorials and user guides provided by the software providers

(PCB123). We also used the help of a previous user of the software - Kaha

Sariashvili. Overall the experience using this new tool was a good and challenging

one. We would recommend this software to future generations although is not

completely user-friendly. Other alternatives such as AutoCad however would not hurt

and are more than welcome for use.