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Gallium Nitride E-Class Power Amplifier ECE402 REPORT Final By Adam Woodward [email protected] Department of Electrical and Computer Engineering Colorado State University Fort Collins, Colorado 80523 Project advisor: Dr. Fernando Tomasel Approved by: Dr. Fernando Tomasel

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Gallium Nitride E-Class Power Amplifier

ECE402 REPORT

Final

By Adam Woodward

[email protected]

Department of Electrical and Computer Engineering

Colorado State University Fort Collins, Colorado 80523

Project advisor: Dr. Fernando Tomasel

Approved by: Dr. Fernando Tomasel

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ABSTRACT

Current amplifier designs using semiconductor N-channel field effect

transistors (FETs) have significant losses at high power. Semiconductor FETs have

losses during current conduction through the channel due to drain-to-source

resistance (Rds ON). Substantial energy is also required to drive the gate capacitor

of the FET to get conduction through the drain-to-source channel. This gate

capacitor requires higher energy as the frequency increases, making it more difficult

at higher frequencies to completely turn on and off the FET.

Lower Rds ON and reduced gate capacitance can benefit the amplifier,

increasing efficiency and speed. New technology is now available that can achieve

both of these two objectives. In 2009 a company called Efficient Power Conversion

(EPC) produced the first commercially available Gallium Nitride eGaN FETs. This

Gallium Nitride material is grown on top of standard silicon wafers. This provides a

platform for new technology at a lower price. The devices created by EPC perform

much better than silicon in several ways. They have lower gate capacitance and

smaller Rds ON. This new technology has created a component that operates at a

high frequency and with low losses.

EPC component specifications have been entered into NL5 an electrical

engineering modeling software package, and show great results. The EPC2012

component rated for 200V Vds and 3A Ids shows up to 52W of RF power each. This

power will then be filtered and combined to produce a 500W RF power supply.

Summer semester will be focused on RF filtering and combining of several E-Class

amplifiers that use eGaN FETs as the main switch.

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TABLE OF CONTENTS

Title 2

Abstract 3

Table of Contents 4

List of Figures and Tables 5

I. Introduction 5

II. Schematic 10

III. Printed Circuit Board 12

IV. Building 13

V. Initial Test 14

VI. Tuning 15

VII. Ethical/Environmental issues 17

VIII. Manufacturability 17

IX. Marketability 18

X. Project Continuation 18

XI. Conclusion 19

References 20

Appendix A – Glossary 21

Appendix B – Budget 22

Appendix C- Time Line for Next Year 24

Acknowledgements 26

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LIST OF FIGURES

Figure 1 eGaN Gallium Nitride Structure 5

Figure 2 Schematic of E-Class Amplifier 6

Figure 3 Voltage and Current Waveforms of E-Class 7

Figure 4 Common Gate Resistance 8

Figure 5 Common Gate Charge 9

Figure 6 Schematic Single FET 11

Figure 7 Schematic Four FET’s 11

Figure 8 Schematic Sixteen FET’s 11

Figure 9 PCB Layout 12

Figure 10 Inductor Test Fixture 13

Figure 11 Infrared Soldering 14

Figure 12 Initial Loading of Board and basic test 15

Figure 13 Finding best impedance match value 15 Figure 14 Drain Waveform (Bad Tuning) 16

Figure15 Drain Waveform (Good Tuning) 16

Figure 16 Final Assembly 17

LIST OF TABLES

Table 1 Product Offering from EPC 7

Table 2 Common Component Data 8

Table 3 Digikey order #1 22 Table 4 Digikey order #2 23 Table 5 Digikey order #3 23 Table 6 PCB order from Advanced Circuits 23

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Introduction

The focus of this project is to investigate Gallium Nitride field effect

transistors (FETs), used in an E-class amplifier. The project was chosen in

collaboration with Advanced Energy to investigate this new technology for future

use in industrial power amplifiers. The team is lead by Dr. Fernando Tomasel a

member of technical staff at Advanced Energy. Team members also include Adam

Woodward a CSU senior, Dr. Don Van Zyl a member of Advanced Energy technical

staff, and Jose Medina an Advanced Energy tech specialist. The project is scheduled

to take two semesters and upon completion have a working power amplifier using

gallium nitride devices. The project will be managed and controlled by Adam

Woodward with Dr. Tomasel overseeing the group.

Gallium nitride devices are important to the future of electrical engineering.

They provide the benefit of lower gate charge and reduced series resistance of the

conducting channel, which results in increased efficiency and lower losses. The

ability to create GaN FETs and diodes has been around for twenty years. However,

for quite a few years the base substrate was very expensive compared to the cost of

silicon-based technology. New technology pioneered by Eudyna Corporation of

Japan in 2004 allows to grow gallium nitride on top of an existing silicon wafer as

seen in figure 1 (PhD). This technology reduces the cost of the gallium nitride

components and may lead to increased use and development of future parts. In June

of 2009 Efficient Power Conversion (EPC) released its first enhancement mode

eGaN on silicon (eGaN) and since then released several more components (PhD).

Figure 1. (Example of EPC eGaN FET) (Davis)

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The E-Class power amplifier was chosen to investigate this new technology because

of its common use in industrial Radio Frequency (RF) amplifiers from 3MHz to

10GHz (Sokal). The E-Class amplifier main components include; gate drive, FET,

voltage source, output load network and a load figure 2 (Sokal). The drain of the

FET has a large series inductor to a direct current (DC) voltage supply, and the gate

of the device is driven with a square wave. This gate drive puts the FET into

saturation because voltage gate to source (VGS) is greater than voltage source to

drain (VDS).

Figure 2. (Simple schematic of E-Class amplifier)(Sokal)

E-Class has several benefits; first, the drain voltage is close to zero during

conduction when the current is highest as seen in figure 3. Ohms law tells us that

power equals voltage times current, so when voltage is low the power is also low.

Second on the next half of the cycle the voltage is high when the current is zero

through the FET, this also calculates to a low power dissipation of the device.

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Figure 3. (Voltage and Current waveforms for the Drain of the FET)(Sokal)

The team has selected the EPC2012 eGaN FET from EPC based on power

output and size. The data sheet lists this component as 200 VDS and 3 amps drain-

to-source current maximum limits as seen in Table 1. Calculations show that this

device could generate about 60 watts of RF power (PhD. Gideon J.J. van Zyl). Since

the goal of the project is to generate 500 watts total, the power amplifier will

require 9-10 eGaN FETs operating individually and a RF combiner to sum the power

of all the stages.

Table 1. (EPC devices in production)(EPC)

There are three main benefits to use eGaN devices. The drain-to-source

resistance (Rds ON) is very low, and in most cases significantly lower than that of

comparable components as seen in figure 4. The eGaN FET also has significantly

lower gate charge than comparable semiconductor components figure 5. The eGaN

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devices are also much smaller in size. The semiconductor components are usually

SOIC-8 or DPAK which range from 31-70 mm2 for PCB layout. The eGaN device is

18.8 times smaller and has a surface area of 1.65 mm2 for its PCB layout. These

improved parameters allow for improved efficiency and smaller size over

equivalent semiconductor designs (PhD).

Table 2.

Semiconductor V DRAIN I DRAIN RDS Q GATE nC PACKAGE

VISHAY SI4490-DY 200 2.85 0.08 42 8-SOIC

FAIRCHILD FQD4N20TM 200 3 1.4 6.5 DPAK

FAIRCHILD FDS2670 200 3 0.13 43 8-SOIC

ROHM 2SK2887TL 200 3 0.9 8.5 DPAK

NXP PSMN165-200K 200 2.9 0.165 40 8-SOIC

FAIRCHILD FDS217ON3 200 3 0.128 36 8-SOIC

Figure 4.

(Davis)

0.08

1.4

0.13

0.9

0.165 0.128

0.1 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 2 4 6 8

RE

SIS

TA

NC

E D

RA

IN

TO

SO

UR

CE

Si and eGaN FET

RESISTANCE OF COMMON FET'S

SI4490-DY

FQD4N20TM

FDS2670

2SK2887TL

PSMN165-200K

FDS217ON3

2012

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Figure 5.

Significant time and energy has been devoted this past semester to the

development of an E-Class amplifier using eGaN FETs. The team has learned some

valuable lessons when working with new technology. The devices that were chosen

for initial test were no longer available once the gate drive schematic was complete.

This caused a significant redesign of the gate drive and change the input circuitry for

the amplifier. The team has learned from this lesson and has now purchased

enough components to build a prototype and extras if device failures occur.

The first semester consisted of investigating the gate drive design and

modeling the amplifier. To build models of the amplifier, the device parameters

have been modeled in an E-Class amplifier using the simulation software Non Linear

Five (NL5). After the tasks associated with creating a schematic and modeling the

amplifier were completed, the second semester focused on building and testing the

design. The activities for the second semester started with the definition of the

components schematic and printed circuit board (PCB) layouts, and continued with

the procurement of parts and construction of the power portion of the prototype.

This includes most of the components from the drain of the eGaN FET to the output

of the amplifier. These tasks involved the completion of the power amplifier and

included the impedance matching network from the eGaN FET to the harmonic filter

network and the output transmission to the load. Thermal characteristics have been

closely monitored and evaluated for operation in manufactures specifications.

42

6.5

43

8.5

40 36

1.8 0

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8

GA

TE

CH

AR

GE

nC

Si and eGaN FET

FET GATE CHARGE

SI4490-DY

FQD4N20TM

FDS2670

2SK2887TL

PSMN165-200K

FDS217ON3

2012

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Power measurement also was monitored as a requirement in the spring semester

and a voltage feedback loop will need to be designed to control the power delivered

to the load.

The completion of the prototype design and PCBs required high dedication.

The team completed hand loading the PCB boards for the gate drive and the power

board. Many of these steps required small surface-mount component soldering and

infrared soldering of the eGaN FETS. The team has also been very involved testing

the power amplifier for efficiency and operation.

Schematic

The schematic had to be transferred from NL5 to PCAD2004. This was

required to continue the project board layout, as the NL5 software we used for

modeling of the entire amplifier does not have a file format that we could transfer to

our schematic capture program PCAD2004. As we can see in (figure 6) the

amplifier was first modeled as a single stage. At this point we optimized tuning and

output power. Then we combined 4 FETs into a single combiner as shown in (figure

7) where this circuit was also optimized for the highest output power. The last step

we combined 16 single stages into one amplifier as shown in (figure 8). At this point

we created a net list that was required to continue forward to design a PCB.

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Figure 6, Single FET Schematic

Figure 7, Four FET Schematic

Figure 8, Sixteen FET Schematic

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PCB Layout

The schematic capture program PCAD2004 is also used to design the

PCB. We had to create a net list from the schematic and import it into the PCB file.

The components were all arranged to be as close as possible to each other while still

allowing enough space to solder and install. The inductor values had been modeled

earlier, and we hand-built them to determine the layout size. The board was a

designed to be two layers, using 1oz copper. We also used a copper pour method on

the bottom layer to tie all the ground connections together with lowest possible

inductance. The board size is 3.075” x 4.250” and cost $49.04 on a quick two day

turn from Advanced Circuits. This price could be reduced significantly with

increased quantity and longer lead times.

Figure 9, PCB Layout

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Building

The E-Class amplifier PCB layout was completed with PCAD-2004.

The layout took three weeks and many hours. The inductors were all hand-wound

using two different mandrels (1/2” and 7/16” diameter), and measured for the

correct value. Different number of turns and different inner diameters created the

appropriate values. In figure 6 we can see the test fixture used connected to a

network analyzer. The frequency of operation was programmed into the analyzer

along with calibrating it for open, short, and 50 ohm load. Then all inductors we

created and measured to be correct values.

Figure 10, Inductor Test Fixture

The entire Bill of Materials (BOM) was ordered through DigiKey.

Component values from the simulations were transferred to the PCB layout. Then

we selected the appropriate size and value from DigiKey to order. Each component

was then had loaded and soldered manually. The triple buffers and eGaN FETs were

soldered using infrared. To accomplish this, a small amount of solder was placed on

each pad with a light coating of flux. Then the board was preheated with the solder

unit until it reached 110 °C. At this point the infrared was turned on for about 20

seconds to reflow the solder and adhere to the PCB. In figure 11 we can see the

infrared station with its white preheating base and the infrared light housing

hanging above.

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Figure 11, Infrared Soldering

Initial Test

Initial tests were performed with a digital multi meter (DMM) looking for

continuity to all components. This procedure checks solder joints and PCB traces

for open connections. Then the body diode of the eGaN FETS was verified with the

DMM to validate the solder joint under the component. The second test was to

apply 5V DC to the active devices (figure 12). There are ten buffer components that

were powered up to make sure they do not draw large amounts of current. Then a

13.56MHz 50% duty cycle square wave was injected into the SMA input connector.

With an oscilloscope probe we checked all the outputs of the buffers to verify

operation. At this point we went through a process of finding the correct load

impedance to match the first stage of the amplifier. The initial analysis showed us

around 29.5 ohms, when we were able to get a nice E-Class waveform the

impedance turned out to be 8.0 ohms (figure 13). This is probably due to the non-

linear drain to source capacitor found on the eGaN FET.

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Figure 12, Initial Loading of Board and basic test

Figure 13, Finding best impedance match value

Tuning

Initially the step or turn on voltage of the device was too high, as shown by

the red trace which represents the drain of the eGaN FET (figure 14). It was

necessary to adjust the impedance matching to make it look like (figure 15). In this

oscilloscope image we can see that the component rings back to zero volts when the

FET turns back on. This limits the stored energy in the output that has to be

dissipated by the FET and the component will run much cooler. We saw the

component temperature drop by 30 °C by retuning the output. This part of the

project required a lot of assistance from the technical leadership at Advanced

Energy, they have developed and tested many amplifiers and had the tribal

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knowledge to improve this units performance. As shown in (figure 16) the amplifier

is completely assembled.

Figure 14, Drain Waveform (Bad Tuning)

Figure 15, Drain Waveform (Good Tuning)

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Figure 16, Final Assembly

Ethical/Environmental Concerns

The E-Class power amplifier was built with all lead-free components.

However the components were all soldered with lead solder. Lead solder melts at a

relatively lower temperature (188 °C) compared to lead-free solder which melts at

217 °C. Lead solder is easier to work with and, because the eGaN FETs and the

buffers both have hidden solder pads, it was much easier to hand load this PCB

using lead. However, lead is a toxic material that must be handled safely and be

properly disposed. This project will have to be properly disposed of and recycled.

Advanced Energy has a PCB recycling program that will be responsible for recycling

the PCB and lead soldered components.

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Manufacturability

The eGaN amplifier has been a challenge because we had to hand-load all the

components. If this project were to be continued, it would be loaded at a printed

circuit board assembly (PCBA) shop. The inductors would have to all be wound at a

magnetic manufacturing facility and sorted by part numbers. If the boards were all

pre-assembled, then only power up and tuning would be required from a test

technician. Due to the tolerance stack up of all the components a test procedure

would have to be developed to assist the technician in tuning the impedance match

inductor directly after the FET and the combining network inductors. Changes in

tuning are done by stretching the inductors to reduce inductance and collapsing the

inductor to increase the inductance value. Potentially if the design was stabilized

the correct values could be loaded with no tuning required.

Marketability

The eGaN FET amplifier has many possible applications. The cell phone

industry and the semi-conductor industry both have several uses for high power RF

amplifiers. The semi-conductor manufactures use many RF power supplies to

produce integrated circuits (ICs). Although from my experience most of their

processes use more than 500 watts of RF power, the amplifier developed in this

project is built very modular and can be expanded upon with additional stages to

increase the total power. Currently the total budget spend on this amplifier was

$743.78, including 10 PCBs and about two full units worth of parts. We estimate

about $400 for a complete 500 watt RF section. The industry usually says for RF

power it is $2 per Watt of RF, so we could charge around $1000 for this supply. Our

bill of materials cost is around 40% of our sales price, this is in line with many

amplifier manufactures.

Project Continuation

The Gallium Nitride E-Class amplifier project collaboration with Advanced

Energy will not be supported for continuation. However future students could

possibly use the premise of the E-Class amplifier design and start a new project. The

development and study of amplifiers is very important and takes a lot of study and

work to get it correct. The design of the RF filter stage and combiner stage are

complex and difficult to complete successfully. Voltage and current measurements

are challenging to get correct values in a noisy RF environment. For all of these

reasons I would suggest any amplifier project to a future student. It is a great way to

challenge a student to work with real world situations and issues of being a power

engineer.

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Conclusion This project has completed its goals of delivering RF power into a broadband

50 ohm load. We have overcome difficulties soldering the small components and

purchasing new components that are often unavailable. The tuning of all sixteen

power stages and the five combining networks took many hours of work. This

project is has been a success and lots of fun learning about new technology and

developing something brand new.

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Works Cited

Davis, Sam. Enhancement Mode Gallium Nitride MOSFET Delivers Impressive Performance. 1 December 2012 <http://powerelectronics.com/power_semiconductors/power_mosfets/enhancement-mode-gallium-nitride-032010/index.html>. PhD, Alex Lidow. Gallium Nitride Technology Overview. 1 December 2012 <http://epc-co.com/epc/documents/papers/Gallium%20Nitride%20GaN%20Technology%20Overview.pdf>. PhD. Gideon J.J. van Zyl. High Power Solid State Amplifier Design. Design Book. Advanced Energy. Fort Collins: PhD. Gideon J.J. van Zyl, 2008. Sokal, Nathan O. CLASS-E HIGH-EFFICIENCY RF/MICROWAVE POWER AMPLIFIERS: PRINCIPLES OF OPERATION, DESIGN PROCEDURES, AND EXPERIMENTAL VERIFICATION . 1 December 2012 <http://www.cs.berkeley.edu/~culler/AIIT/papers/radio/Sokal%20AACD5-poweramps.pdf>.

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APPENDIX A

Glossary DPAK Common name for surface mount package eGaN Enhancement Mode Gallium Nitride EPC Efficient Power Conversion FET Field Effect Transistor IC Integrated Circuit Ids Current Drain to Source PCB Printed Circuit Board Rds Resistance Drain to Source RF Radio Frequency SOIC-8 Surface mount 8 pin package Vds Voltage Drain to Source Vgs Voltage Gate to Source

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APPENDIX B

Budget The budget for this project was expected to be $550.00. The final budget turned out to be $ 743.78. There were a few reasons for this difference, the first is the eGaN FET was different and the quantity needed was increased. The design was behind schedule and for this reason we had to order the PCB on a two day turn for the unit to be built in time for EE days. $213.38 Components $490.40 PCB $40 Miscellaneous (solder, magnet wire, flux) Table 3, Digikey order #1

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Table 4, Digikey order #2

Table 5, Digikey order #3

Table 6, PCB order from Advanced Circuits

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APPENDIX C

Gallium Nitride Power Amplifier Time Line

2012 August

20-Aug

27-Aug Adam Work on project plan

Adam Research equipment available

Adam Investigate E-Class topology

September

3-Sep Adam Choose a method of circuit modeling (NL5)

10-Sep

Adam Don Research Gallium Nitride Components

17-Sep

Adam Don Research Gallium Nitride Components

24-Sep Adam Order and receive components

October

1-Oct

Adam Jose Acquire part numbers and PCB component layout design

8-Oct Adam Test individual component input and output capacitance

15-Oct Adam Use variable frequency drive to see maximum operating point

22-Oct Adam Layout PCB for gate drive

29-Oct Adam Design for power board (RF Filter)

November

5-Nov

Adam Jose Layout the PCB for power board

12-Nov

Adam Don Work on report and build RF Filter

19-Nov

Thanksgiving Break

27-Nov Adam Work on oral presentation

Adam Work on report

December

3-Dec Adam Work on oral presentation

Adam Final revision of report

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Don

10-Dec Finals

Christmas Break

2013 January

21-Jan

Adam Jose Finish Layout for PCB power board

28-Jan Adam Order both driver and power board

February

4-Feb

Adam Jose Order components for project

11-Feb

Adam Jose Solder Components

18-Feb

Adam Jose Solder Components

25-Feb

Adam Don Initial power up and testing

March

4-Mar

Adam Don Deliver power and collect data on amplifier

11-Mar

Adam Don Deliver power and collect data on amplifier

18-Mar Adam Test thermal cooling requirements

25-Mar Adam Work on E-Days display

April

1-Apr Adam Work on E-Days display

8-Apr

Adam Jose Collect all equipment to demonstrate project

15-Apr Adam Present E-Days

22-Apr

29-Apr

May

6-May

13-May Finals

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Acknowledgment

I would like to thank my advisor Dr. Fernando Tomasel for his oversight and help this semester. I would also like to thank my team members Dr. Don Van Zyl and Jose Medina for their help and knowledge this semester, it would not have been possible without a great team. I am also truly appreciative for Advanced Energy’s contributions and the ability to work together on new technology.