Download Final Report - University of Victoria

University of Victoria
Department of Electrical and Computer Engineering
CENG/ELEC/SENG 499
JANUARY – APRIL 2011
Final Report
Project #7:
Project Title: AB3 – DC to DC Convertor for Charging Electric Vehicle Batteries
Date: April 1, 2011
Submitted To: Dr. Ashoka Bhat
Names:
Samuel Abubakker V00159423
Mike Engleman Germain V00705416
Tenli Gregory V00482224
Mohammed Tayel V00183335
Serge Vincent V004821286
Dr. Ashoka Bhat
Professor
Electrical and Computer Engineering
University of Victoria
P.O. Box 1700
Victoria, B.C.
V8W 2Y2
April 1st, 2011
Dear Dr. Bhat,
Please accept the accompanying report entitled “DC-to-DC LCL type series Resonant
Converter for Charging Electric Vehicle batteries” submitted as a required component of
the ELEC/SENG 499 course from January to April 2011.
This report is based on the completion of the design project where the development of a
fast and efficient method of electric vehicle battery charging using an LCL-type resonant
converter configuration was built. We were engaged in various steps of creating a
functional and professional project. This report will provide details of how the project
was accomplished.
This course provided an opportunity to apply theoretical and technical knowledge while
developing communication and technical skills during the completion of a practical
engineered product.
We would like to thank you for your proposal which served as the basis of our project
and for your patience as well as your commitment in supervising us.
Sincerely,
Mohammed Tayel, Samuel Abu Bakker, Tenli Gregory, Serge Vincent, Mike Germain
Table of Contents
Executive Summary ...................................................................................................................... 2
Introduction................................................................................................................................... 3
Discussion ...................................................................................................................................... 4
Voltage Supply .......................................................................................................................... 4
Rectifiers .................................................................................................................................... 4
LCL Type Series Resonant Converter.................................................................................... 5
.................................................................................................................................................... 6
Transformers............................................................................................................................. 7
Switching.................................................................................................................................... 8
Microcontrollers........................................................................................................................ 9
Oscilloscope ............................................................................................................................... 9
Basic Operation - Theoretical Design ....................................................................................... 10
Final Product............................................................................................................................... 12
Control ..................................................................................................................................... 13
Gating Signal ........................................................................................................................... 13
Signal Amplification ............................................................................................................... 14
MOSFET Switching................................................................................................................ 14
Filter ......................................................................................................................................... 14
Transformer ............................................................................................................................ 15
Rectification............................................................................................................................. 15
Load.......................................................................................................................................... 15
Testing.......................................................................................................................................... 16
Recommendations and future Enhancements.......................................................................... 18
Market Analysis .......................................................................................................................... 18
Conclusion ................................................................................................................................... 19
Appendix...................................................................................................................................... 20
References.................................................................................................................................... 27
Executive Summary
The outcome of the project is to develop a fast and efficient method of electric vehicle
battery charging using a series LCL-type resonant converter configuration. To satisfy safety
requirements, the external charging circuit will be magnetically coupled to the vehicle using a
clamping type high frequency transformer. For ease of use, the entire system will be monitored
by a microcontroller, which will control the power output of the charger and monitor the state of
the batteries to prevent any dangerous operations of the circuit. The report discusses the major
components of the device in separate sections as well as addressing the device as a complete and
functional unit.
A modified LCL-type series resonant converter using an inductor parallel to a
transformer’s primary winding was built. The demonstrative model of the device utilizes a 100
VDC power supply based input, which is a scaled down version of the ~300 VDC one would
obtain by rectifying 240 VAC household power outlet. Upon pulse width modulation performed
by an electrical switching circuit composed of a solid state H-bridge built with discrete
components and a clock signal, inductive and capacitive components filtered and defined the
shape of the intermediate signal. The current waveforms were then directed to transformer,
which respectively increased and decreased the peak-to-peak current and voltage from 2 A and
100 V to 4 A and 48 V (for full –load conditions) such that a final full-wave bridge rectifier and
filtering capacitor would convert it into a DC output for the purpose of charging a battery. For
future refinements, an Arduino 16-bit microcontroller will be incorporated to accordingly replace
the manual potentiometer adjustments that vary the power output and hence change the phase
shift between square wave signals of the switching circuit.
2
Introduction
In the coming years, electric vehicles are predicted to dominate large segments of the
automotive industry. The transition from internal combustion to electric motors will necessitate a
shift from distribution of gasoline to that of electricity to automobiles. Presently, there are no
widely accepted standards for electric vehicle charging techniques and many different methods
are in use. The ideal electric vehicle charging system will incorporate the benefits of maximized
safety, efficiency, speed, and ease of use.
Our project is aimed to develop a fast and efficient method of electric vehicle battery
charging using a series LCL-type resonant converter configuration. To satisfy safety
requirements, the external charging circuit will be magnetically coupled to the vehicle using a
clamping type high frequency transformer. For ease of use, the entire system will be monitored
by a microcontroller, which will control the power output of the charger and monitor the state of
the batteries to prevent any dangerous operations of the circuit.
The scope of the project is to design and implement the LCL resonant converter as well
as the high frequency coupling mechanism. We will start by employing a 100 VDC power supply,
which is a scaled down version of the ~300 VDC one would obtain by rectifying 240 VAC
household power outlet.
The report discusses the major components of the device in separate sections as well as
addressing the device as a complete and functional unit.
3
Discussion
Voltage Supply
A power supply is a device that supplies electrical energy to one or more electric loads.
In this project, a regulated power supply was used; a regulated power supply controls the output
voltage or current to specified values. In addition, the controller value will be held constant
despite variations in load current or voltage supplied from power supply’s energy source.[1]
An Alternating Current (AC) powered supply will use a transformer to convert the
voltage from wall outlets to lower voltages used in the circuits. If it is used to produce a Direct
Current supply, a rectifier is used to convert the alternating voltage to pulsating direct voltage.
To provide power for the components of the circuit we will be employing a 100VDC
power supply, which will be a scaled down version of the ~300 VDC one would obtain by
rectifying 240 VAC household power outlet.
Rectifiers
A rectifier is an electrical device that converts alternating current to direct current.
Rectification has various uses which include power supplies and detectors of radio signals.
Rectifiers use a combination of diodes in a specific arrangement for efficiently, converting AC to
DC. A diode can also be used to rectify AC by cancelling negative or the positive parts of a
waveform[2].
In the circuit, the rectifier that was used was known as a ‘Full Wave Bridge Rectifier’ the
schematic can be seen below.
4
Figure 1: Schematic for a Full Wave Bridge Rectifier
A full wave bridge rectifier is a single phase rectifier that uses fours diodes connected in
a closed loop (bridge) configuration. The advantage of this configuration is that it will not
require a centre tapped transformer, this in turn will reduce size and cost. The secondary winding
is connected to one side of the diode bridge network and the load was connected to the other side
[2].
LCL Type Series Resonant Converter
There are two types of high frequency resonant convertors; series resonant and parallel
resonant. While a series resonant convertor has a problem on voltage regulation, parallel
resonant convertors have lower efficiency due to reduced circulating currents. The main
advantages of resonant convertor operating in the above resonance(lagging power factor) is that
the circuit will not require lossy snubbers and di/dt limiting inductors[3]. A dc/dc high-frequency
link LCL-type series resonant converter suitable for operation above resonance. Below, the halfbridge version is shown.
5
Figure 2: Circuit Schematic of the Theoretical LCL-Type Series Resonant Converter [1].
The LCL resonant converter will create a smooth sinusoidal wave from the choppy
square wave output from the switching circuit. The “LCL” term refers to an arrangement of
electrical inductors and capacitors (wire coils and charged plates) which filter and define the
shape of the signal. The actual operation of the convertor can be seen in the wave form shown
below [3].
Figure 3: Waveforms at different points in the LCL-type series resonant converter of Fig. 2 for operation above
resonance.
6
Transformers
A transformer is an electrical device that uses mutual induction to transfer electrical
energy from one circuit to another. That is, when current flows through the primary windings of
the transformer, a magnetic flux is induced in the core of the transformer. This magnetic flux is
transferred through the core thereby creating a magnetic field in the secondary windings of the
transformer. Due to the resulting induced electromotive force (EMF), current will flow through
the secondary windings when connected to a load. In the ideal case, the relation of turn ratio to
voltage is given as follows:
However, in actuality there are losses associated with the alternating magnetic field which cause
fluctuation forces, or vibrations, between the primary and secondary windings. Transformers
come in a wide range of sizes. A high frequency transformer allows for a physically compact
design with fewer needed turns but becomes less efficient [4] [5].
7
Switching
An electrical switching circuit, composed of a solid state H-bridge built with discrete
components and a clock signal, can be used to convert a dc signal into an alternating square wave
signal. The solid state H-bridge is composed of 4 N-channel MOSFETs in an H-configuration as
shown below [6].
Figure 4: H-bridge Configuration
The operation works as follows:
Figure 5: H-bridge operation
8
Using a clock signal to drive the gate voltage of the FETs in conjunction with an isolated singleended resonant converter we can use a single frequency to control variable loads with minimal
switching losses [7].
Microcontrollers
Microcontrollers are small computers that are integrated on a single circuit containing a
processor core, memory and programmable input/output peripherals. The majority of
microcontrollers in use today are embedded in other machinery such as automobiles, telephones,
appliances and peripherals for computer systems. Microcontrollers usually have the following
features; a central processing unit, volatile memory for data storage, inputs/outputs, timers and a
clock generator.
The Arduino platform was chosen primarily due to its low startup cost and extensive
support. Also, due to the popularity and open-source nature of this platform the hardware itself is
readily available in a huge number of configurations. The Arduino platform will be discussed in
detail more on its use and how it applied to the project in the basic operation section.
Oscilloscope
Oscilloscopes are commonly used to observe the exact wave shape of an electrical signal.
In addition, oscilloscopes can show distortion and the time between two events. Originally all
oscilloscopes used cathode ray tubes as their way of displaying elements and linear amplifiers.
The modern oscilloscopes have LCD or LED screens and fast analog-to-digital converters. The
oscilloscope that was used was a 4 channel oscilloscope. The oscilloscope usage in the project is
going to be discussed in more detail in the basic operation section.
9
Basic Operation - Theoretical Design
The project device scheme, which encompasses a series resonant converter accompanied
by an inductor in parallel with the primary winding of the transformer (hence defined as a “LCLType Series Resonant Converter”), exhibits many favourable traits. The above resonance mode
operation of the inverter precludes the need for limiting inductors and lossy snubbers with the
exception of capacitive snubbers across the switch, thus also allowing the use of the internal
diodes of MOSFETs knowing there is sufficient time for them to recover. Given a pre-rectified
input signal of 100 V, pulse-width modulation would be carried out by the MOSFET H-bridge
employing current mode control methodologies (achieved through an Arduino Microcontroller
governed Phase Shift Resonant Controller, who’s data sheet is provided in the Appendix), as the
LCL-type tank circuit would grant significantly high efficiency for small switching frequency
variations in regulating output voltages from near open-circuit to full load conditions. More
specifically it is important to note that the phase shift between square waves generated by the
MOSFETs (which, to reiterate, varies the output current based on the degree of destructive
interference resulting from superimposing signals) did not fall below a margin as well, for it was
necessary to ensure that the lagging power factor was maintained.
A number of assumptions tied to previous literature [3] were made, leading to the
derivation of conservative transformer characteristics as well as inductance and capacitance
specifications. As for the former, a presumed converter gain M = 0.99 along with a chosen
output voltage Vo = 48 V, minimum input supply voltage Vs (min) = 100 V, and output power Po =
200 W lead to the computation of the transformer turn ratio n ≈ 2, realized through choosing a
high triggering or switching frequency fs = 100 kHz at full load and adding 10 turns to the
primary and 5 to the secondary coil. To assure that the peak current at full and light loads would
remain small, a quality factor Qs of 1 was selected. Formulas relating the series inductance Ls,
series capacitance Cs, load resistance RL’, and aforementioned data can be expressed as follows:
RL’ = n2
= (2)2
= 46.08 Ω
Studying the fundamental components of the waveform whilst neglecting harmonics
(equivalently disregarding Fourier series analysis),
10
Qs = 1 =
& ωs = 2π fs =
Cs =
Ls =
=
=
= 34.54 nF
= 73.34 µH
Subsequent measurements for the short-circuit and open-circuit tests of the transformer
were then conducted, as the effective parallel inductance LP = Ls / 0.2 = 366.7 µH (where the
inductor ratio was extracted from known design curves [3]), post-∆-Y transformation inductance
L1 = 12.80 µH, and magnetizing inductance Lm = 749.5 µH were then used to calculate the
parameters:
Ls = Lr + L1
&
LP =
Llp << Lt < Lm)
Lr = Ls – L1
Lt ≈
= (74.34 µH) – (12.80 µH)
= 61.54 µH
≈
≈ 718.0 µH
11
≈
(where
Fig 6: Schematic for the equivalent circuit found at the output of the switching circuit
(preceding terminals A and B) with the inclusion of the HF transformer based
effects and parallel inductor (a) prior to a ∆-Y transformation and (b) after a ∆-Y
transformation [3].
A full-wave bridge rectifier and filter capacitor, the latter of which acted as a low-pass
filter applied to the current signal exiting the secondary coil of the transformer, were then
installed to produce an expected constant voltage and current signal of 48 V and 4 A,
respectively.
Final Product
Our completed LCL-type series resonant DC-DC converter consists of four circuit
boards, a transformer, and a filter which includes two torrid inductors and a small capacitor. The
entire circuit was secured to a long piece of wood for demonstration purposes. The clamping
transformer was clamped through a rectangular hole in a small wooden “wall” to simulate the
boundary between the car and the charger. The setup consists of the stages outlined and
described here:
1. MOSFET gating signal (100kHz) creation using the PCB
2. Gating signal amplification using the PCB
3. MOSFET switching using the fabricated H-Bridge switching board
4. Filtering using the custom designed LCL configuration
5. Voltage step-down using 2:1 high frequency transformer
12
6. Full bridge rectification
7. Load
Figure 7: The completed circuit
Control
Our design originally included an Arduino 16-bit microcontroller to control the power
output of the circuit. Due to lack of time, this portion was not successfully implemented and is
discussed further in the recommendations section of the report.
Gating Signal
A pre-made circuit board, the PCB, was used for creation of the gating signals. The
frequency output was adjusted to just over 100kHz and a potentiometer was added for better
control of the internal voltage ramp. This was necessary to achieve a full square wave with the
existing gain of the circuit. The oscilloscope screen here shows the four gating signals, under the
minimum phase offset conditions.
13
Figure 8: Gating Signals
Signal Amplification
Four signal lines and a common line from the gating signal board are inputs for the signal
amplification board, the Fixed Frequency Phase Shift Controller Board. The gating signals are
optically isolated, and amplified to around 15V, with enough power to switch the power
MOSFETS on the next board.
MOSFET Switching
Four power MOSFETS are used, which are IRF3315; in an H-Bridge configuration, to
create a 100Vpp square wave from a supplied 100VDC power input. The MOSFETS are capable
of conducting 27A [8], but need some improved heat dissipation for high load use. Our design
included screw-on type heat sinks. Under testing, at full load (200W) for around one minute, the
MOSFETS were estimated to have raised to approximately 45 degrees centigrade. Snubber
capacitors are incorporated into the switching circuit to dissipate voltage spikes.
Filter
The calculations, described in detail in the theory section of the report, determined the
proper LCL values for the filter circuit. For Lr, we wound an inductor having a value of 60 µH
and placed this in series with a high voltage capacitor of 33 nF. In parallel with the primary side
of the transformer, a torroidal inductor, wound with a value of 711 µH, was placed. The resulting
current waveform, with the entire circuit under load, was extremely close to a true sinusoidal.
Under testing at full load the series inductor was estimated to have raised to approximately 35
degrees centigrade, which shows that our choice of wire gauge, 18, was acceptable. The
oscilloscope screen here shows the smoothed current (green).
14
Figure 9: Voltage and Current
Transformer
The high-frequency clamping-type transformer was wound with a 10-turn primary coil
and a 5-turn secondary coil for a 2:1 voltage ratio. This low number of turns was acceptable due
to the high frequency used. Due to the thicker wire used for the transformer coils, neither
transformer, nor its coils warmed to any noticeable levels during full load operation.
Rectification
A high voltage full bridge rectifier was constructed using U1510. The resulting DC
waveform was observed to be very smooth at full load. The diodes were barely warm to the
touch under full load, but were also fitted with screw-on heat sinks as a precaution.
Load
A power resistor bank was used to draw over 100W from our circuit. For demonstration
purposes, a “40W 120V” incandescent light bulb was installed parallel to the resistive load. The
light bulb’s resistance of 360 ohms added only about 7W to the total load. For testing purposes,
the light bulb was removed from the circuit.
15
Testing
Our circuit was tested to validate its operation as per our design specifications. We tested
the efficiency of our circuit under several different loads.
The following table shows the results of these tests:
Power (W)
Input
Voltage
(V)
Output
Efficiency
Input
Current(
A)
Output
Input
Output
0
0
100.0%
96.5
55
0
0
191
172
90.1%
96.5
44.7
1.92
3.8
250
200
80.0%
96.5
44.6
2.6
4.38
375
331.6
88.4%
96.5
43.6
3.87
7.5
Table 1: Input/Output Measurements
16
Figure 10: Efficiency plotted against output power
Figure 11: Output voltage plotted against output power
As can be seen in the above figure, the efficiency is fairly steady through different power
outputs. A slight dip in efficiency is seen at the 200W point. The efficiency averages about 85%,
17
which means the 15% of the input power is being dissipated as heat from the components. Some
losses can also be attributed to leakage inductance of the transformer, especially since we are
using a clamping type.
The circuit performs very well when the load is switched off. No measurable current is
seen at the input with no load at the output. The output voltage under load can be seen to drop
from 55V at no load to about 45V at full load.
Recommendations and future Enhancements
Future designs for the LCL-type series resonant DC-DC converter could be improved
mostly by means of ergonomics. Incorporating the first two signal circuit boards into one, with
an on-board embedded system would be ideal. The embedded system would control an 8 or 10bit digital potentiometer which is connected to the gating signal chip, controlling the phase shift
and, thus, the pulse width modulation of the switching. A second circuit board would be
dedicated to the power circuit with the switching MOSFETS and LCL filter, as well as an initial
rectifier for rectifying the 60Hz input power. The two circuit boards would be encased in a
water-proof case, which could plug into a 240VAC socket with a heavy duty cable. A second
heavy duty cable would furnish the primary side of the clamping transformer, safely encased and
slotted to mate with the secondary side, attached to the car or battery bank. The transformer
halves would plug together, completing the magnetic circuit.
Market Analysis
Our LCL-type series resonant DC-DC converter, as it is now cost approximately $236 to
build. This figure includes materials only and does not take into account any labour or overhead
such as tools. To design and build a case for the device, with appropriate heavy-duty cables,
would cost, we estimate, approximately $10,000 with large scale production bringing the perunit cost for casings down to about $18. The cost of the electronics, similarly, would decrease
with mass production. The complete unit, therefore could be expected to cost around $100 given
a run of several thousand devices. With a small retailer make-up, the device would be well
within the budget of any electric car user.
18
Conclusion
In conclusion the outcome of the project was to develop a fast, safe and efficient method
of electric car battery charging using the series LCL-type resonant converter. The project was
completed in timely and orderly fashion. Currently vehicles are predicted to dominate large
segments of the automotive industry and as of now there are no widely accepted standards of
electric charging and the LCL-type series resonant DC-DC converter would be a good start in a
standard battery charging. The project is in functional state but enhancements and upgrades are
needed.
As stated in the recommendations and future enhancements, improvements including
incorporating the first two signal circuit boards into one with an on-board embedded system
would be basic upgrade. Another enhancement that could be added is to use a microcontroller
like the Arduino which would control the power output of the charger and monitor the state of
the batteries to prevent any dangerous operations in the circuit. In addition, a water-proof casing
could be used to have the two circuit boards encased, also a heavy duty cable could be plugged
into a 240VAC socket.
Cost of the project was reasonable and cost around $236, although labour and overhead
charges were not taken into account. In addition, if mass production was taken into consideration
the cost of product would be subsidised. The cost of the device would be well within the budget
of any electric car user.
Overall this project was a success and there is a positive marketing potential thanks to its
efficient design solutions.
19
Appendix
¸ 20
21
22
23
24
Table 1: Bill of Materials [5]
Estimated
# of
Total Cost of
Cost/part
parts
Parts
$30
1
$30
MOSFET - IRF3315
$1.67
4
$6.68
High Voltage Caps
$5
2
$10.00
MOSFET Driver Board
$20
1
$20
Populated Parts
$15
1
$15
$7
1
$7.00
$10
1
$10
Part Description
H-bridge MOSFET
Board
Fixed Frequency Phase
Shift Controller Board
Populated Parts
25
Arduino Microcontroller
$50
1
$50.00
Inductor Core - Lr
$11.38
1
$11
Inductor Core - Lm
$4.46
1
$4.46
$5
1
$5
1
$50
MKP10 33nF, 250V
Cap
HF Clamping
Transformer
$50
Rectifier Board
$5
1
$5
Diode - U1510
$1
4
$4
470 uF cap
$2
1
$2
20 uF cap
$6
1
$6
Total
$236
26
References
[1] Miller, Rex. Electronics The Easy Way, 4th ed. Barron's Educational Series, 2002.
[2] Storr, Wayne, “Electronics Tutorials.ws”,< http://www.electronicstutorials.ws/diode/diode_6.html>, march 2011.
[3] A. K. Bhat, “Analysis and Design of LCL-Type Series Resonant Converter,” IEEE Trans.
Ind. Electron, Vol. 41, No. 1, pp. 118-124, 1994.
[4] Heathcote, Martin (November 3, 1998). J & P Transformer Book, Twelfth edition. Newnes.
pp. 2–3. ISBN 0750611588
[5] Flanagan, William M. (January 1, 1993). Handbook of Transformer Design and Applications.
McGraw-Hill Professional. Chap. 1, p. 1–2. ISBN 0070212910
[6] McManis, Chuck, “H-Bridges: Theory and Practice”,
<http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/>, Dec. 23, 2006
[7] Bhat, A.K.S., and S.B. Dewan, “A generalized approach for the steady-state analysis of
resonant inverters”, IEEE Transactions on Industry Application, vol. 35, no. 2, pp. 326-338,
March/April 1989.
[8] International Rectifier, “IRF3315 HEXFET© Power MOSFET”, <http://www.irf.com>,
12/1998
27