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Compact Stand-Alone-Power-Source (SAPS) for Wireless Applications

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Title:
Compact Stand-Alone-Power-Source (SAPS) for Wireless Applications
Creator:
Olander, Gabriel
Ngo, Khai ( Mentor )
Place of Publication:
Gainesville, Fla.
Publisher:
University of Florida
Publication Date:
Language:
English

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serial ( sobekcm )

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University of Florida
Holding Location:
University of Florida
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All applicable rights reserved by the source institution and holding location.

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Compact Stand-Alone-Power-Source (SAPS) for Wireless Applications

Gabriel Olander


ABSTRACT


A Stand-Alone-Power-Source (SAPS) will be developed to convert electrical energy drawn from a piezoelectric

(PZT) source to a usable form. This form will be miniaturizable so it can be used in wireless applications. A

PZT source that outputs a sinusoidal wave at low frequencies (about 100 hertz) will be converted to a direct

current (DC) in order to charge a battery. The firing angle of circuit controller will be variable. This circuit should

have an efficiency of 50-80% depending on conditions.



INTRODUCTION


With the continued expansion in the realm of miniaturized electronic circuitry, the demand for cheap

autonomous power sources has become an ever important issue. By transformation of ambient energy

sources, wireless miniature electronics can be made without the limitation of single charge batteries.

Piezoelectric materials (PZTs) are currently being researched for their ability to convert mechanical vibrational

energy into electrical power. Since the functional characteristics of PZTs vary greatly with frequency, load, and

many other operating conditions [1], their use requires power conversion circuitry. By developing a control

system that determines when to switch on a battery charging system, high efficiency power harvesting can

be achieved.



The objective of this research is to develop a power source that converts a PZT alternating current (AC) input to

a usable direct current (DC) output. First, a prototype circuit will be developed and simulated. After

successful simulation, a working model will be built and tested in a lab with a PZT simulation circuit, and then

tested with a PZT. The control system used is designed to optimize circuit efficiency while maintaining an ability to

be miniaturized. Without the need for inductors or large capacitors, this circuit system can readily be fabricated on

an IC chip. This realization enables use in applications such as wireless sensors, transmitters, or radio

frequency identification (RFIDs).



STATE OF THE ART


In order to bring ambient energy supplies to circuitry, PZT based energy harvesting has been researched over





the past decade. This research has broadly covered ideas for harvested energy, with tidal flow [2] and human

motion [3] as studied examples. Harvested power levels have ranged from tens of mW to many watts - opening

the door for various applications.



Many wireless sensors and transmitters require relatively small power levels (mW). Ideally, all circuitry of the

SAPS would be designed to enable manufacturing on an integrated chip. In order to cheaply miniaturize a

circuit, inductors or large capacitors must be avoided. Current research has shown some success in

developing circuits that are efficient power converters or able to be miniaturized but little success has been

achieved in both.



The Energy Harvesting Eel [2] explored the possibility of transferring the mechanical energy of water flow (by

means of alternating vortices behind a bluff body) into electrical power. The Eel's design calls for an inductance of

ten Henry's and the use of multiple PZT elements. This will greatly limit the ability of the circuit to be

miniaturized. For these reasons, this technology would be unlikely to be of use for wireless applications (even if

the Eel could conceivably be redesigned to harness airflow). Having successfully tested lower power versions, a

full-scale one watt system that can be strung in series is currently being developed. Such a system would set a

new precedent for PZT energy harvesting.



A very different approach was taken in this optimized PZT Energy Harvesting circuit [4]. Here, a PZT input is

rectified and fed into a step down converter using discontinuous conduction via pulsed width modulation (PWM) of

a switch. To maximize input power over a large range, two separate circuit control systems are used at different

input power levels. This circuit was built upon maximization of input power and did achieve a good efficiency

at higher voltage levels (> 30V) [4]. Unfortunately, this circuit also requires an inductor, limiting its ability to

be miniaturized. Also, the circuit is considerably large and more complicated than the SAPS, making the

device considerably more expensive.



Finally, the last example is a PZT circuit developed for IC wireless RF transmission [5]. This relatively simple

circuit employs no inductors or large capacitors. The prototype is about the size of a quarter. It has

successfully transmitted a signal at 1.9 GHz while being powered by a PZT shaken at 60 Hz [5]. While a

promising candidate for many wireless applications, this research also suffers from several problems. First, the

circuit was tested without a battery. All energy was stored on a small capacitor (3.3 microfarads which was

only charged to 1.2 V). This capacitor stores very small amounts of energy and properly functions only at

high frequencies. This greatly limits the range of applications the circuit could power. Also, the harvested energy

is about .175 mW at peak, which is also too small to be practical for many applications. The efficiency is, at

best, 49.9% [5].



In these examples, we have seen circuitry employ medium to high harvested energy levels ([2] and [4]) and

circuits that can be miniaturized with very low power levels [5]. There is little success in the development of

circuitry that can harvest power in the milliwatt range while also demonstrating high efficiency and an ability to






be miniaturized.


PZT MODELING



The qpl5n QuickPack PZT actuator was chosen to model because of its low input impedance and high levels of

power available for extraction [6].



In order to use a model with a high degree of accuracy, the 100 nF input capacitance of the qpl5n was used to

scale a resonant frequency model of for the PZT [7]. This model (fig. 1) consists of a sinusoidal current source

in parallel with a large resistance and a capacitance.





PZT model


AC
curmnt
NWRCB


Figure 1. PZT Resonant Model For QP15N.



In order to calculate the conjugate match and optimal resistive match load ratios, the power extracted over

these loads need be determined by simulation. To do this, the Thevenin's Series Equivalent Circuit for each

frequency was calculated and simulated.



SYSTEM FUNCTIONING



Given that there are multiple means to realize the power conversion system, selection criteria must be determined

in order to select between given systems. Below are three main systems considered with a synopsis of the

relative performance.



Rectifier


The Rectifier Circuit is the simplest power conversion system considered. Input current is fed through a diode

rectifier and placed directly onto a battery without any specific control.






RECTIFIER


PZT INPUT


I ----I~:


.Cfk D59 -jLObreak


RECTIIER



BATTERY



| 3-2 i

C -i
T


Figure 2. Rectifier System.




The labeled curves (fig. 3) demonstrate functioning and show output voltage and current achieved when using a

1mA input current at 100 Hz. The top two curves show the input voltage and current, labeled input2 (fig. 3), and

the bottom two curves show the corresponding battery voltage and current, labeled r batt2 (fig. 3).


n
P
uI
t ri.^
C
u
OW, r M
r

t


-a -


U 3.2SI





#,: 'a


SEL 0
*�.5" - 3. 1W .. .... . . ... ... .
ta-rWt) T5.* .V(r�_Utt 2)


Figure 3. Rectifier Function with 1mA Input Current at 100 Hz and a 90K Load.




MOSFET Current Source


The MOSFET current source will be used by placing a MOSFET in series with the battery. The MOSFET will be

driven by a separate voltage source which will be swept to achieve optimal circuit performance.


~ 1


\
\


^,....,


\
\





MOS CURRENT SOURCE
077 D� 4 Mlt** e pZT ITTM I Dbr1 5 4



rm 2 R, *7



I I






Figure 4. MOSFET Current Source System.




The voltage source (fig. 4) labeled "test" need be swept in order for an optimal level to be found. This level will

leave the MOSFET in saturation mode and output a steady current when the input voltage is at a high enough

level. Figure 5 shows that sweeping Vgs places the MOSFET into saturation mode, forcing it to conduct as a

current source into the battery. The top set of curves show a set of positive Vds values while the bottom set of

values show a positive set of Vgs- values.











eft1


- ITitV


SEL~ 5


Tift


T~o "we'~


Figure 5. MOSFET Current Source Function with 1mA Input Current at 50

Swept From 4 to 10 V.


Hz and a 90K Load. Vgs is


MOSFET Switch


The MOSFET switch gives direct control of when to connect and disconnect the circuit. This switch will be used by

a MOSFET in series with the battery that is powered as a switch. The gate voltage will alternate between high

and low, effectively setting the MOSFET to "short" and "open."


40"
















r-OdI


D2 Dba� D3i ~ W ~ *
0. J


Figure 6. MOSFET Switch System.


This switch will allow two degrees of freedom, "turn on" and "turn off" (sometimes referred to as "firing

angle"). "Turn on" will designate at what input voltage level the MOSFET will be made a short. "Turn off"

will designate at what input voltage level the shorted MOSFET will be made "open."




For simplicity sake, turn on and turn off will be set by setting time delay (phase) and pulse width (duty ratio) of

a gate driver. Time delay (fig. 7) is set to 2.25ms and pulse width is set to 2ms. The top two curves show the

input voltage and gate signal, which is when the circuit is switched on. Notice that the input voltage drops to

battery voltage while the circuit is switched on, and approaches the open circuit voltage level while the circuit

is switched off. The two bottom curves show battery voltage and current.


3


V(PI 2, RI V 1 IJ 'NM43


t 3 4U t




t ,


- l ~r bt l�) f 1 0 l s


Figure 7. MOSFET Switch Function with 1mA input current at 100 Hz and a 90K Load.


I -


e�. 3�.. 3�. 4�. st







System Performances


Since all three systems discussed above have been shown to successfully transform a PZT simulated AC input

voltage to a DC battery voltage, their relative performances will be evaluated in terms of total power

extracted, efficiency, conjugate load ratio, and optimal resistive load ratio.

Performance measurement definitions are as follows:



1. Power extracted (in or out): The instant value of P(t) = V(t)in or out * Iin or out (1)

2. Joules J= (2)

3. Complex Conjugate Match Ratio is the power extracted over the battery in one period as in equation (1) divided

by the power extracted over the optimal load given the same input conditions. Optimal Load is defined as

the complex conjugate of the input impedance:

Zoptimal load = Rin - j*Xin (3)



Rin is the input resistance of the PZT source. Xin- is the input reactance of the PZT source.



Selection between these three systems will be made by determining which system has the maximum potential

power harvesting ability. Each system's given degrees of freedom will be swept in order to determine if one

individual system has significantly higher power harvesting capability will performing optimally.



Rectifier Circuit - Conjugate Match Load and Energy Harvested


Since there are no degrees of freedom in such a setup, the only parameters to be swept are input magnitude

and frequency. Input magnitude was swept from .5mA up to 10mA. Frequency was swept from 50 Hz up to 400

Hz. The results of extracted energy and conjugate load ratio are shown below in Figures 8 and 9 respectively.



Increasing frequency has a slight lowering effect on joules harvested (fig. 8), though the effect is subtle.

Increasing input current (fig. 8) shows increases joules harvested proportionately.





Joules per Second vs. Frequency for
Standard Rectifier System


IIU,-u~


0.0.

o i-

0
-O.OO,


-.-,5mA
I----1mA
2mA
5mA
I -l0M1


frequency (Hz)


Figure 8. Joules per Second vs. Frequency for a Rectifier Circuit with a Battery Load of 90K.



Joules per Second vs. Input Current
For Standard Rectifier System


0,025
0.02-

0.01 -

000 . -
0,
-0 00i : O.O- 0.01i 0 o1


l - 50 Hz
-- 100 HI
200 HI
400 H:


Input Current (A)


Figure 9. Joules per Second vs. Frequency for a Rectifier Circuit with a Load of 90K.



Increasing frequency drops the conjugate load ratio linearly (fig. 10). There appears to be a cutoff frequency for

each separate input magnitude, above which the circuit will not function and no power is extracted. For

increasing input current (fig. 11) there appears to be a turn-on level, below which the circuit will not function.

This turn-on level is lower for lower operating frequencies.




Conjugate Load Power vs. Frequency for
Standard Rectifier System


40.00%
30.00%
20 00%
10.00%
0.00%


0 200 400 600

frequency (Hz)


-----.5mA
- l--1mA

2mA
5mA
----1 mA


Figure 10. Conjugate Load Power vs. Frequency for a Rectifier Circuit with a load of 90K.


.N . .
mm a

- - -






Conjugate Load Power vs. Frequency for
Standard Rectifier System


40 00%
30.00%
20.00%
10.00%
000%


0 200 400 600

frequency (Hz)


--.5mA
- l--1mA

2mA

5mA

-- 10mA


Figure 11. Conjugate Load Power vs. Input Current for a Rectifier Circuit with a Load of 90K.



Peak energy harvesting is about .02 joules per second and occurs at 10 mA of input current at 50 Hz. Peak

conjugate load ratio is about 35% and .5mA of input current at 50 Hz.



MOSFET Current Source - Conjugate Match Load and Energy Harvested


The gate to source voltage was set (fig. 13) from 4 to 20 V, which is shown to place the MOSFET in saturation

mode at lower values (as a current source should) and reach up to linear mode when the gate voltage is high.

This shows that for low frequency, optimum gate voltage is as high as possible and that the circuit functions

best when the MOSFET has a gate voltage high enough to push it into linear mode, where it functions as a

short. Therefore, at this frequency level, the MOSFET current source gives no advantage.



In order to verify that the above trend holds for some higher frequencies, the simulations (fig. 13) were repeated

at frequencies up to 400 Hz (fig. 14). Again, the best power output is achieved at the highest gate voltage.

This suggests that at these frequencies and power levels the MOSFET current source will not perform better than

the rectifier.



Table 1

Parameters Swept and Which are Shown in Figures for MOSFET Current Source


Parameters Swept and Values


Fig. 12


Fig. 13


frequency range: 50 to 400 Hz 50 Hz 400 Hz

Values: 50, 400 HZ

Amplitude range: .5 mA to 10 mA full range full range

Values of .5mA ImA 2mA 5mA 10mA


VGS range: 4 to 10 volts


4,6, 8, 10

V


4,6,8, 10

V







values of 4, 5, 6, 7, 8, 9, 10, 20 V



Highest Power Extracted when:


U - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -






U



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




V




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


Tim


Figure 12. Output power for four different Vgs levels while sweeping input current at 50 Hz with a


90K Load. See Table 1.




. � - -. .. . ----- . ----- . ---- . ---- . ---- - ---- . ---- . ----- .------ - .----- . ----- -- ------ .----- . ----- - -- -- _















- - - - - - - - - - - - - - - - - - -
,(PJ(rbt t 1 2)-,1(r _btt1)
-------... - -.. - . .-- . ------- . ---- --- ---. - - .-.--- -- -- -- -- -- - ---- --- --------- .--- -

















4 ).
-e 4., ----- -











- i Ur btttZ:)�-I2 rliattt2)





Figure 13. Output power (W) for four different Vgs levels while sweeping input current at 400 Hz with


a 90K Load. See Table 1.


VGS is max


VGS is max





Gate to Source Voltage vs Power Extracted for
Varying Input Currents at 50 Hz


2 00E-04 -
1 .50E-04
1.OOE-04
5.00E-05 -
0.00E+00


0 2 4 6 8


,,,--.5mA
-----1mA
2mA
5mA
- 10mA


VGS (V)


Figure 14. Output power vs. Gate to Source Voltage for Varying Input Current at 50 Hz with a 90K Load.


Gate to Source Voltage vs Power Extracted for
Varying Input Currents at 400 Hz


2.00E-05
1.50E-05
1,00E-05
5.00E-06
0 00E+00


-----



0 2 4 6 )


-*-.5mA
--1mA
2mA
5mA
-3--10mA


VGS (V)


Figure 15. Output power vs. Gate to Source Voltage for Varying Input Current at 400 Hz with a 90K Load.


Power Extracted vs Input Current for varying
Gate to Sources Voltages at 50 Hz


2.00E-04
1,50E-04 ""'
1 .OOE-04



0 5 10 15
Input current (mA)


Figure 16. Gate to Source Voltage vs. Output power for Varying Input Current at 50 Hz with a 90K Load.


---4 (V)
---6 (V)
8 (V)
10 (V)
-iN- 13 (V)
-.-16 (V)
-+-20(V)


UF-

an*





Power Extracted vs Input Current for varying
Gate to Sources Voltages at 400 Hz


2 OOE-D5-
1. 50E-05
1.O0E-06
5.00E-06
OOOE+OQ.-
-5,OOE-06


5 10 1


input current (rnA)


--4 (V)
---6 (V)
8(V)
10(V)
-311-13 (V)
-e-- 16 (V)
--20 (V)


Figure 17. Gate to Source Voltage vs. Output power for Varying MOSFET current at 400 Hz with a
90K Load.


Setting gate voltage to 20 V, which has been demonstrated (fig. 13 & fig. 14) to give approximately
best performance, power levels were swept and conjugate load ratios were computed. If gate voltages were
lower, these values (fig. 15-18) would drop.


The energy (in Joules) harvested for this system was just nearly equivalent to the diode rectifier system (fig. 9-
12) with slightly lower values at the lowest frequency.




Joules per Second vs. Frequency for Mosfet
Current Source


0.03
0.02
0.01
0
-0.01


frequency JHz)


-4- 5rA
1u--lA
2mA
5mA
-I -OmA


Figure 18. Joules per Second vs. Frequency for a MOSFET Current Source in Linear Mode (Vgs of 10
Volts) With a Load of 90K.


[ -


K- -.


r w


� - - Im- A�0 AL


-





Joules per Second vs. Input Current for
Mosfet Current Source


-*-50 Hz
-.-- 100 Hz
200 Hz
400 Hz


Input Current (A)


Figure 19. Joules per Second vs. Frequency for MOSFET Current Source with a Load of 90K.


Conjugate Load Ratio vs. Frequency for
Mosfet Current Source


40.00%
30.00% .
20,00%-
10.00%
0.00% "
0 200 400 600
frequency (Hz)


-4--.5mA
--1lmA
2mA
5mA
----10mA


Figure 20. Conjugate Load Ratio vs. Frequency for a MOSFET Current Source With a Load of 90K.


Conjugate Load Ratio vs. Input Current for
Mosfet Current Source


40.00%
30.00%
20.00%
10.00% ---
0.00%. ,
0 0.005 0.01 0.015
Input current (A)


-*-50 Hz
-a-100 Hz
200 Hz
400 Hz


Figure 21. Conjugate Load Ratio vs. Input Current for a MOSFET Current Source With a Load of 90K.



Peak energy harvesting is about .02 joules per second, and occurs at 10 mA of input current at 50 Hz.
Peak conjugate load ratio is about 34%, and .5mA of input current at 50 Hz.


These graphs (figs. 15-21) show that as compared to the diode rectifier circuit, the current source circuit is not of
any advantage at the tested frequencies and input current levels.


0,03

0.02

0.01
0

-0.01


S 0.005 0.01 0.(





Switch Based Circuit - Conjugate Match Load and Energy Harvested


This circuit has two furthers degrees of freedom that can be swept to determine its viability as compared to the

diode rectifier. These two parameters, "turn on" and "turn off" will be swept in terms of time delay and pulse width.

If this circuit can perform better than the diode rectifier, than it would be expected that a value other than, always

on would be optimal.



To start, time delay will be set to switch the circuit on at a voltage value above the battery voltage and varying

pulse widths will be swept. In other words, "turn on" will be set and turn off will be swept. Below (fig. 22) this is

done with a circuit an input current of 1mA at 50 Hz. Time delay is set to 2ms which is approximately a 5 V "turn

on." After which, the reverse process will be carried out.



The top curve is the open circuit battery voltage. This is the voltage level the PZT will rise to so long as the switch

is kept off. The middle set of curves is the gate voltage that is swept from 2.5ms to 20ms (the entire period,

or always on). The bottom set of curves is the battery voltage, which flows when the circuit is switched on.


P

T
11

t




29U


IJ(MI � 241 0


U(1J29 *~
B


r 1k~


4N 0u N *4 a 5"Fp 54NO Ump "40


Figure 22. Voltage Waveforms For Switch Circuit With a Time Delay of 2ms and Swept Pulse Widths

(from 1.25ms to full period) for a 1mA Input Current at 50 Hz with a 90K Load.




Below (fig. 23) are the power extracted curves for a similar circuit as previously listed (fig. 22) for several

time delays. The top set of curves is for a time delay of lms. The second set of curves is for a time delay of 2ms.

The third set of curves is for a time delay of 3ms. The bottom set of curves is for a time delay of 4ms.



In order to determine if this trend changes over the tested frequency or power range, a similar sweep was

conducted at 10mA of input current at 400 Hz (fig. 24). The results are shown and discussed below.








Table 2

Parameters Swept and Which are Shown in Figures for Switch Circuit


Parameters Swept and Values Fig. 24 Fig. 25

frequency range: 50 to 400 Hz 50 Hz 400 Hz


Values: 50, 400 HZ


Amplitude range: .5 mA to 10 mA 10mA 10mA


Values of .5mA ImA 2mA 5mA 10mA


Time Delay as a percent of period full range full range


values of 10% 20 % 30% 40%


Pulse Width as a percent of Period full range full range


values of 12.5%, 25%, 37.5%, 50% 75%


Results: For all curves, longest switch on gives highest output power


SELO_______________
a - ~ =- -
S(f--at z)-~~at)
s o - - - - - I - - - - - I - - - - - I - - - - - - - - - - - 1 - - - - - - - - - - - - - - - - - - - - - - - - -


* $ ( L(r btt:2) i I( r_latt 2
f---- - ----
$Bu"










Figure 23. Power Extracted Curves For Switch Circuit With a Time Delay 1ms to 4ms (Top to

Bottom) with Swept Pulse Widths (2.3ms to full period) for a 10mA Input Current at 50 Hz with a

90K Load. See Table 2.














S ('(r_batt2 2) -I(rbatt2H)
saw ~ ~ ~ ~ _ - ---------- --- -------- ---






















Figure 24. Power Extracted Curves For Switch Circuit With a Time Delay .25m to 1ms (Top to

Bottom) with Swept Pulse Widths (.625ms to full period) for a 10mA Input Current at 400 Hz with
SOU -- ---- .- --- ---- - - ----. ---- .-- -- .- - -- .---- .---- --- -- ----- ---- --- -- .-- -- .- --- .- --- ---- - .- --- .---- .-- -- .- - -- .---- - -


















a 90K Load. See Table 2.




The figures below are a sample of the collected data for the switch circuit under given conditions (Table 2.).

This sample of data shows a given trend: for low frequency and low input current, the circuit is optimally always
ROu - - .- ---.- ----. --- ------ -- - . ---.----. --- --- - .-- -- ----- ---- --- -- .-- -- .- --- .- --- .---- - ---- .---- .-- -- .- - -- .---- - -





















on (or shorted). As Frequency and input current increase, the time delay and pulse width become an important

factor in harvesting energy.




Extracted Power vs Pulse Width for .SmA
Input Current at 50 Hz for Varying Time Delays

1 20E-05
1 00E-05 -i -------Tm
8 00E-06 - 10 %
6 90K Load. SE-e Tab6 le 2.





puon (or shorted). Alse width (as percent ofcurrent increase, the time delay and pulse width become an important

total pein harvesting eneod


Figure 25. Extracted Power vs. Pulse Width for Varying Time Delays with .5mA Input Current at

50Hz over a 90K Loadr
I 20E-05 - I--------
















50Hz over a 90K Load.


200V-





Extracted Power vs Pulse Width for .SmA
Input Current at 60 Hz for Varying Time Delays
1 20E-05
1 OOE-05 . - Delay
8 00E-06- 10%
6 OOE-06- - (of
A OOE-06 - --- %
2 00E-. _-06
0 OOE+00
0.00% 50.00% 100.00%
pulse width (as percent of o%
total period)


Figure 26. Extracted Power vs. Pulse Width for Varying Time Delays with .5mA Input Current at

400Hz over a 90K Load.


Extracted Power vs Pulse Width for .5mA
Input Current at 50 Hz for Varying Time Delays
1 20E-05 - 1
1 OOE-05 . -- 0elay
8 00E-06. 10%
6 OOE-06 - - -(of
4 OOE-06 ----2
2 00E-06 -
0 OOE+O0 O
0.00% 50.00% 100.00% 30%
pulse width (as percent of 5o%
total period)


Figure 27. Extracted Power vs. Pulse Width for Varying Input Current with a 20% Time Delay at

50Hz over a 90K Load.


Extracted Power vs Pulse Width for .5mA
Input Current at 50 Hz for Varying Time Delays
1.20E-05 -
1 00E-05 m-'- T
OOE-05 Delay
8.00E-06 - 10%
6.00E-06- - / (of
perinc
4.00E-06 - - o%
2.OOE-06
0.OE+00
0,00% 50,00% 100.00%
pulse width (as percent of
total period)


Figure 28. Extracted Power vs. Pulse Width for Varying Input Current with a 20% Time Delay at

400Hz over a 90K Load.





Extracted Power vs Pulse Width for .SmA
Input Current at 60 Hz for Varying Time Delays
1 20E-05 -
1 OOE-05 . - Delay
8 00E-06-o- 10%
6 00E-06- - (of
4 00E-06 --- ---2
A OOE-06 - -V - I
2 00E -06
000OE+-0
0.00% 50.00% 100.00%
pulse width (as percent of o%
total period)


Figure 29. Extracted Power vs. Time Delay with Varying Pulse Widths for .5mA Input Current at

50Hz over a 90K Load.


Extracted Power vs Pulse Width for .5mA
Input Current at 50 Hz for Varying Time Delays
1 20E-05 - 1
1 OOE-05 . -- delay!
8 00E-06. - 10%
6 00E-06 - (of
c OOE-06 - --- --20%
2 00E-06 r -- r
0 OOE+00 _
0.00% 50.00% 100,00% 30%
pulse width (as percent of 5o%
total period)


Figure 30. Extracted Power vs. Time Delay with Varying Pulse Widths for .5mA Input Current at

400Hz over a 90K Load.



At lower frequencies and input currents, the switch circuit has little advantage over the diode rectifier.

When frequency and input current are increased, switching becomes an important parameter.



With an optimized time delay and pulse width, the switching circuit can provide a higher power output and thus

a greater conjugate match ratio. This advantage is show across all swept current levels for low and high

frequency (fig. 31 & 32).




Extracted Power vs Pulse Width for .5mA
Input Current at 50 Hz for Varying Time Delays
1 20E-05 -
1 OOE-05 -. - e
8 00E-06 - 10%
600E-06 -- (of
4 00E-06 --- r
2 00E-06 -
0 OOE +00
0,00% 50.00% 100.00%
pulse width (as percent of
total period)






Fig 31. Optimal Switching and Diode Rectifier Extracted Energy vs. Input Current over a 90k Load.


Extracted Power vs Pulse Width for .5mA
Input Current at 50 Hz for Varying Time Delays
1.20E-05
1.00E-05 - . -.
8.00E-06- - 10%
6 00E-06 - (Of
4.00E-06 - --
2 OOE-06
0000E+00
0.00% 50.00% 100,00% 30
pulse width (as percent of so
total period)


Fig 32. Optimal Switching and Diode Rectifier Conjugate Load Ratio vs. Input Current over a 90k

Load.



CONCLUSION


Of the three types of circuit systems researched for this report, there is no clear winner in terms of energy

harvesting and conjugate load ratio for simulated situations.



When sweeping gate voltage, the current source system proved to be unnecessary at such low frequencies and

input current levels. In fact, best performance was obtained when the gate voltage was at a level such that

the MOSFET went from saturation to linear region.



For higher frequencies and higher input current levels, the switch circuit system was shown to extract more

energy and have a better conjugate load ratio than the alternative circuit systems researched. This circuit

also showed more limitations, however, as it required a substantially higher input in order to function at all. At

a frequency of 400 Hz, a .5mA and 1mA input current gave no output (fig 32).



The simple diode rectifier functioned at the greatest range of conditions of frequency and input and is the

simplest circuit requiring fewer components. With the least control, the diode rectifier had the best performance

at lower frequencies.



The decision of which circuit system to employ would best be made in consideration of operating frequency and

input current levels.


REFERENCES





1. Ben Yaakov, S. lineykin, et al., "Frequency tracking to maximum power of piezoelectric transformer HV converters

under load variations." Power Electronics Specialists Conference, 2002. pesc02.2002 IEEE 33rd Annual

2. G. W. Taylor, et al., "The energy harvesting eel: a small subsurface ocean/river power generator." IEEE J. Oceanic

Engineering, Vol. 26, pp 539-547, 2001

3. P. Niu, et al., "Evaluation of Motions and Actuation Methods for biomechanical Energy Harvesting." Power

Electronics Specialists Conference, 2004. IEEE 35th Annual

4. G. K. Ottman, et al., "Optimized Piezoelectric Energy harvesting Circuit Using Step-Down Convert in Discontinuous

Conduction Mode." IEEE Transaction on Power Electronics, VOL. 18, NO. 2, MARCH 2003

5. S. Roundy, et al., "A 1.9GHz RF Transmit Beacon using Environmentally Scavenged Energy." ISPLED 2003, August

25-27, 2003, Seoul Korea.

6. QP15N QUICKPACK, MIDE. http://www.mide.com/quickpack_poweract/qpl5n.html

7. Shengwen Xu, et al., "Converter and Controller for Micro-Power Energy harvesting." Manuscript # 10822d





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