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Bridging the power gap between energy
harvesters & peak power applications using
supercapacitors
7th June 2010
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© CAP-XX 2010
The problem
• Energy: the amount of work that can be done
• Power: the rate at which work is done (or at which
energy is delivered)
• You can store energy, not power
• Energy harvesting sources have infinite energy but
limited power.
• Periodic wireless transmission requires bursts of energy
delivered at higher power than the energy harvester can
manage.
• Supercapacitors bridge that gap:
they are charged at low power from the harvester and
deliver burst of energy at high power to the load.
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© CAP-XX 2010
Agenda
• Supercapacitor Properties
What’s inside
Power buffer
Leakage current
Charge current
Ageing
• Supercpacitor circuits
• Interfacing the supercapacitor to your energy source
• Sizing your supercapacitor
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© CAP-XX 2010
What is a Supercapacitor?
4
Electrode:Aluminum foil
Nanoporous carbon:Large surface area > 2000m2/gm
Separator:Semi-permeable membrane
-
-
-
-
-
-
-
+
+
+
+
+
+
+
++veve --veve
Separation distance:Solid-liquid interface (nm)
Basic Theory:Capacitance is proportional to
the charge storage area,
divided by the charge
separation distance (C α A / d)
As area (A) , and
charge distance (d)
capacitance (C)
No dielectric, working voltage
determined by electrolyte
Electrolyte:Ions in a solvent
Basic Electrical Model:Electric Double Layer Capacitor (EDLC)
A supercapacitor is an energy storage device which utilizes high surface area
carbon to deliver much higher energy density than conventional capacitors
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© CAP-XX 2010
Supercapacitor as a power buffer
• A supercapacitor buffers the load from the source. Source
provides low average power, supercapacitor provides peak
power to the load.
• Average load power < Average source power
• Source sees constant power load, set at maximum power point
• Load sees low impedance source that delivers high peak power
for duration needed
Low ESR: high power
High C: delivered for duration needed
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Energy
Harvesting
Source
Interface
Electronics
Constant
low power DC:DC
Converter
(if needed)
LOAD
High Peak Power
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© CAP-XX 2010
Excellent performance across a
wide temperature range
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© CAP-XX 2010
Leakage Current
0
5
10
15
20
25
30
35
40
0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 180.00
Time (hrs)
Le
ak
ag
e C
urr
en
t (u
A)
CAP-XX GZ115 0.15F
CAP-XX GZ115 0.15F
CAP-XX HS230 1.2F 5V
CAP-XX HS230 1.2F 5V
Maxwell PC10 10F
Maxwell PC10 10F
Powerburst 4F
Powerburst 4F
PowerStor 1F
PowerStor 1F
Nesscap 3F
Nesscap 3F
AVX 0.1F 5V
AVX 0.1F 5V
Supercapacitor Leakage Current
8
Diffusion current: Ions
migrate deeper into the
pores of the carbon
electrode.
It takes many hours for
diffusion current to decay and
leakage current to settle to its
equilibrium value.
Leakage current behaviour
means a minimum charge
current is required to charge a
supercapacitor. Leakage current
increases exponentially with
temp & decreases exponentially
with voltage.
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© CAP-XX 2010
Charging at 20uA
0
0.5
1
1.5
2
2.5
3
0 50 100 150 200 250 300
Time (hrs)
Su
pe
rca
pa
cit
or
Vo
lta
ge
(V
)
CAP-XX HZ102 0.18F
CAP-XX HZ102Theoretical
Maxwell PC10 10F
Maxwell PC10 10F
Maxwell PC10 Theoretical
Powerburst 4F
Powerburst 4F
Powerburst 4F Theoretical
PowerStor 1F
PowerStor Theoretical
Nesscap 3F
Nesscap 3F
Nesscap 3F Theoretical
Supercapacitors need a minimum
initial charging current …
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Knee at ~1.4V shows current is
consumed reacting with water
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© CAP-XX 2010
…but a short initial charge at
“high”current can overcome this …
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HZ102 charging to 2.5V at 5uA after varying time at pre-charge at 2.5V, 10mA
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 10 20 30 40 50 60 70 80 90 100
Time (hrs)
Su
pe
rca
pa
cit
or
Vo
lta
ge
(V
)
5uA theoretical
5uA 0 min - 1
5uA 0 min - 2
5uA 1 min - 1
5uA 1 min - 2
5uA 5 min - 1
5uA 5 min - 2
5uA 10 min - 1
5uA 10 min - 2
5uA 20 min - 1
5uA 20 min - 2
5uA 50 min - 1
Theoretical: V = ICONST x t/C
Even 1 min pre-charge reduces charge time by half
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© CAP-XX 2010
Self discharge characteristic
empirically determined
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0
0.5
1
1.5
2
2.5
3
0 200 400 600 800 1000 1200 1400 1600 1800
Su
pe
rca
pa
cit
or
Vo
lta
ge
(V
)
Time (hrs)
Long Term HS108 Supercapacitor Self Discharge
Sample1
Sample2
Sample3
Sample4
Sample5
Sample6
Sample7
Sample8
Sample9
Sample10
Sample11
Sample12
Estimate
Est 2
)(025317.0 hrstVinitV
Estimate (based on diffusion):
Est 2, based on RC time constant and estimate for R as resistor in parallel with supercapacitor to model self discharge
FCMR
eVinitV RCst
8.1,5.5
/)(sec
Figure 12: Supercapacitor self discharge characteristic
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© CAP-XX 2010
Take ageing into account when
sizing your supercapacitor
• Supercapacitors use physical not electrochemical
charge storage
• Ageing is a function of time at temperature and
voltage, not no. of cycles
• Determine expected ageing from operating profiles
(voltage and temp combination) and their duty cycle
• Size supercapacitor so you have required C & ESR at
end of life after allowing for ageing
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© CAP-XX 2010
Supercapacitor ageing – C loss
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GW214@ 3.6V, 23C, Ambient RH
y = 1.136E-01e-1.416E-05x
R2 = 9.889E-01
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Time (hrs)
Ca
pa
cit
an
ce
(F
)
C Loss rate = 1.4%/1000hrs
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© CAP-XX 2010
Supercapacitor ageing – ESR rise
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GW214 ESR @ 3.6V, 23C, Ambient RH
y = 0.0027x + 71.706
R2 = 0.9168
0
20
40
60
80
100
120
140
160
180
200
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Time (hrs)
ES
R (
mO
hm
s)
ESR rise rate of
2.7mOhms/1000hrs, or 3.4%
of initial ESR/1000hrs
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© CAP-XX 2010
Cell balancing is needed
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Leakage Current
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.5 1 1.5 2 2.5 3 3.5 4
Cell Voltage (V)
Le
ka
ge
Cu
rre
nt
(uA
)
Cell 1 Voltage
Cell 2 Voltage
Example data only,
dual cell supercap,
cells C1 and C2
Assume both cells have identical C. At
initial charge up to 4.6V, voltage across
each cell is inverse ratio of their C. If C1
= C2, then voltage across each cell =
2.3V, however at that voltage, CI would
have a leakage current of 0.8uA and C2
of 1.6uA. But this is not a possible
equilibrium condition, since ILeakage1
must = ILeakage2
Voltage across C2 reduces to 1.9V,
Voltage across C1 increases to 2.8V to
achieve equilibrium leakage current =
1.1uA for both C and C2. However, C2
will be damaged at this voltage and the
device will eventually fail.
2.3V,
1.6uA
2.3V,
0.8uA
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© CAP-XX 2010
Simplest balancing is a pair of resistors
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VSCAP
0V
balancing
resistor, RB
balancing
resistor, RB
Leakage Current Cell1,
IL1
Leakage Current Cell2
IL2
Balancing
Current
Balance Resistor1 Current,
IBR1
Balance Resistor2 Current,
IBR2
Fig 3: Balancing resistor circuit
VM
The purpose of this circuit is to maintain VM close to VSCAP/2.
VM = RBxIBR2 = RBx(IBR1-Balancing Current).
For this circuit to work, Balancing Current must be > VSCAP/2 or
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© CAP-XX 2010
Active balance circuit
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U1A
MAX4470
-2
+3
V-4
V+8
OUT1
R1
2M2
R2
2M2
R3
470
R4
100R5
100
V_SUPERCAP
J1
12
J2
12
SC1
CAP-XX SupercapacitorC2
10nF
C1
100nF
2 capacitor cells in
series need voltage
balancing, otherwise
slight differences in
leakage current may
result in voltage
imbalance and one
cell going over
voltage.
Low current rail-rail
op amp, < 1A
Can source or sink
current, 11mA
Supplies or sinks the
difference in leakage
current between the
2 cells to maintain
balance.
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© CAP-XX 2010
Active balance – very low currents
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Active Balance HW207 @ 23C
0
0.5
1
1.5
2
2.5
3
100.00 1000.00 10000.00 100000.00 1000000.00
Time (s)
Vo
ltag
e (
V)
-0.000020
0.000000
0.000020
0.000040
0.000060
0.000080
0.000100
Cu
rren
t (A
)
Bottom Cell V Top Cell V Balance Current OP Amp Supply Current
Top Current Bottom Current Total Current
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© CAP-XX 2010
Supercapacitor Interface Circuits
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Design principles:
1. Must start charging from 0V
2. Must provide over voltage protection
3. Must prevent the supercapacitor from discharging into the source
4. Should be designed for maximum efficiency
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© CAP-XX 2010
Inrush current
• Discharged supercapacitor will look like a short circuit to your
energy source
• Most energy harvesters don’t care – will deliver current into a
short circuit
• Definitely a problem for batteries, some power supplies
Interface electronics must manage this
Either inherent in the design to keep the source near its maximum
power point
or
Separate current limit
o AAT4610
o http://www.cap-xx.com/resources/app_notes/an1002.pdf
or
Supercapacitor charging IC
o LTC3225
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http://www.cap-xx.com/resources/app_notes/an1002.pdfhttp://www.cap-xx.com/resources/app_notes/an1002.pdfhttp://www.cap-xx.com/resources/app_notes/an1002.pdf
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© CAP-XX 2010
Example1: Microgenerators
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Generator Output Power against Generator Output Voltage
0
100
200
300
400
500
600
700
0 1 2 3 4 5 6 7 8 9 10
Generator Output Voltage (V)
Outp
ut P
ow
er
(µW
)
Output Power
28mm
Perpetuum microgenerator is a high impedance source
Power match circuit to keep PMG17 @ ~5V, supercap
charge current regulated so PMG17 o/p voltage ~5V
But what this data sheet curve doesn’t tell you …
Operate between
4V - 5V
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© CAP-XX 2010
Microgenerators: characterise the
device for yourself …
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Typical Micro Generator Output
Perpetuum PMG17
0
500
1000
1500
2000
2500
3000
0 1 2 3 4 5 6 7 8 9
Microgenerator Output Voltage (V)
Cu
rre
nt
(uA
)
0
2
4
6
8
10
12
14
16
18
Po
we
r (m
W)PMG17 Current
PMG17 Power
Delivers current into a short cct for rapid charging of a supercapacitor
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© CAP-XX 2010
Which leads to a very simple cct …
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Over voltage protection
Active
balance
U2 is open drain, so when Vout < 1.8V (Vss min), U1 o/p is open cct and R8 keeps M1 off.
Hysteresis on U2, turn M1 on when Vout > 5.35V, turn M1 off when Vout < 5.25V
Equivalent cct of microgenerator shows it is not possible for the supercap to discharge back
into it when its o/p voltage < Vscap, therefore do not need any reverse current protection
Quiescent current of MAX4470 1A, TLV3011 3A
Choose RL to set rate of supercapacitor discharge and to ensure that RL x Imicrogen < Vmax,
typical value ~1K
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© CAP-XX 2010
Piezoelectric interface
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o/p voltage
select
1F, 6V
CSTORAGE25V
10F,
6V
10H
4.7F
6V Supercapacitor,
e.g. HZ292
Active
Balance
Linear Technology LTC3588-1Block diagram of IC from LTC3588-1 datasheet
Piezoelectric
transducer
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© CAP-XX 2010
Thermoelectric generator
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Micropelt MPG-D571, Vtherm =140mV/K, Rs = 300
Texas Instruments TPS61200 has min Vin = 300mV, starts charging Vaux with load
disconnected, when Vaux = 2.5V IC starts functioning.
Has true load disconnect (back to back PFETs as o/p switch), so supercap cannot discharge
back into the source
Uses output PFETs in linear mode to limit inrush current or limit o/p voltage if Vin > Vout.
Typically draws 50A during operation
Active
Balance
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© CAP-XX 2010
Thermoelectric generator 2
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Perpetuapower Power Puck, several volts with high impedance (Ks)
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© CAP-XX 2010
Single or dual cell supercapacitor?
• Single cell:
No balancing circuit needed
Lower current
Smaller, thinner
Lower cost
• Dual Cell:
Higher energy storage ½ CV2
5V – 1.2V, 1F 11.8J
2.5V – 1.2V, 2F 4.8J
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© CAP-XX 2010
Sizing your supercapacitor
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ESR
C
VSUPERCAP
VLOAD
ILOAD
PLOAD
ESR
PESRVVI
PIVESRI
IESRIVP
IVP
ESRIVV
SUPERCAPSUPERCAP
LOAD
LOADLOADSUPERCAPLOAD
LOADLOADSUPERCAPLOAD
LOADLOADLOAD
LOADSUPERCAPLOAD
2
4
0
)(
2
2
Energy balance approach often used: Avge Load Power x time = ½ C(Vinit2 – Vfinal
2)
but
If the supercapacitor is supplying a constant power load, such as a DC:DC
converter, where supercapacitor current increases as supercapacitor voltage
decreases, to maintain V x I constant, then supercapacitor ESR may become
significant, and you should solve:
If load current is very small, then ILOAD•ESR
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© CAP-XX 2010
Example: GPRS class 10 transmission
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Average load power = 25% x 2A x 3.6V/85% = 2.11W. Load duration = 100 frames x 4.6ms = 0.46s
Load energy = 0.97J. Supercap Vinit = 5V, Vfinal = 2.5V
C = 2 x 0.97/(52-2.52) = 0.1F. Nothing about ESR
Quadratic solution:
C = 0.1F
ESR = 200m
Does not work.
Vfinal 1.5V
Will only support
transmission for
0.36s
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© CAP-XX 2010
Example: GPRS class 10 transmission
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Average load power = 25% x 2A x 3.6V/85% = 2.11W. Load duration = 100 frames x 4.6ms = 0.46s
Load energy = 0.97J. Supercap Vinit = 5V, Vfinal = 2.5V
C = 2 x 0.97/(52-2.52) = 0.1F. Nothing about ESR
Quadratic solution:
C = 0.13F
ESR = 200m
Solution works
Vfinal 2.7V
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© CAP-XX 2010
About using supercapacitors
• Ideal power buffer
• Low voltage, multiple cells, need cell balancing
• Leakage current decays over time, charging may take longer
than expected
• Allow for ageing when selecting initial C & ESR
• Can pre-charge the supercapacitor to greatly reduce charging
time
• Four principles of supercap energy harvesting circuits
• Interface ccts for vibration & thermoelectric harvesters
• Remember ESR when sizing the supercapacitor
• Supercap selection based on: min load reqmts, ageing &
application life, time to charge, leakage current, single or dual
cell?
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© CAP-XX 2010
About CAP-XX
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• World leader in thin, flat, small supercapacitors suitable for portable
electronic devices
• Research-based, market-driven electronic components
manufacturer. Founded in Australia in 1997. Listed on AIM in
London, April 2006
• Turn-key power design solutions
• Production in Sydney & Malaysia
• Significant sales to big brand customers in Europe, Asia and North
America
• CAP-XX supercapacitor technology licensed to Murata in 2008
• Distributors throughout USA, Europe and Asia
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© CAP-XX CONFIDENTIAL 2008
For more information, please contactPierre Mars
VP Applications Engineering
Email: [email protected]
Web: www.cap-xx.com
mailto:[email protected]:[email protected]:[email protected]://www.cap-xx.com/http://www.cap-xx.com/http://www.cap-xx.com/