Using a Small Solar Cell for Harvesting and a
Supercapacitor for Power Management in a
Wireless Sensor
9th June 2010
© CAP-XX 2010
Agenda
• Supercapacitor Properties
What’s inside
Power buffer
Leakage current
Charge current
Ageing
• Solar cell characteristics
• Circuits to charge supercpacitors from solar cells
• Sizing your supercapacitor
2
© CAP-XX 2010
What is a Supercapacitor?
3
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
© 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 low power load
• Load sees low impedance source that delivers high peak power
for duration needed
Low ESR: high power
High C: delivered for duration needed
5
Energy
Harvesting
Source
Interface
Electronics
Constant
low power DC:DC
Converter
(if needed)
LOAD
High Peak Power
© CAP-XX 2010 6
Excellent performance over a wide
temperature range: Power Delivery
© CAP-XX 2010 7
Excellent performance over a wide
temperature range: Energy Storage
© CAP-XX 2010
Poor frequency response …
8
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…But excellent pulse response
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© CAP-XX 2010
Excellent pulse reponse
10
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Ceffective as a function of PW
11
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Ceffective data
12
© CAP-XX 2010
Required C reduces faster than Ceff
• HS206, DC Capacitance = 600mF
• V = I. t/Ceff
• 2A for 1ms
Ceff = 6.5% of 600mF = 39mF
V = 2A x 1ms/39mF = 51mV
• 2A for 10ms
Ceff = 26% of 600mF = 156mF
V = 2A x 10ms/156mF = 128mV
• 2A for 100ms
Ceff = 70% of 600mF = 420mF
V = 2A x 1ms/39mF = 476mV
13
© 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
14
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.
© CAP-XX 2010
Supercapacitors need a minimum
initial charging current …
15
Knee at ~1.4V shows current is
consumed reacting with water
Charging at 50uA
0
0.5
1
1.5
2
2.5
3
0 20 40 60 80 100 120 140 160 180 200
Time (hrs)
Su
pe
rcap
acit
or
Vo
lta
ge
(V
)
CAP-XX HZ102 0.18F
CAP-XX HZ102 0.18F
Theoretical HZ102
CAP-XX HS130 2.4F
CAP-XX HS130 2.4F
CAP-XX HS130 Theoretical
Maxwell PC10 10F
Maxwell PC10 10F
Maxwell PC10 Theoretical
Powerburst 4F
Powerburst 4F
Powerburst 4F Theoretical
PowerStor 1F
PowerStor 1F
PowerStor 1F Theoretical
Nesscap 3F
Nesscap 3F
Nesscap 3F Theoretical
© CAP-XX 2010
Self discharge characteristic
empirically determined
16
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
© 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
17
© CAP-XX 2010
Supercapacitor ageing – C loss
18
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
© CAP-XX 2010
Supercapacitor ageing – ESR rise
19
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
© CAP-XX 2010
Cell balancing is needed
20
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
© CAP-XX 2010
Simplest balancing is a pair of resistors
21
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 << IBR1, IBR2.
VM must be prevented from going >> VSCAP/2 or <<VSCAP for any significant length of time.
SIMPLE but HIGH CURRENT SOLUTION (~100A)
© CAP-XX 2010
Active balance circuit
22
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.
© CAP-XX 2010
Active balance – very low currents
23
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
© CAP-XX 2010
Solar cell characteristics
24
Simplified Circuit Model of a Solar Cell
IPH generates current light falling on the cell
If no load connected all the current flows
through the diode whose forward voltage = VOC.
RP represents leakage current
RS represents connection losses, usually not
significant
XOB17, 22mm x 7mm x 1.6mm
used for measurements in
following slides
Will deliver current into a short
circuit (discharged supercapacitor)
Will discharge the load if light level
drops
© CAP-XX 2010
Simple to characterise your solar
cell in your conditions
25
XOB17-04x3 IV Low Current IV Curves
0
50
100
150
200
250
300
350
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Solar Cell Vout (V)
So
lar
Ce
ll I
ou
t (u
A)
Vo, Isc = 100uA
Vo, Isc = 200uA
Vo, Isc = 300uA
Curves provided in data sheet
Curves developed in lab in our light conditions
© CAP-XX 2010
Simple to characterise small solar cells
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© CAP-XX 2010
Supercapacitor Interface Circuits
27
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
© CAP-XX 2010
Simplest charging circuit
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Starts charging from 0V
Voc < 2.7V at max light level in the application. I-V curves of slide 25 showed
Voc 1.5V
D1 prevents supercapacitor discharging back into solar cell when light levels fall
BAT54 chosen for D1 due to low VF. VF at currents < 10A, <0.1V
HS130 provides excellent energy storage & power delivery at low voltage
© CAP-XX 2010
Single cell supercapacitor with over
voltage protection
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When VLOAD > 2.75V, turn M1 ON, VSOLAR drops & D1 does not conduct.
When VLOAD < 2.71V turn M1 OFF, can charge solar cell
TLV3011 is open drain, so o/p is open cct when Vscap < 1.8V (min Vss)
TLV3011 quiescent current ~3A
© CAP-XX 2010
Dual cell supercapacitor with over
voltage protection and active balancing
30
Optional over voltage control, only
needed if VOC > VSCAP MAX (= 5.5V)
Active
balance
© CAP-XX 2010
Supercapacitor charging
31
Supercapacitor Charging by Solar Bit XOB17- 04 x 3 Mini Solar Array
(Over Voltage Limiting Circuit Used)
0
0.5
1
1.5
2
2.5
3
0 10 20 30 40 50 60 70 80
Time (hrs)
Su
pe
rcap
acto
r V
olt
ag
e (
V)
CAP-XX HZ202 0.09F
CAP-XX HS130 2.4F
Isc = 300uAIsc = 200uA
Isc = 100uA
© CAP-XX 2010
High Power Transmission
• Previous circuits ideal for low power transmission:
Supercapacitor at solar cell voltage
with voltage regulation on load side of the supercapacitor
Regulator is small
• For high power, place regulator between solar cell and
supercapacitor:
Regulator is small, low power (solar cell o/p power)
Supercapacitor charged to the RF PA supply voltage,
supplies the RF PA directly
Supercapacitor must have low ESR for power delivery as well
as enough energy storage to support the transmission for its
duration.
32
© CAP-XX 2010
High power circuit
33
CAP-XX HS230,
1.2F, 50m
Active
Balance
High power
load, e.g. GPRS
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
© 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
34
© CAP-XX 2010
Sizing your supercapacitor
35
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 << VSUPERCAP and can use an energy
balance to size the supercapacitor. Otherwise, use a spread sheet to solve the above
and simulate V & I over time, or use SPICE.
© CAP-XX 2010
Example: GPRS class 10 transmission
36
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
© CAP-XX 2010
Example: GPRS class 10 transmission
37
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
© CAP-XX 2010
About using supercapacitors
with solar cells
• 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
• Solar cells deliver max current into a short circuit – ideal for
charging a supercapacitor from 0V
• Need to prevent supercapacitor discharging back into the solar
cell when light falls
• Single or dual cell supercapacitor?
• Low power and high power circuits
• Remember ESR when sizing the supercapacitor
38
© CAP-XX 2010
About CAP-XX
39
• 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
© CAP-XX CONFIDENTIAL 2008
For more information, please contactPierre Mars
VP Applications Engineering
Email: [email protected]
Web: www.cap-xx.com