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

9

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Excellent pulse reponse

10

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Ceffective as a function of PW

11

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Ceffective data

12

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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: pierre.mars@cap-xx.com

Web: www.cap-xx.com