Bridging the power gap between energy

harvesters & peak power applications using

supercapacitors

7th June 2010

© 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.

2

© 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

3

© 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

© 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

6

Energy

Harvesting

Source

Interface

Electronics

Constant

low power DC:DC

Converter

(if needed)

LOAD

High Peak Power

© CAP-XX 2010

Excellent performance across a

wide temperature range

7

© 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.

© 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 …

9

Knee at ~1.4V shows current is

consumed reacting with water

© CAP-XX 2010

…but a short initial charge at

“high”current can overcome this …

10

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

© CAP-XX 2010

Self discharge characteristic

empirically determined

11

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

12

© CAP-XX 2010

Supercapacitor ageing – C loss

13

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

14

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

15

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

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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

© CAP-XX 2010

Active balance circuit

17

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

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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

© 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

© 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

20

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

© 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

© CAP-XX 2010

Microgenerators: characterise the

device for yourself …

22

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

© 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

© CAP-XX 2010

Piezoelectric interface

25

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

© CAP-XX 2010

Thermoelectric generator

26

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

© CAP-XX 2010

Thermoelectric generator 2

27

Perpetuapower Power Puck, several volts with high impedance (Ks)

© 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

28

© CAP-XX 2010

Sizing your supercapacitor

29

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

© CAP-XX 2010

Example: GPRS class 10 transmission

30

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

31

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

• 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?

32

© CAP-XX 2010

About CAP-XX

33

• 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

mailto:pierre.mars@cap-xx.commailto:pierre.mars@cap-xx.commailto:pierre.mars@cap-xx.comhttp://www.cap-xx.com/http://www.cap-xx.com/http://www.cap-xx.com/