Supercapacitors Enable µPower Energy Harvesters to Power Wireless Sensors

and do other useful things

(Everything you wanted to know about supercapacitors and were afraid to ask)

1

Sensors Expo June 2019Pierre MarsVP Quality & Applications Engineeringwww.cap-xx.com

Energy Harvesting

The environment has infinite energy, sensors are everywhere, so why have small energy harvesters not taken off as forecast?

2

Some Forecasts Forecast 1, https://www.prnewswire.com/news-releases/energy-

harvesting-market-worth-42-billion-by-2019-global-industry-analysis-size-share-growth-trends-and-forecast-2012---2018-237000391.html

• Energy Harvesters markets at $131.4 million in 2012 are projected to increase to $4.2 billion in 2019. Growth is anticipated to be based on demand for micro power generation that can be used to charge thin film batteries. Systems provide clean energy that is good for the environment. Growth is based on global demand for sensors and wireless sensor networks that permit control of systems.

Forecast 2, https://www.marketwatch.com/press-release/global-energy-harvesting-market-2019-industry-analysis-size-share-strategies-and-forecast-to-2023-2019-03-28

• $500M in 2018 and forecast $1030M in 2025• List of key players shows this report also deals with micro power.

3

Hurdles for Small Energy Harvesters

Price (CR2032 coin cell ~US$0.3)

10’s – 100’s µW

Knowledge: • How do I get enough power with EH?• How do I keep size, cost down?• How do I avoid complexity?• Efficiency?

Solution:

Supercapacitors

Case Studies – visit us at booth 942

4

Batteries Low cost

Good energy density

But

Need replacing, proper disposal (high cost)

RoHS, SVHC, REACH (e.g. Mouser would not ship CR2032 to Australia, Element14 warning:

High Internal impedance,

May have complex charging algorithm & capacity estimation

Self discharge / leakage current

5

Hazardous ItemThere maybe additional transit time on this item. Delivery of other items on your order will be unaffected.

especially when cold

Supercapacitors “Infinite cycle life”, physical charge storage

Excellent power density

Wide temperature range, -40C to +85C

Low leakage current [CAP-XX ~1µA/F]

Great round trip efficiency (~99%)

From small thin prismatic form-factors to large cans

Simple to charge: Show me the current

An ideal power buffer

But

Low voltage: may need cells in series → Need cell balancing

Not SMD

6

Supercapacitor: an Ideal Power Buffer

Supercapacitor charged at low average power

Supplies peak power bursts (low ESR)

And/or

Backup power in case of energy loss (high C)

Regulate duty cycle so Avge Pwr Out ≤ Avge Pwr In

7

Tx 1 message/hr,

150mW for 0.6s, avge pwr = 25µW

80% efficiency

→ ~30µW 150mW

Supercapacitor Properties

8

What makes a Supercap “Super”?

9

A supercapacitor is an energy storage device which utilizes high surface area carbon to deliver much higher energy density than conventional capacitors

Electrode:Aluminum foil

Nanoporous carbon:Large surface area > 2000m2/gm

Separator:Semi-permeable membrane

-

-

-

-

-

-

-

+

+

+

+

+

+

+

+ve -ve

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) , andcharge distance (d) capacitance (C)

No dielectric, working voltage determined by electrolyte

Electrolyte:Ions in a solvent

Basic Electrical Model:Electric Double Layer Capacitor (EDLC)

What makes CAP-XX “Super”?

Very small, very thin form factors

Ultra-low impedance (ESR)

• High power delivery (CAP-XX has world’s highest power density)

Very high capacitance (C)

• Provides the energy needed to keep delivering the power

Easy to charge

• Just need a charge current (from 20uA) & over-voltage protection

Very low leakage current (<1µA)

Unlimited cycle life (physical charge storage, no chemical reactions)

Excellent low temperature performance

Good frequency response

10

High Power & Energy Density

11

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02

Po

we

r D

en

isty

(W

/L)

Energy Density (Wh/L)

Volumetric Power Density vs Energy Density: SupercapacitorsCap-xx

Maxwell Boostcap

Illinois Capacitor - Prismatic

Illinois Capacitor - Radial

Maxfarad

Murata

Nesscap

NEC/Tokin

NEC/Tokin - Prismatic

AVX

Cooper Bussmann / Eaton

Cellergy

Panasonic

Tecate - Prismatic

Tecate - Radial

Nichicon

ELNA - Radial

ELNA - Prismatic

KEMET

Rubycon

Taiyo Yuden

0%

50%

100%

150%

200%

250%

300%

-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90

No

rmali

sed

ES

R t

o r

oo

m t

em

pera

ture

Temperature (C)

Normalised ESR vs. temperature

H Series

G series

Poly. (H Series)

Poly. (G series)

0%

20%

40%

60%

80%

100%

120%

-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90

No

rmali

sed

DC

cap

acit

an

ce t

o r

oo

m t

em

pera

ture

(%)

Temperature (Celsius)

Normalised DC capacitance vs. temperature

G Series

H series

Supercapacitors operate over a wide temp range

12

13

0

5

10

15

20

25

30

35

40

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0

Le

akag

e C

urr

en

t (u

A)

Time (hrs)

Leakage Current

CAP-XX GZ115 0.15F

CAP-XX GZ115 0.15F

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

…translates to Low Charge Current

14

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12 14 16 18 20

Vo

lta

ge

(v)

Time(hrs)

Low Current Charging Behaviour of GA109

200uA

100uA

50uA

20uA

10uA

10uA theoretical

20uA theoretical

50uA theoretical

Shallow discharge steady state ILEAKAGE

15

0

20

40

60

80

100

120

140

100 101 102 103 104 105 106 107 108 109 110

IL(µ

A)

Time (Hrs)

HW109 Single Cells ILeakage following a shallow dischargeBlue Cells precharged for 100 hours at 2v7, Red Cells had no pre-charging time.

Discharged from 2v7 to 2v0 every hour then recharged through 2k2Ω while monitoring ILeakage

IL No-Chg 1

IL No-Chg 3

IL No-Chg 5

IL No-Chg 7

IL Pre-Chg 1

IL Pre-Chg 3

IL Pre-Chg 5

IL Pre-Chg 7

TheoreticalCharge Current

1.25µA2.8µA

3.7µA

Poor Frequency Response ...

16

-100

-80

-60

-40

-20

0

20

40

60

80

100

0.01

0.10

1.00

10.00

100.00

1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Ph

as

e(d

eg

ree

)

Ma

gn

itu

de

(Ω)

Frequency(Hz)

Magnitude

Phase

... But excellent pulse response

17

0

0.5

1

1.5

2

2.5

3

3.5

4

4

4.05

4.1

4.15

4.2

4.25

4.3

4.35

4.4

-1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Cu

rre

nt(

A)

Vo

ltag

e(V

)

Time(ms)

1.8A 1.1ms pulses supported by GW209, source current limited to 0.5A

Voltage (V)

Source Current (A)

Load Current (A)

What is effective capacitance?

18

Effective capacitance is a time domain representation of freq response that can be used for a quick estimate of pulse response

Effective Capacitance (GW209)

19

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.0001 0.001 0.01 0.1 1 10

Pe

rce

nta

ge o

f D

C c

apac

itan

ce(%

)

Pulse Width (s)

Efficient Charge / Discharge Cycle

20

• Losses = I2R x duration during charge, discharge

• Energy in/out = 2 x ½ C (V2MAX – V2

MIN)

• Efficiency = Energy in/out = Energy in/out_____ Energy in/out + Losses Energy in/out + i2.ESR.t

=

12𝐶(𝑉2

2 − 𝑉12

12𝐶 𝑉2

2 − 𝑉12 + 𝑖2. 𝐸𝑆𝑅. 𝐶

𝑉2 − 𝑉1𝑖

=𝑉2 + 𝑉1

𝑉2 + 𝑉1 + 2. 𝑖. 𝐸𝑆𝑅

• Ex 1: GPRS 2A disch from 3.8V to 3.2V, ESR = 50mΩ, = 97.2%

• Ex 2: Charge @ 50mA from 3.2V to 3.8V, = 99.9%

For i = constant

Cell Voltage Balancing

Supercapacitors are low voltage devices

Modules containing 2 or more supercapacitors in series are needed to achieve higher operating voltages

Multi-cell modules need voltage balancing to ensure that slight differences in leakage current do not cause voltage imbalances between the cells

Without adequate voltage balancing, one cell may go over-voltage, leading to accelerated ageing & premature failure

Balancing can be: Passive (simple, but costly in terms of energy lost), or Active (to achieve the minimum possible leakage current)

21

No balancing

22

0

1

2

3

4

5

6

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

3.1

3.3

0 10 20 30 40 50 60 70 80 90 100

Te

rmin

al V

olt

ag

e (

V)

Mid

Po

int

Vo

ltag

e (

V)

Time (hrs)

HA202 supercapacitors at 5V with no balancing

Teminal VoltageMid Point Voltage

With balancing

23

1.7

1.75

1.8

1.85

1.9

1.95

2

0 200 400 600 800 1000 1200 1400 1600

Mid

po

int

Vo

ltag

e(V

)

Time (Hrs)

GS206 @ 3.6V with balancing, midpoint voltages

68KΩ Resistive BalancingActive Balancing

Active Balancing with an Op Amp

24

Low current rail-rail op amp, ~500nA

Can source or sink current, 4.7mA

Supplies or sinks the difference in leakage current between the 2 cells to maintain balance

Total current, supercapacitor leakage + balancing circuit ~2µA

Low cost op amp

Or Use a Single Cell

If your circuit can run at 2.7V (prismatic) or 3V (cylindrical) or less, use a single cell

• Simpler (no balancing required)• Cheaper• Thinner

If source > 2.7V (or 3V) use a low power buck, e.g. TPS62743 (Iq 360nA), or LDO TPS78227 (Iq 500nA)

Depends on energy / power required

It’s a cost / size trade-off

3V prismatic cell coming !!

25

Sizing the Supercapacitor

Energy balance approach often used:Avg Load Power x Time = E = ½ C(Vinit

2 – Vfinal2),

C = 2E/(Vinit2 – Vfinal

2)

But this implicitly assumes ESR = 0!

This may lead to undersizing the supercapacitor.

For constant current pulse of duration T: Vdrop = ILOAD x [ESR + T/Ceff(T)]

For constant power it will be worse as ILOAD increases as Vsupercap decreases to keep V x I = const. See CAP-XX website for tools that solve this problem

26

Equation for Constant Power

27

ESR

C

VSUPERCAP

VLOAD

ILOAD

PLOAD

ESR

PESRVVI

PIVESRI

IESRIVP

IVP

ESRIVV

SUPERCAPSUPERCAP

LOAD

LOADLOADSUPERCAPLOAD

LOADLOADSUPERCAPLOAD

LOADLOADLOAD

LOADSUPERCAPLOAD

2

4

0

)(

2

2

If load current is very small, ILOAD x ESR << VSUPERCAP so use energy balance approach. Otherwise, use a spreadsheet to solve the above & simulate V & I over time, or use SPICE

𝑉𝑆𝑈𝑃𝐸𝑅𝐶𝐴𝑃 𝑡 + 𝑑𝑡 = 𝑉𝑆𝑈𝑃𝐸𝑅𝐶𝐴𝑃 𝑡 −𝐼𝐿𝑂𝐴𝐷 𝑡 . 𝑑𝑡

𝐶

Iterate:

Constant Power Example

28

0

0.5

1

1.5

2

2.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Load P

ow

er

(W)

Superc

apacitor

Voltage (

V),

Load C

urr

ent(

A)

Time (secs)

HS130: 2.4F, 25mΩ supercapacitor module suppyling 1W for 4s

Voltage

Current

Pwr Chk

https://www.cap-xx.com/resources/design-aids/

Supercapacitor Ageing Supercapacitors will slowly lose C and increase ESR over time.

The rate of C loss, ESR rise is a function(V,T)

CAP-XX has placed a range of parts at different voltage-temp combinations for ~1yr to determine ageing rates as functions of V, T.

29

y = 2.2831E+00e-3.3817E-06x

R² = 9.6769E-01

y = 2.2028E+00e-7.5813E-06x

R² = 9.8497E-01

y = 2.0922E+00e-1.8428E-05x

R² = 9.7950E-01

0

0.5

1

1.5

2

2.5

3

0

0.5

1

1.5

2

2.5

3

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

C(F

)

Time (Hrs)

GS130N 1.8V C

50C50C Median

23C23C Median

C loss rate is at 0.3376% per thousand hours from the previous value, at 23°C 1.8V

C loss rate is at 0.7553% per thousand hours from the previous value, at 50°C 1.8V

70C70C Median

C loss rate is at 1.98% per thousand hours from the previous value, at 70°C 1.8V

y = 2.9019E-05x + 1.0947E+01

y = 4.9123E-04x + 1.2169E+01

y = 2.0099E-04x + 1.0231E+01

0

2

4

6

8

10

12

14

16

18

20

0

2

4

6

8

10

12

14

16

18

20

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

ESR

(m

Ω)

Time(Hrs)

GS130N 1.8V ESR

70CMedian 70C

ESR increases at 0.4912mΩ every 1000 hourOr 4.036% increase from initial ESR per thousand hours, at 70°C 1.8V

50CMedian 50C

23CMedian 23C

ESR increases at 0.029mΩ every 1000 hourOr 0.265% increase from initial ESR per thousand hours, at 23°C 1.8V

ESR increases at 0.2010mΩ every 1000 hourOr 1.83% increase from initial ESR per thousand hours, at 50°C 1.8V

Life Estimates Life is not a fixed end date. It is when the supercapacitor has

lost C, increased ESR so it no longer supports your load.

You can increase life by starting with a higher C, lower ESR supercapacitor

You should have a typical V-T operating profile:• Using the corner case (e.g. 5V, 70) is lazy and won’t

work

Common Arrhenius assumptions:• Ageing rate halves for every 10C decrease and every

0.2V decrease• Only applicable if reactions stay the same – they don’t!

CAP-XX has regressed eqns for Closs, ESRrise with V, T.

30

Temperature Voltage % Time

-40°C 5 0%

-30°C 5 0%

-20°C 5 5%

-10°C 5 5%

0°C 5 10%

+10°C 4.8 20%

+20°C 4.8 20%

+30°C 4.8 10%

+40°C 4.6 10%

+50°C 4.6 10%

+60°C 4.5 5%

+70°C 4.2 5%

100%

Arrhenius Ridiculous

Claim: 0.77yrs @ 5V, 85C

At 3.6V, using assumption of life doubling for every 0.2V reduction, life increased by 2(5V-3.6V)/0.2V = 27= 128

Life at 3.6V, 85C = 0.75 x 128 = 99yrs

Reducing temperature to 25C increases life by a factor of 2(85-25)/10 = 26 = 64

Life at 3.6V, 25C = 99 x 64yrs = 6308yrs!

31

Size for Ageing

Use real estimates of Closs, ESRrise

Determine min C / max ESR that will support your application

• I.ESR Vdrop

• Constant power or constant current

• Ceff for short PW

Estimate a realistic V-T operating profile

Apply C loss / ESR rise factor over required life to EOL C, ESR to determine initial C, ESR.

32

Supercapacitor ChargingA supercapacitor charging circuit must:

1. behave gracefully into a short circuit since a discharged supercapacitor will look like a short, or the in-rush current will have to be limited

2. be able to charge from 0V

3. provide over voltage protection for the supercapacitor

4. prevent the supercapacitor from discharging into the source when VSOURCE < VSUPERCAP

and

5. should be designed for maximum efficiency

33

SMALL SOLAR CELL REPORTING OVER A WIDE

CAMPUS AREA

LoRa Case Study

34

Small Solar Cells

Power output for solar cells is typically defined at 50,000 lux (bright sunlight) or 100,000 lux (1kW/m2)

If used indoors, light levels are MUCH lower:Typically between 300 lux (minimum for easy reading) & 500 lux (well it office) AND with different spectrum (LED/CCFL)

Select & characterise your solar cell for the conditions in which it will be used!

35

Indoor use: LED/CCFL spectrum, typically 200 – 400 lux. Silicon cells will not do well!

0

50

100

150

200

250

300

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Po

we

r (u

W)

Cu

rre

nt

(uA

)

Voltage (V)

Epishine EH18-15 solar cells

V-I @ 500 lux

V-I @ 50 lux

V-I @ 500 lux with Halogen

P-V @ 500 lux

P-V @ 50 lux

P-V @ 500 lux with Halogen

Characterise your Solar Cell

36

Epishine: Organic solar cell, lower cost and more responsive to indoor LED / CCFL. 250µW @ 500 lux LED spectrum, 29mm x60mm, 14.4µW/cm2. Voc, Vpp approx. constant

Solar cell

6 digit DVM

RSENSE

DVMSimple cct to characterise solar cell

10Ω

Vpeak_pwr = 2.7V @ 500 lux

Vpeak_pwr = 2.4V @ 50 lux

What is the best charging circuit?

It depends: Direct charging or use a Power Management IC?

Direct Charging• Simple, low cost

• Sufficient light when application needs to run, e.g. working hrs in a

supermarket, office, factory; in daylight

Energy Harvesting PMIC• Boost with Max Peak Power Tracking

• Will still charge when light low, Vsolar < Vmin for application to work

• May include cell balancing

• May include duty cycle regulation

37

Simplest Solar Charging Circuit

38

Single cell supercapacitor. No balancing. Starts charging from 0V

Vsolar – Vdiode ≥ Vload_min when the application needs to run

Vsolar_oc ≤ Vscap_rated at the maximum light level in the application (2.7V in this case) [Vdiode 0 as supercap fully charged]

D1 prevents the supercapacitor from discharging back into the solar cell when light levels fall

BAT46 chosen for D1 due to low VF. VF at currents < 10A, <0.1V

BAT46

Direct Charging Solar Charging Circuit

39

Single cell supercapacitor. No balancing. Starts charging from 0V

Vsolar – Vdiode ≥ Vload_min when the application needs to run

Shunt regulator limits Vload ≤ Vscap_rated.

Low current

No losses when Vsolar < Vscap_rated

D1 prevents the supercapacitor from discharging back into the solar cell when light levels fall

BAT46 chosen for D1 due to low VF. VF at currents < 10A, <0.1V

BAT46

Shunt Regulator

Direct Charge or use EH PMIC?

40

-50

0

50

100

150

200

250

0

0.5

1

1.5

2

2.5

3

0.00 10.00 20.00 30.00 40.00 50.00 60.00

Cu

rre

nt

(uA

)

Su

perc

ap

ac

ito

r vo

ltag

e

Time (hrs)

Comparison of Boost PMIC (AEM10941) and direct charging with indoor solar cell at 400 lux

APPEB1012: Charge with AEM10941

APPEB1011: Direct charge

Direct Charge Current

Direct charge supercapacitor is 0.262F

Capacitor charged with AEM10941 is 0.263F

If Voc > Vtargetat all light levels when app must run then direct charging is a good option.

Epishine Vocnot dropping significantly at lower light levels is suitable for direct charging.

PMIC has ~80% - 90% efficiency

LoRa Setup Supply voltage range, 3.3V – 1.8V

Use Epishine solar cell to directly charge a HA102 supercapacitor to 2.7V, using CAP-XX eval board APPEB1011, see www.cap-xx.com

Supercapacitor can support multiple transmissions

Regulate duty cycle so Vscap does not discharge below 2.4V

41

LoRa Module RFM9xDirect charging circuit

HA102, 240mF, 60mΩ single cell, 2.7V supercapacitor

Solar cell

CAP-XX controller PCB

Sizing the Supercapacitor: LoRa Transmission

42

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

2.4

2.45

2.5

2.55

2.6

2.65

2.7

2.75

2.8

-2 0 2 4 6 8 10

Lo

ad

Cu

rre

nt

(A)

Su

pe

rca

pa

cit

or

Vo

lta

ge

(V

)

Time (secs)

Sizing the Supercapacitor: discharge while tranmsitting

Vcap

Icap

Avge current = 10mA for 5.2sPeak Current = 62mA. PW 60ms

Set voltage range 2.7V - 2.4V

1st approx (ignore ESR)

C ≥ 10mA x 5.16s / 0.3V≥ 172mF

Select HA102, 240mF, 60mΩESR

dV = 10mA x 5.16s/0.24F + 0.062*0.06 = 0.22V

2.67V

2.42V

Supercapacitor Direct Charging Circuit

43

• AN1007 Charging a Supercapacitor from a Solar Cell Energy Harvester

• APPEB1011 Direct Solar Cell Charging• APPEB1012 Solar cell charging with PMICVisit www.cap-xx.com , register,

look at Design Services.

0

0.5

1

1.5

2

2.5

3

0:00:00 0:00:30 0:01:00 0:01:30 0:02:00 0:02:30 0:03:00 0:03:30 0:04:00 0:04:30 0:05:00 0:05:29 0:05:59

Su

perc

ap

ac

ito

r vo

ltag

e(V

)

Elapsed Time

LoRa demo Duty Cycle Regulation at 500lux

Supercapacitor charged, load connected

Transmission burst, supercapacitor discharging

Supercapacitor discharged to lower voltagethreshold, disconnect load.

Load disconnected, solar cell charging supercapacitor

Solar cell selected with direct charging of a CAP-XXX HA102 240mF single cell supercapaciror supports 4 transmission bursts lasting 5.2s every 2 mins

Operation

44

Duty cycle managed by µController, turn load on when supercap

reaches upper threshold, turn load off when supercap reaches

lower threshold.

Lower threshold set above brown out voltage – clean reset.

Interval between transmit bursts depends on light level, brighter

light → shorter interval

See the demo, visit us at booth 942

SMALL SOLAR CELL SUPPORTING CELLULAR IoT

NB IoT/LTE CAT M1 Case Study

45

NB IoT / LTE CAT M1 Setup Report messages from the Arduino µC managing a Ublox IoT module

Supply voltage range, 4.5V – 3.3V

Use GaAs solar cell to charge 2.5F supercapacitor to 4.5V with a PMIC using CAP-XX eval board APPEB1012, see www.cap-xx.com

46

Ublox IoT module

Charging circuit with PMIC

2 x 5F, 2.5V supercapacitor cells in series

Solar cell

CAP-XX controller PCB

Characterise your solar cell

47

Lightricity: GaAs solar cell, most efficient in indoor LED / CCFL light. 687µW @ 500 lux LED spectrum, 48mm x41mm, 34.9µW/cm2.

4s2p configuration, Voc < Vtarget, use a boost to charge the supercap

0

100

200

300

400

500

600

700

800

0

50

100

150

200

250

300

350

400

450

0 0.5 1 1.5 2 2.5 3

Po

we

r (u

W)

Cu

rre

nt

(uA

)

Voltage (V)

Lightricity 4S2P Solar Module

V-I @ 50 lux

V-I @ 200 lux

V-I @ 300 lux

V-I @ 500 lux

P-V @ 50 lux

P-V @ 200 lux

P-V @ 300 lux

P-V @ 500 lux

Selecting Your Charging IC

48

Attribute AEM10941 Comment

Min cold start

voltage

380mV The lower the better.

Cold start

charge

Rapid. Boost charges

22µF (typ) cap.

How the IC boosts the input voltage during cold start to reach the internal voltage

required to run as a boost.

Cold start

power

3µW The lower the better, but must be < power available from the solar cell at min light

levels at which the unit must charge.

Cold start

threshold

380mV Voltage at which the IC starts operating as a boost converter, the lower the better

Vin min after

start up

50mV Min i/p voltage for the boost to keep operating once it has started

Quiescent

current

< 1µA Current drawn by the IC while operating as a boost. Vbatt ≥ 2.5VThis is reflected in the

low power efficiency.

Max Peak

Power

Tracking

Samples Vsolar_oc

every 5s to set MPPT

Periodically disconnecting the i/p to sample Voc is the preferred method. MPPT then

set as % of Voc. Can set MPPT at 70%, 75%, 85%, 90% of Voc. Some ICs set this as

a fixed value which only works in constant light.

SCAP Bal. Yes Includes balancing cct for dual cell supercaps

Duty Cycle Ctrl Yes Status bit that reflects if supercap within desired voltage range

Efficiency ~90% at Peak Pwr Pt 90% is excellent efficiency at such low power. Vscap > Vsolar

Hysteric

operation

Yes The boost converter turns off when Vcap reaches its desired voltage and turns on

again when Vcap has discharged to a lower threshold. Saves power.

Max current 100mA From energy harvester to boost converter

Supercapacitor charging cct with PMIC

49

Refer to: AN1007 Charging a Supercapacitor from a Solar Cell Energy HarvesterAN1012 Supercapacitor powered NB IoT LTE CAT-M1User Manual for APPEB1012 Solar cell charging with PMIC

Duty cycle regulation integrated in PMIC

50

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0

0.5

1

1.5

2

2.5

3

-2.5 2.5 7.5 12.5 17.5 22.5

Cu

rre

nt

(A)

Vo

lta

ge

(V

)

Time (s)

APPEB1012 AEM10941 Duty Cycle Regulation

Vout

Vscap

STATUS[0]

STATUS[1]

Current

STATUS[0] turns on as Vscap > Vchrdy

STATUS[0] and STATUS[1] go LOW and LDO shuts down

Vscap < Vovdis, STATUS[1] turns on to warn of LDO shutdown in 600ms

STATUS[0] controls connection of load to supercap

Vscap > Vsolar, PMIC starts operating as a boost

Load Power & Energy, Sizing the Scap

51

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

-5 0 5 10 15 20 25 30 35 40 45

Lo

ad

Cu

rre

nt(

A)

Su

perc

ap

ac

ito

r V

olt

ag

e(V

)

Time(s)

LTE CAT-M1 Demo running with a 2.5F dual cell

Supercapacitor Voltage

Load Current

Avge Current = 63mA

Final peak = 180mA

Vdrop = 63mA x 36.6s / C < 4.5V - 3.3V = 1.2V

C > 0.063 x 36.6 / 1.2 = 1.92F

Select 2 x GY12R710020S505R, 5F, 170mΩ cells in series: C = 2.5F, ESR = 340mΩ

Vdrop = 0.063A x 36.6s / 2.5F + 0.18A x 0.34Ω = 0.98V

Vchrdy = 3.67V, Vovdis = 2.8V

Hard cutoff at 2.8V prevents brown out

Regulating the load, Tx interval

At 500 lux, solar cell delivers 687µW

Supercapacitor discharge (excl ESR drop) = 0.92V

Energy loss = ½ x 2.5F x (4.52 – 3.62) = 9.1J

Time to re-charge supercapacitor = 9.1J / 687µW = 13246s = 3.7hrs

Fine if this is suitable for whatever you are monitoring, e.g. a slowly moving variable

Otherwise increase power by: larger solar cell, more light, other energy source, etc.

Large energy requirement for Tx since unit disconnected between transmissions to reduce power, so need to log on to network every time. If you can increase EH power to support unit being always connected to the network, then energy for each transmission greatly reduced.

52

SUPPORTING a WiFi GATEWAY by WIRELESS CHARGING A

SUPERCAPACITOR

53

WiFi Gateway Setup Report light level from the Adafruit to a web page

Supply voltage range, 5V – 3.3V

Use NFC Wireless Charger, see App Notes AN1009 Wireless Charging and AN1014 Supercapacitor Powered Wifi IoT, www.cap-xx.com

54

WiFi moduleWireless charging circuit

2.5F, 5V supercapacitor module

NFC Transmitter 13.56MHz

CAP-XX controller PCB

Charging antenna

Characterising the Charger

55

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

Cu

rre

nt(

mA

)/P

ow

er(

mW

)

Voltage(v)

NFC coil V-I plot with custom NFC charger

I

P

Max pwr 94mW

V

I

Charging Circuit

56

Diode Bridge prevents

supercapacitor

discharging back into

the coil

Shunt regulator

protects supercapacitor

from over voltage.

Load Profile, Sizing the Supercapacitor

57

0

0.1

0.2

0.3

0.4

0.5

0.6

0

1

2

3

4

5

6

-2 0 2 4 6 8 10 12 14 16 18

Cu

rre

nt(

A)

Vo

ltag

e(V

)

Time(s)

WiFi IoT with 2.5F 5.5V module transmission waveform

V

I

Avge I = 73mA

Transmission time varies from 5s to 40s depending on signal strength and time taken to log on to WiFi network.

Average current during transmission = 73mA

Vdrop < 5V = 3.3V = 1.7V

Peak at the end = 88mA

C > 40s x 0.073A / 1.7V = 1.7F.

Select CAP-XX 5.5V 2.5F, 340mΩ module, GY25R51022S255R0.

Vdrop = 0.073A x 40s / 2.5F + 0.088A x 0.34Ω = 1.2V

Charging the supercapacitor

58

0

0.02

0.04

0.06

0.08

0.1

0.12

0

1

2

3

4

5

6

-20 30 80 130 180 230 280

Cu

rre

nt(

A)

Vo

ltag

e(V

)

Time(s)

NFC charging 2.5F Supercapacitor for WiFi IoT

Supercapacitor Voltage

Charging Current

COMING SOONTHIN 3V PRISMATIC CELLS

SAMPLES Q4 2019

We have 3V cylindrical cells now.

Thin prismatic cells are more challenging.

Ideal across CR2032 3V coin cells (no cct reqd) or with a small EH to store more energy or to enable direct charging.

59

3V cells, excellent life: 3V, 70C

60

0

20

40

60

80

100

120

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 200 400 600 800 1000 1200

ES

R(m

Ω)

Cap

ac

ita

nc

e (

F)

Elapsed Time (hrs)

Test Cells C & ESR over time at 3V, 70C

Sample 1 C

Sample 2 C

Sample 3 C

Sample 1 ESR

Sample 2 ESR

Sample 3 ESR

~13% Closs, IEC62391 Endurance spec is 30%

1.12 x ESRnit, IEC62391 Endurance spec is 4 x ESRinit

3V Supercapacitor Cell

Target specs:• 3V, -20C to +70C• 500mF• ESR < 200mΩ• IL < 2µA

HA130 size (20mm x 18mm x 1.7mm)• Fits over CR2032 coin cell

Place directly across a 3V coin cell• No extra circuit required

Higher energy for EH apps• 3V→ 2V 50% more energy

61

3V Battery

3V

scap

than 2.7V → 2V

Take Aways

62

Supercapacitors enable µPower energy harvesters to power wireless sensors with high power bursts

Supercapacitors are ideal power buffers• High C (enough energy to do the job)• Low ESR (high power delivery)• Wide temperature range of

operation• Easy to charge (show me the current)• Low leakage current & Excellent

round trip efficiency Supercapacitor Charging

• Into a S/C, from 0V, O/V protection, cannot discharge into the source

Characterise your Energy Harvester

Sizing the supercapacitor• Allow for I.ESR voltage drop• Constant current or constant power?• Ceff for narrow pulse• Easy to charge (show me the current)• Low leakage current & Excellent round

trip efficiency• Ageing (need real data, Arrhenius a

gross approximation) Case studies

LoRa Cellular IoT (LTE CAT M1) WiFi IoT

Visit www.cap-xx.com for App Notes, EVBs. Visit us at booth 942

63

For more information, contact:

Pierre Mars

VP Quality & Applicatons Engineering

pierre.mars@cap-xx.com

or visit us at:www.cap-xx.com