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Aluminum Electrolytic Capacitors

Design and Characteristics

1V

10V

1 pF 100 pF 10 nF 1 uF 100 uF 10 mF 1 F

100V

1KV

10KV

VO

LT

AG

E

Ceramic

Film

C A P A C I T A N C E

EDLC

Tantalum

Ta Polymer

Technology

AluminumElectrolytic

© 2019 KEMET Corporation

Aluminum Element

13

Al

ALUMINUM

26.9815923

Aluminum: Chemical Symbol: Al Atomic Number: 13Atomic Mass: 26.9815923Discovered in 1825


AluminumProperties

• Most abundant element to be found in earth’s crust

(8.1%, after O & Si)

• Reasonable corrosion resistance due to the

formation of air-formed Aluminum oxide in nature

• Thicker Aluminum oxide using electrochemical

method (anodizing)

• Aluminum oxide is a good dielectric substance

Useful properties of Al:

Light weight;

Good electrical conductivity

Good thermal conductivity

Non-toxic

Non-magnetic

No reaction with alcohol & other organic solvent

High reflectivity of light

Environmentally friendly


Aluminum ElectrolyticManufacturing Process

Testing

Standard MeasurementsAgingSealingImpregnation

Deck WeldingWindingSlitting


Aluminum Electrolytic CanConstruction

Cathode

Foil

Anode

Foil

Separator

Paper

Foil

Tabs

Safety

Vent


Anode Foil Etching

Super pure plain aluminum foil (70 – 125 µm thick) undergoes

electrochemical reactions to dissolve the metal substrate of the

foil in the form of a dense network of microscopic channels, in

order to increase the overall surface area providing a maximum

capacitance for a given electrode surface size.

• Electrochemical tunnel “Etching” of 99.99% aluminum

• Tunnel initiation strongly influenced by impurities

• Tunnel diameter 1um - 2um across

• Tunnel length 15um - 50um long

• Density 10 - 25 million tunnels per cm2

• Foil gains 50 to 100 (actual area / plain surface area)The disadvantage of Surface “Etching” is the

reduction of the capacitors ability to

withstand HIGH DC currents.


Anode FoilFormed Cross-Section

Surface of foil

Dark area in the

middle is solid core (Al)

3500 times

magnification


Foil Technology Evolution

Foil Capacitance Evolution (550 VF)

YearHistoric (µF/cm2)

Current & Prediction (µF/cm2)

1993 0.57

1994 0.63

2001 0.65

2003 0.68

2008 0.71

2011 0.74

2015 0.77

2018 0.79

2019 0.83

Al Anode Foil Capacitance Evolution

~ Foil for 400V Capacitor ~


• By etching the anode foil, we increase the capacitance by increasing the surface area

• Thicker dielectric reduces maximum capacitance potential

• Dielectric thickness determines voltage rating

Aluminum ElectrolyticBasic Model

0

Electrolyte

Aluminum Foil

Cathode Plate

Aluminium Oxide Dielectric

Aluminum Foil

Anode Plate

Aluminium Oxide

Dielectric

Separator


Electrolyte Composition

• High conductivity, neutral pH

– Acid + Base -> Salt

• Wide operational temperature range

– Stay conductive across range

• Provides ability to reform oxide

– Controlled level of water

• Compatibility with paper and deck material

– Hydrogen gas absorber

• Low flammability, low toxicity


Aluminum

Anode

Plate

Forward bias is same as formation bias.

If dielectric gets thin enough, the forward bias voltage will form new dielectric

Thinner than original because VFormation > VRating > Vapplication

Wet

Electrolyte

Reformed dielectric region

Die

lectr

ic

Red

ucti

on

O-2

O-2

Reform

Traditional Aluminum ElectrolyticThe Electrolyte Allows for the Dielectric to Reform (or “Heal”)


Separator Papers

• Materials (pulp)

– Kraft, manila, esparto, hemp

– Combinations of pulp are often used

• Properties

– Thickness 12µm to 90µm

– Density 0.3 to 0.8 g/cm3, uniform density, minimum pinholes

– Simplex – single type of paper

– Triplex – three papers joined together

• Usage

– 1 to 3 papers used between anode and cathode

– Up to 5-6 actual layers when triplex paper used


Construction

• Can– Diameter and Length

– Vent

– Mounting

• Common Termination styles– Screw Terminals

– 2-5 pin Solder Pin, Pin Tag, Snap-In

– Axial, Radial

– SMD

– Press-Fit

• Sleeve– PVC

– PET; UL recognized

– Polyolefin


New Press-Fit Termination

Capacitor is pressed into the PCB, not soldered

Specific poke yoke terminations

Eliminates the problems of soldering on thick PCB copper tracks

Eliminates fractured solder joints

Quick exchange of components

Press-Fit Pin Material

Material:

Copper Nickel Silicon Alloy CuNiSi R580 (C19010)

Plating:

• Ni 1.5-3μ all over

• Sn 100% mat 0.4-1.1μ on press-fit area

• Sn 100% mat 3-6μ on the remaining area

16© 2019 KEMET Corporation

Press-FitSolution for Multiple Issues

Soldering Problems

• Heavy copper tracking on the PCB acts as a heat sink which makes soldering difficult

• This can cause cold spots, voids, splatter, cracks etc

Washing Issues

• More aggressive washing of pcb after reflow soldering can force water

under insulating sleeves of electrolytics

• Avoiding reflow / washing means expensive and additional processes

such as hand soldering, automated selective soldering, or hand washing

of components

Field work

• Preventive maintenance of soldered components often means replacing the entire

(and costly) PCB

1

2

3


Characteristics


Electrolytic Parameters

• Capacitance

• Equivalent Series Resistance, ESR

• DC Leakage

• Shelf Life & Reforming

• Operational Lifetime

• End of Life


Terms and Definitions


Basics

The basic principle of the capacitor is to store electrical charge

(Q in coulombs). The potential charge it can hold is determined

by the capacitance (C in Farads) and voltage (V in volts)

An electric equivalent schema of an electrolytic capacitor can be

described as an equivalent series resistance (ESR), equivalent

series inductance (ESL), the capacitance (C) and a parallel

resistance for the leakage current (Rleak).

Rleak depends on the quality of the dielectric.𝑸 = 𝑪 × 𝑽

ESL

Rleak

ESR

C


Electrolytic ParametersCapacitance (RC-Ladder)

The RC-Ladder is an effect of the tunnel

created to increase the area of the foil.

The capacitive elements are distributed

along the walls to the bottom of the tunnel,

connected to a cathode extension created

by the electrolyte

As frequency increases more capacitive

elements will ‘drop out’, eventually getting

to point where only those elements near the

surface of the foil

The electrolyte (or cathode) contact has a

resistance that is extremely dependent on

temperature.

As the temperature decreases, the mobility

of the ions in the electrolyte slow down.

Because of this the capacitance drop occurs

at lower frequencies for lower temperatures.


Effective capacitance reduces as temperature decreases

(Capacitance change with temperature is greater for lower rated voltages)

Effect of Frequency Change:

Effective capacitance reduces as frequency increases

𝐶 =1

(2𝜋𝑓𝑍)

d

AC r 0=

( )

=

TfOAlr

132

( )

132 OAlAcontacted

=

Tf

1

Electrolytic Parameters Capacitance Change - Frequency and Temperature

Effect of Temperature Change:


Capacitance and Capacitance VariationTotal Capacitance of the Capacitor

By design, a non-solid aluminum electrolytic capacitor has a second aluminum foil, the so-called

cathode foil, for contacting the electrolyte. This structure of an aluminum electrolytic capacitor results

in a characteristic result because the second aluminum (cathode) foil is also covered with an insulatin

oxide layer naturally formed by air. Therefore, the construction of the electrolytic capacitor consists of

two single series-connected capacitors with capacitance CA of the anode and capacitance CK of the

cathode. The total capacitance of the capacitor Ce-cap is thus obtained from the formula of the series

connection of two capacitors:

CK is much higher than CA

𝑪𝒆 − 𝒄𝒂𝒑 =𝑪𝑨 ×

𝑪𝑲𝑪𝑨+ 𝑪𝑲

𝟏

𝑪𝒕𝒐𝒕𝒂𝒍=

𝟏

𝑪𝒂𝒏𝒐𝒅𝒆+

𝟏

𝑪𝒄𝒂𝒕𝒉𝒐𝒅𝒆


Capacitance and Capacitance VariationTemperature Dependencies

The temperature has a considerable

effect on the capacitance. With

decreasing temperature, the viscosity

of the electrolyte increases, thus

reducing its conductivity.

The resulting typical behavior is

shown in the figure.


Capacitance and Capacitance VariationFrequency Dependencies

Frequency Dependence of the Capacitance

The AC capacitance depends not only on the temperature but also on the measuring frequency. The

figure below shows the typical behavior. Typical values of the effective capacitance can be derived

from the impedance curve, as long as the impedance is still in the range where the capacitive

component is dominant.

C - Capacitance [F]

𝒇 - Frequency [Hz]

Z - Impedance [Ω]

Standardized measuring conditions for electrolytic capacitors are an AC

measurement with 0.5V at a typically frequency of 100Hz or 120Hz and

a temperature of 20°C.

𝑪 =𝟏

𝟐𝝅𝒇𝒁


Electrolytic ParametersCapacitance Change - Frequency

Freq 20C 50C 85C

20.00 4278.11 4378.17 4504.13

25.00 4271.29 4370.10 4492.97

35.00 4261.24 4356.87 4477.68

50.00 4250.85 4343.88 4461.13

60.00 4245.75 4337.37 4452.97

70.00 4241.73 4331.77 4445.95

80.00 4238.24 4327.49 4440.40

100.00 4232.13 4319.64 4429.39

200.00 4215.23 4296.63 4400.46

400.00 4199.96 4278.80 4375.35

1000.00 4100.00 4125.00 4200.00

2000.00 3650.00 3725.00 3800.00

4000.00 3000.00 3070.00 3400.00

10000.00 1500.00 1750.00 2500.00

20000.00 800.00 1050.00 1700.00

40000.00 400.00 550.00 1000.00

100000.00 150.00 225.00 400.00

1000000.00 10.00 18.00 40.00

1

10

100

1,000

10,000

10 100 1,000 10,000 100,000 1,000,000

Cap

acit

ance

[u

F]

Frequency [Hz]

4700uF/400V, 85°C Screw Terminal

20C

85C

50C

Characteristics of Wet

Technology


Electrolytic Parameters Capacitance over time

CAP

Time

Cause of Cap Change: Solution:

Chemical changes within the

electrolyte and drying

• More stable electrolytes

• Lower gassing & diffusion rates

Accelerated with

leakage current

• Improved quality anode foil

giving a lower leakage current

• Consistent quality anode foil

Ripple current and ESR causes

power loss giving higher

temperature

• Lower ESR designs

• Improved thermal conductivity


ESR – Equivalent Series Resistance

The equivalent series resistance is the resistive component of the equivalent series circuit. The ESR value

depends on frequency and temperature and is related to the dissipation factor by the following equation:

Lower ESR, Higher Current

Higher ESR, Lower Ripple Current

𝑬𝑺𝑹 =tan δ

ω × 𝑪𝑺

ESR: Equivalent Series Resistance Ω

tan δ: Dissipation Factor

ω: Angular Frequency rad/s

CS: Series Capacitance F


ESR – Equivalent Series ResistanceMain Contributors

• Oxide Resistance‒ Determined by oxide thickness, oxide characteristics

and surface area

‒ Frequency dependent

• Electrolyte + Paper Resistance‒ Determined by electrolyte and paper characteristics

and surface area

‒ Temperature dependent

• Foil Resistance‒ Determined by foil type and length

‒ Temperature & frequency independent

• Tabbing & Connection Resistance‒ Determined by number of tabs & construction method

‒ Temperature & frequency independent


• The equivalent series resistance (ESR) is a single resistance representing all of the ohmic losses of the

capacitor and connected in series with the capacitance.

• Standardized measuring conditions for electrolytic capacitors are AC measurement with maximum 1 V at

a typical frequency of 100 Hz or 120Hz and a temperature of 20 °C.

ESR – Equivalent Series ResistanceMain Contributors and Dependencies


ESR – Equivalent Series ResistanceDependencies

ESR decreases with temperature rising which causes the:

• Electrolyte conductivity to increase.

• Electrolyte viscosity to decrease.

ESR decreases with frequency increment which has a:

• Direct influence of the oxide resistance at low

frequencies.


Electrolytic ParametersESR Change (Frequency & Temperature)

ESR Contributors:

• Electrolyte

• Oxide

• Tabbing

• Tissue Density

• Thickness

20°C 50°C 85°C

20.00 34.76 28.84 30.22

25.00 31.28 24.21 24.84

35.00 27.13 19.51 19.15

50.00 23.88 15.76 14.78

60.00 22.64 14.30 13.00

70.00 21.74 13.28 11.88

80.00 21.07 12.53 10.97

100.00 20.11 11.43 9.71

200.00 18.37 9.33 7.23

400.00 17.50 8.31 6.02

1000.00 16.94 7.70 5.32

2000.00 16.72 7.55 5.10

4000.00 16.51 7.47 5.03

10000.00 16.39 7.42 5.04

20000.00 16.41 7.55 5.15

40000.00 16.60 7.88 5.49

100000.00 17.26 8.98 6.49

0

5

10

15

20

25

30

35

40

10 100 1,000 10,000 100,000

ESR

[m

Oh

m]

Frequency [Hz]


20C

50C

85C


• Distorted polarization of dielectric (Aluminum oxide layer)

• Resolution and formation of dielectric

• Moisture absorption by dielectric

• Breakdown of dielectric due to the existence of chlorine or iron particles

Electrolytic Parameters Leakage Current – Main Causes in Electrolytic Capacitors

Leakage Current

Temperature

Voltage

Ripple Current

Time under Voltage

Storage Time (0V)

The DC leakage current is a small current that flows through a capacitor when voltage is

applied, between the two conductive plates.

Leakage current is primarily caused by imperfections in the oxide layer. This current varies

mainly depending on the applied voltage, time, and capacitor temperature.

The leakage current of an Aluminum electrolytic capacitor increases when the component is

stored for a long period of time.

The specified leakage current value is measured after the rated voltage of the capacitor is

applied at room temperature for a specified time period.

0

0.5

1

1.5

2

2.5

3

3.5

124

47

70

93

116

139

162

185

208

231

254

277

300

323

346

369Le

akag

e C

urr

en

t (m

A)

Hours of Discharge


DCL – DC LeakageDependencies

1. DCL increases with temperature rising and the internal temperature TCore must be considered.

2. DCL decreases with time and reaches a stable value which remains quite constant for a long period of time.

3. The leakage current is an increasing function of the applied voltage Vop, and rises quite strongly when Vop exceeds the

rated value VR.

1 2 3


Electrical Characteristics Summary

Temperature Increases

Frequency Increases

Voltage Increases

Time under Voltage

Capacitance ~ ~

ESR ~ ~

Leakage Current ~


End of Life Criteria

• Catastrophic Failure:

− Open or short circuit

• Mechanical Failure:

− Operation of safety vent often seen as split sleeve

• Parametric Failure*:

− Capacitance change 25-100V – ±20%; +100V 15%

− ESR >3x limit or

− Impedance >3x limit or

− Leakage current > specified limit

Comparisons between

capacitor manufacturers

should be made using

the same criteria.

From catalog *


Expected LifeParameters

• Ta → Ambient Temperature

• Tc → Core Temperature

• Vop → Operational Voltage

• Ripple Currents at Frequencies

Airflow significantly

increases the life of

the capacitor.


Expected LifeTemperature

If we use the derivative of Arrhenius Law, we can predict the impact of change in

temperature on the expected life of the capacitor.

𝐿𝑜𝑝 = 𝐿𝑜𝑝𝑟 × 2 ×(𝑇𝑚𝑎𝑥 −𝑇𝑎𝑚𝑏)

10

• Since we understand that temperature plays such a large factor, we should also factor in the losses (self-heating)

of the capacitor.

• Ripple current is the main contributor to power loss and self-heating.

• The temperature in the hottest area inside the capacitor, “the hot-spot” has most impact on operational life.

• The “hot-spot” temperature is dependent upon several factors:

‒ Power loss caused by ripple current

‒ Thermal resistance between the hot-spot and the ambient

‒ Ambient temperature and capacitor cooling condition


Life Time CalculationHot-Spot Temperature

Calculation of Hot-Spot Temperature

From previous slide we see:

𝑃𝐿𝑜𝑠𝑠 = 𝑅 × 𝐼𝑎𝑐2

• Since ESR is dependent on frequency 𝑓, our 𝑃𝐿𝑜𝑠𝑠 should also consider the

complex current waveform.

• With complex current waveform it is therefore necessary to calculate the

contribution from each harmonic frequency to the power loss.

• Once we have our PLoss value we can calculate the Thot-spot temperature.

Thot-spot = Tamb + 𝑃𝐿𝑜𝑠𝑠 x Rth

𝑃𝐿𝑜𝑠𝑠 = 𝐸𝑆𝑅 𝑓1 × 𝐼𝑎𝑐2 𝑓1 + 𝐸𝑆𝑅 𝑓2 × 𝐼𝑎𝑐2 𝑓2 +⋯(𝑊)


Life Time CalculationThermal Model

The thermal resistance Rth, (°C/W) is defined from the power loss (P) and

temperature difference.

ΔT, between the hot-spot and the ambient temperature

ΔT = P x Rth

ΔT = Th–Ta

Power (P) is assumed to be generated in the hot-spot.

Rth (total thermal resistance) can be divided in two parts:

• Rthhc → thermal resistance between the hot-spot and the case

dependent on the capacitor design

• Rthca → thermal resistance between the case and the ambient

dependent on cooling conditions


Life Time CalculationHot-Spot Temperature

0

5

10

15

20

25

30

60 65 70 75 80 85 90

Life

Tim

e [y

]

T hot-spot [°C]



Thermocouples:

We recommend to measure the temperature in the middle

of the cap and also the ambient near the parts.

Snap-In

Screw Screw

Operational Life Time and Hot Spot CalculationHow to Measure the Core Temperature


Foil tabs

Tissues

Cathode foil

Anode foil

Extended cathode

Laser welded tabs to deck

• Can spigot for supporting wind

• Ribs for thermal connection of extended cathode

• Anode formation type (oxide layer)

• Number of tab foil connections

• Reduce ESR

Operational Life Time and Hot Spot CalculationHow to Improve Life

Life Time CalculationVoltage vs. Temperature

Voltage Derating

• If Electrolytic capacitors are operated at a voltage below their rated value then the component will be under less operating stress.

• Reduced stress and lower leakage current provides an improvement in the life expectancy.

• Since leakage current increases with temperature, the benefit of a reduced operating voltage is more pronounced at higher temperatures.

Le(Vop) - Life expectancy at operating voltage

Le(VR) - Life expectancy at rated voltage

Kv - Voltage derating factor

Le(Vop) = Le(VR) x Kv


Life Time CalculationVoltage vs. Temperature

Voltage derating factor (Kv) for products with a rated temperature of 85°C and core temperatures (Tc ) of 45°C, 65°C and 85°C

Voltage Derating

Rated: 400V, 105⁰C, 7,000h

Operational voltage de-rated for 15% → 340V

At 105⁰C, the Kv: 1.4 (provided by KEMET)

Le(Vop) - Life expectancy at operating voltage

Le(VR) - Life expectancy at rated voltage

Kv - Voltage derating factor

Le(Vop) = Le(VR) x Kv

Recommended derating %

7,000 X 1.4

10,000 h

ECAD

Electrolytic Innovation Center


ECAD – Electrolytic Innovation Center

• In-house developed integrated design system

• Product design, specification & costing

• Design history, issue & control

• Bills of material & routings

• Integrated with Oracle manufacturing

• Manufacturing batch cards

48© 2019 KEMET Corporation

ECAD: Screw Terminal, Snap-In & Press-Fit

Theoretical Simulation:

Thermal conditions (ambient, core temperature)

Ripple currents/frequencies

ESR, impedance

Operational life

Airflow

Samples for testing with thermocouples

Measuring the core temperature of the capacitor

Performance in a real application environment

Test data for evaluation

1

2

3

Samples:


Electrolytic Capacitor Life Calculatorengineeringcenter.com

Market & Applications Features & Benefits

Applications

• UPS Systems

• Power Supplies

• Data Storage

• Smart Meters

• Wind & Solar

• Drives

• Transportation

• Welding

Features & Benefits

• Long Life Expectancy

• High Transient Voltage Performance

• High Ripple Current

• Wide Range of Mounting & Assembly Options

• Full Design Flexibility / Customized Solutions

• Leading the Design In – Fast Approach

Market Segments

• Automotive

• Industrial

• Alternative Energy

• Medical

• Consumer

• Computer


Electrolytic Product Portfolio

Axial/Radial Screw Terminal Snap-In

Radial / Single Ended SMD

Press-Fit

Conductive Polymer

Single EndedConductive Polymer

SMD

Varistors

Relays


Thank You!

aluminum electrolytic capacitors - ec.kemet.com · total capacitance of the capacitor by design, a...

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