Aluminum Electrolytic Capacitors

Design and Characteristics Technology

10KV

1KV Film

100V

Ceramic VOLTAGE Aluminum 10V Tantalum Electrolytic Ta Polymer EDLC 1V 1 pF 100 pF 10 nF 1 uF 100 uF 10 mF 1 F

CAPACITANCE Aluminum Element

13 Al ALUMINUM 26.9815923

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

Useful properties of Al:

• Most abundant element to be found in earth’s crust (8.1%, after O & Si) ✓ Light weight; ✓ Good electrical conductivity • Reasonable corrosion resistance due to the ✓ Good thermal conductivity formation of air-formed Aluminum oxide in nature ✓ Non-toxic • Thicker Aluminum oxide using electrochemical ✓ Non-magnetic method (anodizing) ✓ No reaction with alcohol & other organic solvent • Aluminum oxide is a good dielectric substance ✓ High reflectivity of light ✓ Environmentally friendly

Slitting Winding Deck Welding

Cathode Foil

Anode Foil

Separator Paper

Foil Tabs

Safety Vent

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 The disadvantage of Surface “Etching” is the • Foil gains 50 to 100 (actual area / plain surface area) reduction of the capacitors ability to withstand HIGH DC currents.

Surface of foil

Dark area in the 3500 times middle is solid core (Al) magnification

Al Anode Foil Capacitance Evolution Foil Capacitance Evolution (550 VF) ~ Foil for 400V Capacitor ~ Historic Current & Year (µF/cm2) 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

Aluminium Oxide Dielectric Electrolyte Aluminium Oxide Dielectric

Aluminum Foil Aluminum Foil Anode Plate Cathode Plate

 0

Separator

• 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

• 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

➢ 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

Reform Aluminum -2 O Wet

Anode O-2 Dielectric

Reduction Electrolyte Plate

Reformed dielectric region

• 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

• 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

➢ 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)

1 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

2 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

3 Field work

• Capacitance

• Equivalent Series Resistance, ESR

• DC Leakage

• Shelf Life & Reforming

• Operational Lifetime

• End of Life

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). 푸 = 푪 × 푽 Rleak depends on the quality of the dielectric. Rleak

ESL ESR

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.

Effect of Frequency Change: 1 퐶 = (2휋푓푍) ➢ Effective capacitance reduces as frequency increases

Effect of Temperature Change:  1   (Al O ) = f   r 2 3 T  1  r 0 A A (Al O ) C = contacted 2 3   1  d  = f    T 

➢ Effective capacitance reduces as temperature decreases (Capacitance change with temperature is greater for lower rated voltages) © 2019 KEMET Corporation Capacitance and Capacitance Variation Total 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: 푪 푪 푪 = 푨 × 푲 풆 − 풄풂풑 푪푨 + 푪푲 ퟏ ퟏ ퟏ = + CK is much higher than CA 푪풕풐풕풂풍 푪풂풏풐풅풆 푪풄풂풕풉풐풅풆

➢ 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.

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] ퟏ 풇 - Frequency [Hz] 푪 = Z - Impedance [Ω] ퟐ흅풇풁

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.

4700uF/400V, 85°C Screw Terminal 10,000

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 1,000 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 Characteristics1000. 00of41 00Wet.00 4125 .00 4200.00 100 2000.00 3650.00 3725.00 3800.00 20C 4000.00 3000.00 3070.00 3400.00 Technology10000.00 1500.00 1750.00 2500.00 85C 20000.00 800.00 1050.00 1700.00

40000.00 400.00 550.00 1000.00 Capacitance [uF] Capacitance 100000.00 150.00 225.00 400.00 50C 1000000.00 10.00 18.00 40.00 10

1 10 100 1,000 10,000 100,000 1,000,000 Frequency [Hz]

Cause of Cap Change: Solution: CAP Chemical changes within the • More stable electrolytes electrolyte and drying • Lower gassing & diffusion rates

• Improved quality anode foil Accelerated with giving a lower leakage current leakage current • Consistent quality anode foil

Ripple current and ESR causes • Lower ESR designs power loss giving higher • Improved thermal conductivity Time temperature

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:

tan δ ESR: Equivalent Series Resistance Ω 푬푺푹 = tan δ: Dissipation Factor ω × 푪 ω: Angular Frequency rad/s 푺 CS: Series Capacitance F

Lower ESR, Higher Current

Higher ESR, Lower Ripple Current

• 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 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.

4700uF/400V, 85°C Screw Terminal 40

ESR Contributors: 35 20°C 50°C 85°C 20.00 34.76 28.84 30.22 25.00 31.28 24.21 24.84 • Electrolyte 35.00 27.13 19.51 19.15 30 50.00 23.88 15.76 14.78 60.00 22.64 14.30 13.00 • Oxide 70.00 21.74 13.28 11.88 80.00 21.07 12.53 10.97

] 25 100.00 20.11 11.43 9.71 • Tabbing 200.00 18.37 9.33 7.23 400.00 17.50 8.31 6.02 20 1000.00 16.94 7.70 5.32 20C • Tissue Density mOhm 2000.00 16.72 7.55 5.10 4000.00 16.51 7.47 5.03 50C 10000.00 16.39 7.42 5.04 ESR [ ESR 15 • Thickness 20000.00 16.41 7.55 5.15 85C 40000.00 16.60 7.88 5.49 100000.00 17.26 8.98 6.49 10

0 10 100 1,000 10,000 100,000 Frequency [Hz]

3.5 3 • Distorted polarization of dielectric (Aluminum oxide layer) 2.5 2 • Resolution and formation of dielectric 1.5 1 • Moisture absorption by dielectric 0.5

Leakage Current (mA) Current Leakage

24 47 70 93

323 139 162 185 208 231 254 277 300 346 369 • Breakdown of dielectric due to the existence of chlorine or iron particles 116 Hours of Discharge Leakage Current ➢ The DC leakage current is a small current that flows through a capacitor when voltage is Temperature applied, between the two conductive plates.  ➢ Leakage current is primarily caused by imperfections in the oxide layer. This current varies Voltage mainly depending on the applied voltage, time, and capacitor temperature. 

➢ The leakage current of an Aluminum electrolytic capacitor increases when the component is Ripple Current stored for a long period of time. 

➢ The specified leakage current value is measured after the rated voltage of the capacitor is Time under Voltage applied at room temperature for a specified time period.  Storage Time (0V) © 2019 KEMET Corporation  DCL – DC Leakage Dependencies

1 2 3

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.

Temperature Frequency Voltage Time under Increases Increases Increases Voltage Capacitance   ~ ~ ESR   ~ ~ Leakage Current  ~  

• Catastrophic Failure: − Open or short circuit

• Mechanical Failure: − Operation of safety vent often seen as split sleeve Comparisons between capacitor manufacturers • Parametric Failure*: should be made using − Capacitance change 25-100V – ±20%; +100V 15% the same criteria. − ESR >3x limit or − Impedance >3x limit or − Leakage current > specified limit

• Ta → Ambient Temperature

• T → Core Temperature c Airflow significantly increases the life of • V → Operational Voltage op the capacitor. • Ripple Currents at Frequencies

If we use the derivative of Arrhenius Law, we can predict the impact of change in temperature on the expected life of the capacitor. (푇푚푎푥 −푇푎푚푏) 퐿표푝 = 퐿표푝푟 × 2 × 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

Calculation of Hot-Spot Temperature From previous slide we see:

2 푃퐿표푠푠 = 푅 × 퐼푎푐

• Since ESR is dependent on frequency 푓, our 푃퐿표푠푠 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.

2 2 푃퐿표푠푠 = 퐸푆푅 푓1 × 퐼푎푐 푓1 + 퐸푆푅 푓2 × 퐼푎푐 푓2 + ⋯ (푊)

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

Thot-spot = Tamb + 푃퐿표푠푠 x Rth

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

4700uF/350V, 85°C Screw Terminal 30

15 Life Time [y] Time Life

0 60 65 70 75 80 85 90 T hot-spot [°C]

Screw Screw

Thermocouples: Snap-In

We recommend to measure the temperature in the middle of the cap and also the ambient near the parts.

Extended cathode

Anode foil

Foil tabs Laser welded tabs to deck

Tissues

Cathode foil • Can spigot for supporting wind • Ribs for thermal connection of extended cathode • Anode formation type (oxide layer) • Number of tab foil connections • Reduce ESR

Voltage Derating Le(Vop) = Le(VR) x Kv

• If Electrolytic capacitors are operated at a voltage below their rated value then the component will be Le - Life expectancy at operating voltage under less operating stress. (Vop) Le(VR) - Life expectancy at rated voltage K - Voltage derating factor • Reduced stress and lower leakage current v 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. Life Time Calculation Voltage vs. Temperature

Voltage Derating

Le(Vop) = Le(VR) x Kv

Le(Vop) - Life expectancy at operating voltage Le(VR) - Life expectancy at rated voltage Kv - Voltage derating factor

Rated: 400V, 105⁰C, 7,000h

Operational voltage de-rated for 15% → 340V

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

Recommended derating %

Voltage derating factor (Kv) for products 7,000 X 1.4 with a rated temperature of 85°C and core temperatures (Tc ) of 45°C, 65°C and 85°C 10,000 h

• 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

Theoretical Simulation: Samples:

➢ Thermal conditions (ambient, core temperature) 1 Samples for testing with thermocouples Measuring the core temperature of the capacitor ➢ Ripple currents/frequencies 2 ➢ ESR, impedance Performance in a real application environment ➢ Operational life 3 ➢ Airflow Test data for evaluation

Market Segments Applications Features & Benefits • Automotive • UPS Systems • Long Life Expectancy • Power Supplies • Industrial • High Transient Voltage Performance • Data Storage • Alternative Energy • High Ripple Current • Smart Meters • Medical • Wide Range of Mounting & Assembly Options • Wind & Solar • Consumer • Full Design Flexibility / Customized Solutions • Computer • Drives • Leading the Design In – Fast Approach • Transportation • Welding Electrolytic Product Portfolio

Press-Fit Varistors Axial/Radial Screw Terminal Snap-In

Conductive Polymer Conductive Polymer Relays SMD Radial / Single Ended Single Ended SMD