AEappGUIDE

1 Aluminum Electrolytic Capacitor Application Guide This guide is a full handbook on aluminum electrolytic capacitors, of course with emphasis on Cornell Dubilier’s types. It covers construction in depth and discloses the latest information on performance and application for the major aluminum electrolytic types made worldwide. We encourage you to tell us what more you’d like to know, so we can improve this guide. CONTENTS PAGE Capacitor Construction 2 Other Types of Capacitors Comparison 4 Characterization and Circuit Model 5 TABLES PAGE Temperature Range 6 Capacitor Parameter Formulas 6 Capacitance 7 Base Lives and Max Core Temperatures 14 Dissipation Factor (DF) 7 Thermal Resistance Screw Terminal Capacitors 17 Equivalent Series Resistance (ESR) 8 Thermal Resistance for Snap-in Capacitors 19 Impedance (Z) 8 Pressure Relief Devise Clearance 21 Low-Temperature Impedance 8 Screw Tightening Torque for Screw Terminals 21 DC Leakage Current (DCL) 8 Maximum Currents for Screw Terminals 21 Voltage Withstanding 9 Tightening Torque for Nylon Mounting Nuts 22 Ripple Current 10 Inductance 10 Self-Resonant Frequency 10 Dielectric Absorption 11 Insulation and Grounding 11 Elevation & External Pressure 11 Vibration Withstanding Capability 11 Safety Considerations 11 Capacitor Bank Configurations 12 Non-Polar and Motor Start Capacitors 13 Reliability and Lifetime 13 Cooling and Thermal Resistance 16 Process Considerations 19 Mounting 20 Disposal of Capacitors 22 ALUMINUM ELECTROLYTIC CAPACITOR OVERVIEW Except for a few surface-mount technology (SMT) aluminum electrolytic capacitor types with solid electrolyte systems an aluminum electrolytic capacitor consists of a wound capacitor element, impregnated with liquid electrolyte, connected to terminals and sealed in a can. The element is comprised of an anode foil, paper separators saturated with electrolyte and a cathode foil. The foils are high-purity aluminum and are etched with billions of microscopic tunnels to increase the surface area in contact with the electrolyte. While it may appear that the capacitance is between the two foils, actually the capacitance is between the anode foil and the electrolyte. The positive plate is the anode foil; the dielectric is the insulating aluminum oxide on the anode foil; the true negative plate is the conductive, liquid electrolyte, and the cathode foil connects to the electrolyte. However, just as the anodic-oxide dielectric insulates the anode foil from the electrolyte, so too the cathode is insulated from the electrolyte by the low voltage air oxide on the cathode foil and the double-layer ionic barrier. This makes the cathode a capacitor in series with the anode. In high voltage capacitors the cathode capacitance is hundreds of times the anode capacitance and does not measurably affect the overall capacitance, but in capacitors of less than about 50 V the anode capacitance begins to approach the value of the cathode capacitance and requires use of higher capacitance cathode to avoid needing to increase the anode length to achieve the rated 2 Miniature, Radial-Leaded Type Lead Wire Aluminum Tabs Rubber Sleeve over Aluminum Can Capacitor Element Snap-in Type Sleeve over Aluminum Can Terminal Aluminum Tabs Phenolic/ Rubber Disc Capacitor Element Rivet Tape capacitance. Aluminum electrolytic capacitor construction delivers colossal capacitance because etching the foils can increase surface area more than 100 times and the aluminum- oxide dielectric is less than a micrometer thick. Thus the resulting capacitor has very large plate area and the plates are intensely close together. These capacitors routinely off er capacitance values from 0.1 µF to 3 F and voltage ratings from 5 V to 550 V. Up to 700 V are commercially available. They are polar devices, having distinct positive and negative terminals, and are off ered in an enormous variety of styles which include molded and can-style SMT devices, axial- and radial-leaded can styles, snap-in terminals styles and large-can, screw-terminal styles. Representative capacitance-voltage combinations include: 330 µF at 100 V and 6,800 µF at 10 V for SMT devices, 100 µF at 450 V, 6,800 µF at 50 V and 10,000 µF at 10 V for miniature-can styles, 1200 µF at 450 V and 39,000 µF at 50 V for snap-in can styles and 9000 µF at 450 V and 390,000 µF at 50 V for large-can, screw-terminal styles. If two, same-value, aluminum electrolytic capacitors are connected in series with the positive terminals or the negative terminals connected together, the resulting single capacitor is a non-polar capacitor with half the capacitance. The two capacitors rectify the applied voltage and act as if they had been bypassed by diodes. When voltage is applied, the correct- polarity capacitor gets the full voltage. In non-polar aluminum electrolytic capacitors and motor-start aluminum electrolyte capacitors a second anode foil substitutes for the cathode foil to achieve a non-polar capacitor in a single case. Snap-in Type CAPACITOR CONSTRUCTION Tabs Phenolic / Nylon Cover w/ Centering Peg Aluminum Centering Peg Sleeve Over Aluminum Can Extended Cathode Terminals Centering Peg Aluminum Centering Peg Rills Rubber Gasket Tape Aluminum Stiffening Ribs Tabs Phenolic / Nylon Cover w/ Centering Peg Aluminum Centering Peg Sleeve Over Aluminum Can Extended Cathode Terminals Rubber Gasket Tape Aluminum Stiffening Ribs Thermal Pak Construction Rilled Construction Thermal Pak Construction Rilled Construction 3 These fi gures show typical constructions of the non-surface- mount aluminum electrolytic capacitors. All Cornell Dubilier capacitors use compression-fi t construction so there is no thermoplastic potting compound to interfere with safety- vent operation. Thermal Pak™ and Rilled are Cornell Dubilier’s exceptional constructions for screw terminal capacitors. Compared to conventional, potted construction, Thermal Pak operates cooler, provides longer life, withstands higher shock and vibration, delivers more reliable safety vent operation and is lighter weight. Rilled off ers superior shock and vibration withstanding, typically withstanding more than 15 g acceleration forces. ETCHING The anode and cathode foils are made of high purity, thin aluminum foil, 0.02 to 0.1 mm thick. To increase the plate area and the capacitance, the surface area in contact with the electrolyte is increased by etching the foils to dissolve aluminum and create a dense network of billions of microscopic tunnels penetrating through the foil. For maximum increase in surface area in higher voltage capacitors the anode foil is 99.99% high purity, high cubicity aluminum that allows the billions of microscopic etch tunnels to be parallel and mostly perpendicular to the foil surface. Etching involves pulling the aluminum foil on rollers through a chloride solution while applying an AC, DC or AC-and-DC voltage between the etch solution and the aluminum foil. Surface area can increase as much as 200 times for foil in low-voltage capacitors and up to 60 times for high-voltage capacitors. FORMING The anode foil carries the capacitor’s dielectric. The dielectric is a thin layer of aluminum oxide, Al2O3, which is chemically grown on the anode foil during a process called “formation.” Formation is accomplished by pulling the anode foil on rollers through an electrolyte bath and continuously applying a DC voltage between the bath and the foil. The voltage is 135% to 200% of the fi nal capacitor’s rated voltage. The thickness of the aluminum oxide is about 1.4 to 1.5 nm for each volt of the formation voltage, e.g., the anode foil in a 450 V capacitor may get a formation voltage in excess of 600 V and have an oxide thickness of about 900 nm. That’s about a hundredth of the thickness of a human hair. Formation reduces the eff ective foil surface area because the microscopic tunnels are partially occluded by the oxide. The tunnel etch pattern is adjusted by choice of foil and etching process so that low-voltage anodes have dense tunnel patterns compatible with thin oxide and high-voltage anodes have coarse tunnel patterns compatible with thick oxide. The cathode foil is not formed and it retains its high surface area and dense etch pattern. SLITTING Foil is etched and formed in jumbo rolls of 40 to 50 cm wide and then slit into various widths according to the lengths of the fi nal capacitors. WINDING The capacitor element is wound on a winding machine with spindles for one-to-four separator papers, the anode foil, another set of one-to-four separator papers and the cathode foil. These are wound into a cylinder and wrapped with a strip of pressure-sensitive tape to prevent unwinding. The separators prevent the foils from touching and shorting, and the separators later hold the reservoir of electrolyte. Before or during winding aluminum tabs are attached to the foils for later connection to the capacitor terminals. The best method is by cold-welding of the tabs to the foils with tab locations microprocessor controlled during winding so that the capacitor element’s inductance can be less than 2 nH. The older method of attachment is by staking, a process of punching the tab through the foil and folding down the punched metal. Cold welding reduces short-circuit failures and performs better in high-ripple current and discharge applications in which the individual stakes may fail from high current like buttons popping off one at a time from a fat-man’s vest. CONNECTING TERMINALS In SMT capacitors and miniature capacitors with rubber-bungs, extensions of the tabs are the capacitor terminals. But in large- can capacitors like snap-ins and screw-terminal styles, the tabs are riveted or welded on the underside of the capacitor tops to terminal inserts. Welding produces the lowest contact resistance and highest current handling. Both resistive welding and ultrasonic welding are used. The up to 12 tab pairs that may be used in large screw-terminal capacitors often require more mechanical support during assembly so the tabs in such capacitors may be both riveted to post extensions on the terminals and then welded. In an axial-leaded capacitor the cathode tab is welded to the can before sealing. IMPREGNATION The capacitor element is impregnated with electrolyte to saturate the paper separators and penetrate the etch tunnels. 4 The method of impregnation may involve immersion of the elements and application of vacuum-pressure cycles with or without heat or, in the case of small units, just simple absorption. The electrolyte is a complex blend of ingredients with different formulations according to voltage and operating temperature range. The principal ingredients are a solvent and a conductive salt – a solute – to produce electrical conduction. The common solvent is ethylene glycol (EG) and is typically used for capacitors rated –20 ºC or –40 ºC. Dimethylformamide (DMF) and gammabutyrolactone (GBL) are often used for capacitors rated –55 ºC. Common solutes are ammonium borate and other ammonium salts. Water in the electrolyte plays a big role. It increases conductivity thereby reducing the capacitor’s resistance, but it reduces the boiling point so it interferes with high temperature performance, and it reduces shelf life. A few percent of water is necessary because the electrolyte maintains the integrity of the aluminum oxide dielectric. When leakage current flows, water is broken into hydrogen and oxygen by hydrolysis, and the oxygen is bonded to the anode foil to heal leakage sites by growing more oxide. The hydrogen escapes by passing through the capacitor’s rubber seal. SEALING The capacitor element is sealed into a can. While most cans are aluminum, phenolic cans are often used for motor-start capacitors. In order to release the hydrogen the seal is not hermetic and it is usually a pressure closure made by rolling the can edge into a rubber gasket, a rubber end-plug or into rubber laminated to a phenolic board. In small capacitors molded phenolic resin or polyphenylene sulfide may replace the rubber. Too tight a seal causes pressure build up, and too loose a seal shortens the life by permitting drying out, loss of electrolyte. AGING Here the capacitor assembly comes full circle. The last manufacturing step is “aging” during which a DC voltage greater than the rated voltage but less than the formation voltage is applied to the capacitor. Usually the voltage is applied at the capacitor’s rated temperature, but other temperatures and even room temperature may be used. This step reforms the cut edges and any damaged spots on the anode foil and covers any bare aluminum with aluminum oxide dielectric. Aging acts as burn- in and reduces or eliminates early life failures (infant mortals). Low, initial DC leakage current is a sign of effective aging. COMPARISON TO OTHER TYPES OF CAPACITORS CERAMIC CAPACITORS Ceramic capacitors have become the preeminent, general- purpose capacitor, especially in SMT chip devices where their low cost makes them especially attractive. With the emergence of thinner-dielectric, multilayer units with rated voltages of less than 10 V capacitance values in the hundreds of microfarads have become available. This intrudes on the traditional, high- capacitance province of aluminum electrolytic capacitors. Ceramic capacitors are available in three classes according to dielectric constant and temperature performance. Class 1 (NPO, COG) is suitable for low capacitance, tight tolerance applications in the range of 1 pF to a few mF. Class 2 (X7R, X5R, Y5V) has 20 to 70 times as much capacitance per case size, but capacitance typically varies about ± 10% over its –55 to 125 ºC temperature range. The maximum change is +15 % to –25%. Class 3 (Z5U) with about 5 times the capacitance of Class 3 has wild swings of capacitance with voltage and temperature. The temperature range is –25 ºC to 85 ºC, and capacitance varies about +20% –65% over the range. All classes of ceramic capacitors are available in a variety of physical forms, ranging from round disc or rectangular single layer to multilayer types as well as tubular and feed-through types. Ceramic chip capacitors are brittle and sensitive to thermal shock, so precautions need to be taken to avoid cracking during mounting, especially for high-capacitance large sizes. The typical temperature range for aluminum electrolytic capacitors is –40 ºC to 85 ºC or 105 ºC. Capacitance varies about +5% –40% over the range with the capacitance loss all at cold temperatures. Capacitors rated –55 ºC generally only have –10 % to –20 % capacitance loss at –40 ºC. Cold temperature performance for rated voltages of 300 V and higher is often worse, and temperature performance varies by manufacturer. Thus Class 1 and 2 ceramic capacitors perform better than aluminum electrolytic capacitors at cold temperatures, and Class 3 ceramic capacitors perform worse at all temperatures. Aluminum electrolytic capacitors readily deliver much more capacitance. Aluminum electrolytic capacitors give more capacitance and energy storage per unit volume than ceramic capacitors for all types except for low-voltage, Class 3 ceramic SMT chip capacitors. While tolerances of ±5% and ±10% are routine for ceramic capacitors, ± 20% and –10% +50% are the norms for aluminum electrolytic. This makes aluminum electrolytics the choice for high-capacitance applications like rectification filters and power hold up where more capacitance is a bonus. Ceramic capacitors are not polarized and therefore can be used in AC applications. The low DF and high capacitance stability of Class 1 and 2 are especially suited to AC and RF applications. By comparison, aluminum electrolytic capacitors are polarized and cannot withstand voltage reversal in excess of 1.5 V. While non- polar aluminum electrolytics are available for momentary-duty 5 AC applications like motor starting and voltage-reversing applications, the high DF of aluminum electrolytic capacitors – from 2% to 150% – causes excess heating and short life in most AC applications. Ceramic capacitors are generally no more reliable than aluminum electrolytic capacitors because aluminum electrolytics self heal. Since high-capacitance ceramic capacitors may develop micro-cracks, aluminum electrolytic capacitors are preferred for high capacitance values. However, small sizes of aluminum electrolytic capacitors may have limited life due to dry out, and so consider reliability in your choice for applications operating at high temperatures, over 65 ºC. FILM CAPACITORS Film capacitors offer tight capacitance tolerances, very low leakage currents and small capacitance change with temperature. They are especially suited to AC applications through their combination of high capacitance and low DF that permits high AC currents. However, they have relatively large sizes and weights. The popular polymers used for plastic-film dielectric capacitors are polyester and polypropylene. The popular polymer for SMT devices is polyphenylene sulfide (PPS). While film/foil construction is often used for small capacitance values – less than 0.01 µF – and for high-current applications, metallized-film is usually preferred because it gives smaller size, lower cost and is self healing. Film capacitors are general-purpose capacitors for through-hole applications and have special uses for tight- tolerance, AC voltage, high voltage and snubbing. Polyester film capacitors operate from –55 ºC to 85 ºC at rated voltage; 85 ºC to 125 ºC with linear voltage derating to 50% rated voltage. The typical capacitance change over the entire range is less than –5% +15% with ±1% from 0 ºC to 50 ºC. Capacitance values are readily available up to 10 µF with special large sections to 100 µF. Generally available voltages are 50 to 1000 Vdc and 35 to 600 Vac. AC current handling is limited by polyester’s high-temperature DF of about 1%. Polypropylene film capacitors operate from is –55 ºC to 85 ºC at rated voltage; 85 ºC to 105 ºC with linear voltage derating to 50% rated voltage. The typical capacitance change over the entire range is less than +2% –4% with ±1% from –20 ºC to 60 ºC. Capacitance values are readily available up to 65 µF with special large sections to 1000 µF. Generally available voltages are 100 to 3000 Vdc and 70 to 500 Vac. AC current handling permits use in motor-run and other continuous duty AC applications. Compared to aluminum electrolytic capacitors, film capacitors take the lead in high voltage, AC voltage and tight tolerance applications. Aluminum electrolytics excel in capacitance and energy storage. However, there is growing use of power film capacitors as replacements for aluminum electrolytic capacitors as dc-link, bus capacitors in high-voltage inverter power systems. While generally power film capacitors are more than four times the price for capacitance as aluminum electrolytic capacitors, film capacitors are perceived as more reliable because failures are relatively benign and without the incidence of explosion and ignition that can accompany aluminum electrolytic capacitor failures in large high-voltage banks of 10 or more capacitors. Cornell Dubilier now provides special aluminum electrolytic capacitors with improved self-healing to deliver the needed reliability for these applications. SOLID TANTALUM CAPACITORS Like aluminum electrolytic capacitors solid tantalum capacitors are polar devices (1 V maximum reverse vo