Design of Electrolytic Capacitor

Design of Electrolytic Capacitor

Introduction

Today electrolytic capacitors or as they are more correctly termed, aluminium electrolytic capacitors are used in huge quantities. They are very cost effective and able to provide a larger capacitance per volume than other types of capacitor. This gives them very many uses in circuits where high currents or low frequencies are involved. Aluminium electrolytic capacitors are typically used most in applications such as audio amplifiers of all types (hi-fi to mobile phones) and in power supply circuits.

Like any other capacitor, it is necessary to understand the advantages and limitations of these capacitors to enable them to be used most effectively.

Electrolytic capacitor development

The electrolytic capacitor has been in use for many years. Its history can be traced back to the very early days or radio around the time when the first broadcasts of entertainment were being made. At the time, valve wireless sets were very expensive, and they had to run from batteries. However with the development of the indirectly heated valve or vacuum tube it became possible to use AC mains power. While it was fine for the heaters to run from an AC supply, the anode supply needed to be rectified and smoothed to prevent mains hum appearing on the audio. In order to be able to use a capacitor that was not too large Julius Lilienfield who was heavily involved in developing wireless sets for domestic use was able to develop the electrolytic capacitor, allowing a component with sufficiently high capacitance but reasonable size to be used in the wireless sets of the day.

Construction of electrolytic capacitors

The plates of an electrolytic capacitor are constructed from conducting aluminium foil. As a result they can be made very thin and they are also flexible so that they can be packaged easily at the end of the production process. The two plates, or foils are slightly different. One is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte is placed between them. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil act as cathode.

In order to package them the two aluminium foils with the electrolyte soaked paper are rolled together to form a cylinder, and they are placed into an aluminium can. In this way the electrolytic capacitor is compact while being robust as a result of the protection afforded by the can.

There are two geometries that are used for the connection leads or tags. One is to use axial leads, one coming from each circular face of the cylinder. The other alternative is to use two radial leads or tags, both of which come from the same face of the cylinder.

The lead styles give rise to the descriptions used for the overall capacitors. Descriptions of axial and radial will be seen in the component references.

Electrolytic capacitor properties

There are a number of parameters of importance beyond the basic capacitance and capacitive reactance when using electrolytic capacitors. When designing circuits using electrolytic capacitors it is necessary to take these additional parameters into consideration for some designs, and to be aware of them when using electrolytic capacitors.

1.    ESR Equivalent series resistance:   Electrolytic capacitors are often used in circuits where current levels are relatively high. Also under some circumstances and current sourced from them needs to have a low source impedance, for example when the capacitor is being used in a power supply circuit as a reservoir capacitor. Under these conditions it is necessary to consult the manufacturers datasheets to discover whether the electrolytic capacitor chosen will meet the requirements for the circuit. If the ESR is high, then it will not be able to deliver the required amount of current in the circuit, without a voltage drop resulting from the ESR which will be seen as a source resistance.

2.    Frequency response:   One of the problems with electrolytic capacitors is that they have a limited frequency response. It is found that their ESR rises with frequency and this generally limits their use to frequencies below about 100 kHz. This is particularly true for large capacitors, and even the smaller electrolytic capacitors should not be relied upon at high frequencies. To gain exact details it is necessary to consult the manufacturers data for a given part.

3.    Leakage:   Although electrolytic capacitors have much higher levels of capacitance for a given volume than most other capacitor technologies, they can also have a higher level of leakage. This is not a problem for most applications, such as when they are used in power supplies. However under some circumstances they are not suitable. For example they should not be used around the input circuitry of an operational amplifier. Here even a small amount of leakage can cause problems because of the high input impedance levels of the op-amp. It is also worth noting that the levels of leakage are considerably higher in the reverse direction.

4.    Ripple current:   When using electrolytic capacitors in high current applications such as the reservoir capacitor of a power supply, it is necessary to consider the ripple current it is likely to experience. Capacitors have a maximum ripple current they can supply. Above this they can become too hot which will reduce their life. In extreme cases it can cause the capacitor to fail. Accordingly it is necessary to calculate the expected ripple current and check that it is within the manufacturers maximum ratings.

5.    Tolerance:   Electrolytic capacitors have a very wide tolerance. Typically this may be -50% + 100%. This is not normally a problem in applications such as decoupling or power supply smoothing, etc. However they should not be used in circuits where the exact value is of importance.

Polarisation

Unlike many other types of capacitor, electrolytic capacitors are polarised and must be connected within a circuit so that they only see a voltage across them in a particular way. The capacitors themselves are marked so that polarity can easily be seen. In addition to this it is common for the can of the capacitor to be connected to the negative terminal.

It is absolutely necessary to ensure that any electrolytic capacitors are connected within a circuit with the correct polarity. A reverse bias voltage will cause the centre oxide layer forming the dielectric to be destroyed as a result of electrochemical reduction. If this occurs a short circuit will appear and excessive current can cause the capacitor to become very hot. If this occurs the component may leak the electrolyte, but under some circumstances they can explode. As this is not uncommon, it is very wise to take precautions and ensure the capacitor is fitted correctly, especially in applications where high current capability exists.

Electrolytic capacitors rating and anticipated life

Great care should be taken not to exceed the rated working voltage of an electrolytic capacitor. Normally they should be operated well below their stated working value. Also in power supply applications significant amounts of current may be drawn from them. Accordingly electrolytic capacitors intended for these applications have a ripple current rating which should also not be exceeded. If it is, then the electronic component may become excessively hot and fail. It is also worth noting that these components have a limited life. It can be as little as 1000 hours at the maximum rating. This may be considerably extended if the component is run well below its maximum rating.

Electrolytic SMD capacitors

Electrolytic capacitors are now being used increasingly in SMD designs. Their very high levels of capacitance combined with their low cost make them particularly useful in many areas. Originally they were not used in particularly high quantities because they were not able to withstand some of the soldering processes. Now improved capacitor design along with the use of reflow techniques instead of wave soldering enables electrolytic capacitors to be used more widely in surface mount format.

Often SMD electrolytic capacitors are marked with the value and working voltage. There are two basic methods used. One is to include their value in microfarads (m F), and another is to use a code. Using the first method a marking of 33 6V would indicate a 33 mF capacitor with a working voltage of 6 volts. An alternative code system employs a letter followed by three figures. The letter indicates the working voltage as defined in the table below and the three figures indicate the capacitance on picofarads. As with many other marking systems the first two figures give the significant figures and the third, the multiplier. In this case a marking of G106 would indicate a working voltage of 4 volts and a capacitance of 10 times 10^6 picofarads. This works out to be 10 mF 

LETTER	VOLTAGE
e	2.5
G	4
J	6.3
A	10
C	16
D	20
E	25
V	35
H	50

Voltage codes for SMD electrolytic capacitors

Reforming aluminium electrolytic capacitors

It may be necessary to re-form electrolytic capacitors that have not been sued for six months or more. The electrolytic action tends to remove the oxide layer from the anode and this needs to be re-formed. Under these circumstances it is not wise to apply the full voltage as the leakage current will be high and may lead to large amounts of heat being dissipated in the capacitor which can in some instances bring about its destruction.

To reform the capacitor, the normal method is to apply the working voltage for the capacitor through a resistor of around 1.5 k ohms, or possibly less for lower voltage capacitors. (NB ensure that it has sufficient power rating to handle the capacitor in question). This should be applied for an hour or more until the leakage current drops to an acceptable value and the voltage directly on the capacitor reaches the applied value, i.e. virtually no current is flowing through the resistor. This voltage should then be continued to be applied for a further hour. The capacitor can then be slowly discharged through a suitable resistor so that the retained charge does not cause damage.

Inventory of Tools for Ergonomic Evaluation

Inventory of Tools for Ergonomic Evaluation

Introduction

This is an inventory of tools for description, evaluation, and design of working environment / ergonomics. This ‘Tool Inventory’ is intended to assist practitioners in identifying potentially useful methods for evaluating working environment in their professional work (or perhaps evaluating their own working environment!). The emphasis here is on tools that can be used to evaluate a workplace or a potential design for a workplace, often in some kind of quantitative way. Evaluation methods and change tools such as focus groups, photo-safari, or dialogue conferences are not included here. A number of such improvement tools are described in a related SMARTA project. There are many evaluation tools ‘out there’ and this list includes only some of these. The aim of this report is to provide those seeking evaluation approaches with a broad overview of available tools and connections to information that can help choose the tool best suited to their needs. 

 

Methods

The tools listed here were identified in a number of ways:

·         Tools and methods known to the author either directly or through reports & literature

·         Tools were identified in connection with the associated SMARTA evaluation review project tools that had been used in the scientific studies of ergonomics interventions were included

·         Tools identified by web searches and from measurement text-books

·         Tools & tips provided by colleagues who viewed early versions of this list

A classification scheme was developed as tool were collected based on characteristics of the tool and how the tool might be used. Whenever possible internet links and references are provided giving the reader access to more information and sometimes even copies or software of the method itself. This scheme is sequenced roughly in order of stages of development. Thus simulation tools, that can be used before a workplace is built, are listed before checklists that usually require existing workplaces. 

Results

Tool were categorised as follows:

·         Tools for Strategic Decision making

·         Tools for Work System & Product Design

o   Complex Human Simulation Models

o   Simpler Computerised Human Biomechanical Models

o    Design Checklists and Other Design tools

o   Flow Simulation Tools

o   Tools for Product Design

·          Instruments for Evaluating Work Environment

·         Computer Based Evaluation Tools

·          Checklists for Workplace Evaluation

·         Questionnaires on Risk Factor Perceptions

o   Physical Risk Factors

o   Psychosocial & Psychophysical

·         Questionnaires on Health & Wellbeing

o    Fatigue, Motivation, Satisfaction etc.

o   Pain, Disability & Symptom surveys

·         Economic Models

Tools for evaluating work environment were found in all categories except for the support of strategic decision-making. The most common tool identified was the ‘Checklist’ type tool, many of which exist in computerized formats.

Discussion

Use the right tool for the job. There is no ‘best’ tool. After all a hammer is not a ‘better’ tool than a saw. The choice of tool depends therefore on what needs to be done. Early in a design phase it may be more suitable to use simulation approaches that can be based on early design specifications. If a workplace is running, and a specific concern is to be addressed, then a simple paper and pencil tool may be more cost effective. When choosing a tool considers:

What is the purpose of the evaluation?

 Who will gather the information?

 Who will use or act on the information?

 Just because you have a hammer in your hand doesn’t mean your problem is a nail.

These tools aim to evaluate risk and consequences. Most of the tools listed here attempt to quantify the physical load or psychosocial conditions for the employee. Several tools are oriented to quantifying outcomes such as pain or disability. A few tools consist of economic models that try to evaluate a potential change in terms of productivity, costs, and financial benefits.

Every tool has a ‘blind spot’. No tool is perfect. Carpenters have dozens of tools for different tasks. Remember that there are often ‘intangible’ effects from change projects. It is helpful to try and capture these with more qualitative approaches – by interviewing the people involved. This can provide insight into the effects (and process) of change that might not be clear from a particular tool. Operator and supervisor descriptions of how the system is working and where improvements might be made can support and shed light on the results of your analysis.   

It’s the skill of the carpenter not just the sharpness of the saw that counts.  Of course a good tool makes a big difference, but how the tool is used is also critical. The way you use a tool and the process by which the information is used by you and by others can always be improved. Think of the tool as supporting your organisations continuing development efforts.

Conclusions

There are many tools available for evaluating ergonomics at different stages in the development process. Checklists (often implemented on computers) for evaluating current working situation appear to be the most common tool type. Research is needed to examine the extent to which tools are being used by practitioners, the process by which tools are used, and their experience of the benefits and drawbacks of various tools.

Design of Chip Ceramic Capacitor

Design of Chip Ceramic Capacitor

Ceramic Capacitor Basics

• A capacitor is an electrical device that stores energy in the   electric field between a pair of closely spaced plates
• Capacitors are used as energy-storage devices, and can also be used to differentiate between high-frequency and low-frequency signals. This makes them useful in electronic filters
• Capacitance Value: Measure of how much charge a capacitor can store at a certain voltage
• MLCC: Multilayer Ceramic Chip Capacitor L-ayers of ceramic and metal are alternated to make a multilayer chip

Process of Making Capacitor:

The process of making ceramic capacitors involves many steps.

Mixing: Ceramic powder is mixed with binder and solvents to create the slurry, this makes it easy to process the material.

Tape Casting: The slurry is poured onto conveyor belt inside a drying oven, resulting in the dry ceramic tape. This is then cut into square pieces called sheets. The thickness of the sheet determines the voltage rating of the capacitor.

Screen Printing and Stacking: The electrode ink is made from a metal powder that is mixed with solvents and ceramic material to make the electrode ink. The electrodes are now printed onto the ceramic sheets using a screen printing process. This is similar to a t-shirt printing process. After that the sheets are stacked to create a multilayer structure.

Lamination: Pressure is applied to the stack to fuse all the separate layers, this created a monolithic structure. This is called a bar.

Cutting: The bar is cut into all the separate capacitors. The parts are now in what is called a ‘green’ state. The smaller the size, the more parts there are in a bar.

Firing: The parts are fired in kilns with slow moving conveyor belts. The temperature profile is very important to the characteristics of the capacitors.

Termination: The termination provides the first layer of electrical and mechanical connection to the capacitor. Metal powder is mixed with solvents and glass frit to create the termination ink. Each terminal of the capacitor is then dipped in the ink and the parts are fired in kilns.

Plating: Using an electroplating process, the termination is plated with a layer of nickel and then a layer of tin. The nickel is a barrier layer between the termination and the tin plating. The tin is used to prevent the nickel from oxidizing.

Testing: The parts are tested and sorted to their correct capacitance tolerances.

At this point the capacitor manufacturing is complete. The parts could be packaged on tape and reel after this process or shipped as bulk.

Types of Material Systems Used to make capacitors

There are two material systems used today to make ceramic capacitors: Precious Metal Electrode and Base Metal Electrode. The precious metal system is the older technology and uses palladium silver electrodes, silver termination, then nickel and tin plating. Today this material system is mostly used on high voltage parts with a rating of 500V and higher. The base metal system is a newer technology and uses nickel electrodes, nickel or copper termination, and nickel and tin plating. This material system is typically used for parts with voltage ratings lower than 500VDC.

Precious Metal Vs Base Metal System

MLCC Basics

The capacitance value of a capacitor is determined by four factors. The number of layers in the part, the dielectric constant and the active area are all directly related to the capacitance value. The dielectric constant is determined by the ceramic material (NP0, X7R, X5R, or Y5V). The active area is just the overlap between two opposing electrodes.

The dielectric thickness is inversely related to the capacitance value, so the thicker the dielectric, the lower the capacitance value. This also determines the voltage rating of the part, with the thicker dielectric having a higher voltage rating that the thinner one. This is why the basic trade off in MLCCs is between voltage and capacitance.

Critical Specifications

Material	Dielectric Constant	% Capacitance Change	DF
NP0	15-100	<0.4% (-55 to 125C)	0.1%
X7R	2000-4000	+/-15% (-55 to 125C)	3.5%
Y5V	>16000	Up to 82% (-30 to 85C)	9.0%

• Dissipation factor: % of energy wasted as heat in the capacitor
• Dielectric Withstanding Voltage: Voltage above rating a capacitor can withstand for short periods of time
• Insulation resistance: Relates to leakage current of the part (aka DC resistance)

The critical specifications of a capacitor are the dielectric constant, dissipation factor, dielectric withstanding voltage, and insulation resistance. Dielectric constant: this depends on the ceramic material used. The table shows different dielectrics and some of their specifications. As you can see NP0 has the lowest dielectric constant, followed by X7R which has a significantly higher constant, and Y5V which is higher still. This is why the capacitance values for X7R capacitors are much higher than NP0 capacitors, and Y5V has higher capacitance than X7R. The capacitance change vs temperature is very small for NP0 parts from -55C to 125C, and gets larger for X7R, then even larger for Y5V. So, the more capacitance a material provides, the lower the stability of capacitance over temperature. Dissipation Factor: this is the percentage of energy wasted as heat in the capacitor. As you can see, NP0 material is very efficient, followed by X7R, then Y5V which is the least efficient of the three materials. Dielectric withstanding voltage: this refers to the momentary over voltage the capacitor is capable of withstanding with no damage. Insulation resistance: this is the DC resistance of the capacitor; it is closely related to the leakage current.

Characteristics of Ceramic Capacitors

Low impedance, equivalent series resistance (ESR) and equivalent Series Inductance (ESL). As frequencies increase, ceramic has bigger advantage over electrolytic

The final part of this presentation will cover the characteristics of ceramic capacitors. MLCCs have low impedance when compared with tantalum and other electrolytic capacitors. This includes lower inductance and equivalent series resistance (ESR). This allows ceramic capacitors to be used at much higher frequencies than electrolytic capacitors.

Temperature Coefficient: Describes change of capacitance vs. temperature. Ceramic materials are defined by their temperature coefficient

Voltage Coefficient: Describes change of capacitance vs voltage applied. Capacitance loss can be as much as 80% at rated voltage. This is a property of ceramic materials and applies to all manufacturers

Voltage Coefficient of Capacitance: describes change of capacitance vs DC voltage applied. This is a property of ceramic materials and applies to all manufacturers. The graph shows typical voltage coefficient curves for 500VDC rated X7R and NP0 capacitors. Note that the capacitance of the NP0 remains stable with applied voltage, while the X7R material can have a capacitance loss of 80% at rated voltage.

Aging: X7R, X5R, and Y5V experience a decrease in capacitance over time caused by the relaxation or realignment of the electrical dipoles within the capacitor.

• For X7R and X5R the loss is 2.5% per decade hour and for Y5V it is 7% per decade hour, NP0 dielectric does not exhibit this phenomenon
• De-Aging: aging is reversible by heating the capacitors over the “Curie Point” (approx 125°C), the crystalline structure of the capacitor is returned to its original state and the capacitance value observed after manufacturing.

Aging: X7R, X5R, and Y5V experience a decrease in capacitance over time caused by the relaxation or realignment of the electrical dipoles within the capacitor. For X7R and X5R the loss is 2.5% per decade hour and for Y5V it is 7% per decade hour, NP0 dielectric does not exhibit any aging. Aging is reversible by heating the capacitors over the “Curie Point” (approx 125°C), the crystalline structure of the capacitor is returned to its original state and the capacitance value observed after manufacturing.

Johanson Part Number Breakdown

  

This slide is for reference and shows the Johanson Dielectrics part number breakdown.

Summary

• Manufacturing process and basic structure of ceramic capacitors
•  Material systems and basic specifications of ceramic capacitors

o   Precious Metal vs Base Metal

o   Critical Specifications of MLCCs

•        Characteristics of ceramic chip capacitors

o   Low impedance, temperature coefficient, voltage coefficient, aging