deki electronics limited design guidelines for film ... · title: r&d copy no. : doc. no. :...

DEKI ELECTRONICS LIMITED Design Guidelines for Film Capacitors

TITLE: R&D Copy No. :

DOC. No. : Issue status : of 34

Issue Date Approved by Documentation

1/1/2011

Department Signed

Prepared by:

Mr. Vishnu

R&D and Engg.

Checked & Approved

by:

Mr. Sankar

R&D and Engg.

History

Revision Description Date

Only the latest version is valid at present.

Distribution list: Division Head, Marketing, R&D Engg., Production, Maintenance,

Quality Assurance and Purchase





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Contents:

General Introduction

Annexure I:

1 Guidelines for selection of dielectric film

2 Guidelines for selection of construction type of capacitor

3 Guidelines for selection of film type

4 Design sheet verification

5 Guidelines for selection of spray coating material

6 Guidelines for selection of lead wire

7 Guidelines for selection of outer encapsulation

Annexure II:

10.1 Dv/dt

10.2 Temperature co-efficient α

10.3 Humidity co-efficient β

10.4 Dielectric absorption

10.5 Time constant & Insulation Resistance





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DESIGN GUIDELINES FOR FILM

CAPACITORS

General Introduction:

Capacitors are an essential component in any circuit requiring coupling/de coupling,

Bypassing, timing circuits, tuning, filtering, charge storage and many functions. Today a

wide range of capacitors are available to cover the broad range of requirements in

electronics. The capacitor selection depends on various parameters. This design

guidelines discusses the design rules for film capacitors with respect to the requirements

Key terms: Competitor analysis, capacitance, IR, Tanδ, ESR, ESL, Self-healing, BDV,

∆T, Inductive/Non-inductive capacitors.

There are two kinds of customer specific requirements

1. The code number of product of competitor will be given and they require the

product with same characteristic features.

2. The technical specifications of capacitor with waveforms will be given, after

analysis a suitable product will be suggested by the vendor/manufacturer.

Case 1:

1. The code number of product of the competitor should be given.

2. The technical details like capacitance, rated voltage, material type & construction,

tolerance and size must be analyzed.

3. Whether the product is available in the existing design should be checked; if yes,

the product will be suggested otherwise the samples of the competitor should be

taken and the competitor analysis should be made.

4. The results of competitor analysis are fed into the existing design sheet and the

dimensions are matched.

5. The trials should be made and all the necessary tests should be performed.

6. The design should be reviewed and finalized.

Case 2:

1. The technical specifications of product with waveforms are given (if necessary).

In this case, all the sections 1 to 3 in annexure I are followed to choose the

dielectric film and construction type.

2. Whether the product is present in the existing design should be checked; if yes,

the dimensions are compared and finalized.

3. If not, then the sections from 4 to 7 in annexure I should be followed in order to

make a new design.

4. The trials should be made and all the necessary tests should be performed.

5. The design should be reviewed and finalized.





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Annexure I

1 Guidelines for selection of dielectric film.

To design a film capacitor, the first step is to choose the dielectric film

Commercially available dielectric films,

Polyethylene Terephthalate (PET)

Polypropylene (PP)

Polyethylene Naphthalate (PEN)

Polyphenylene Sulphide (PPS)

In cost, PPS>PEN>PP>PET film. So the best cost effective film should be chosen for the

required application.

1.1 Operating temperature range

For PET film:

Dielectric constant Vs Temperature

Tanδ Vs Temperature





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Insulation resistance Vs Temperature.

For PP film:

Dielectric constant Vs Temperature





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Insulation resistance Vs Temperature

Tanδ Vs Frequency in Hz





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For PEN & PPS film:

Capacitance change ∆C/C Vs Temperature






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Insulation resistance Vs Temperature

If the maximum operating temperature <=105oC, then first preference is polypropylene

(PP) but if the dimensions or any other electrical parameter requirement then PET film is

recommended.

If the maximum operating temperature <=120oC, then polyethylene Terephthalate (PET)

should be the choice.

If the maximum operating temperature reaches 150 oC, then polyethylene naphthalate

(PEN) is recommended.

If the maximum operating temperature goes beyond 150 oC, then polyphenylene sulphide

(PPS) is the only choice.

The above two films are called as high temperature category films but it is very

expensive and the difficulty lies in the self-healing.

So generally, PET and PP films are used.





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Note: If temperature >110oC is required with low tanδ (PP Film) in inductive type, then

two undercoats and two outer coats can be made to withstand upto 120 oC but

compromising with some increase in dimension.

1.2 Analyze the application

DC Application

AC Application

Pulse Application

DC Application:

For the case of DC applications, the rated voltage of capacitor should be slightly greater

than the DC voltage of the signal. For dc applications, leakage current should be very less

otherwise the capacitor will not be triggered at the required voltage point. So the circuit

operation will fail. So leakage should be less, which means insulation resistance should

be very high. In DC applications, Insulation resistance is a critical parameter and tanδ is

not considered much for dc applications.

Film can be chosen from temperature coefficient, for positive temperature coefficient the

choice is PET film and for negative temperature coefficient the choice is PP film.

AC Application:

For AC applications tanδ is very important rather than insulation resistance, since it

changes with respect to frequency. Here, in AC applications because of the change of

positive and negative half cycles the insulation resistance is not much critical.

AC applications categorized into two,

Low frequency applications

High frequency applications

For high frequency applications, ∆T is a parameter called self-heating need to be

considered. For any chosen type of film, the ∆T must be checked after finishing all

manufacturing process and it should be less than 10 oC. If suppose the ∆T is more than 10

oC, then increase the rated voltage of the capacitor. So dimension will increase.

Pulse Applications:

This is also AC application but in this case, the capacitor undergoes with high current

pulses at short durations randomly. Hence the self-healing is the major role for pulse

applications so as to withstand for pulses.





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AC and Pulse Applications:

For AC applications, Vrms, operating frequencies and waveforms during start phase &

run phase should be given.

1.2.1 Rated voltage Vdc of the capacitor > Vpeak of the given signal

For AC rated voltage, rated Vac of capacitor > peak Vrms of the given signal.

1.2.2 Peak current of the capacitor > Peak current of the given signal

Peak current of the capacitor can be calculated by,

Ip= C*dV/dt

1.2.3 At Operating frequency, the continuous Vrms applied to the capacitor should

satisfy the voltage derating of the capacitor with frequency. This is related to the

capacitor construction and it must be checked while choosing the type of

construction.

1.2.4 Calculate the dV/dt from the waveforms of start phase and continuous run phase.

Compare the calculated dV/dt with capacitor dV/dt. Calculated value must be

lesser than the capacitor dV/dt.

To calculate dV/dt, refer section 10.1 in annexure II.

1.3 Temperature Co-efficient α:

Temperature co-efficient is a factor which change with respect to the temperature and

which is a property of the dielectric film.

In order to choose the appropriate film with temperature, the temperature co-efficient

should be given

Film Temp co-eff. Α (*10

-6/Kelvin)

PP -250

PET +600

So, if the temperature co-efficient is negative, the appropriate film is Polypropylene (PP)

and if the temperature co-efficient is positive and very high, the appropriate film is

Polyethylene Terephthalate (PET).

If the temperature co-efficient is positive and low, then the PEN and PPS films are

considered.

For explanation, how to calculate the temperature co-efficient refer section 10.2 in

annexure II





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1.4 Humidity co-efficient β:

Humidity co-efficient is a factor which changes with respect to the humidity of

environment and which is a property of the dielectric film

If the capacitor is box type, it is not necessary to consider the humidity co-efficient. But if

the capacitor is DIP type in epoxy material, humidity plays a major role.

When the relative humidity increases, capacitance value increases and insulation

resistance decreases.

Film β (*10-6

per % of RH)

PP 40 to 100

PET 500 to 700

Hence, polypropylene is having less effect with humidity comparable to other films.

For explanation, how to calculate the humidity co-efficient refer section 10.3 in annexure

II

1.5 Dielectric absorption:

It is the property of the dielectric. When the capacitor is charged for a long period of time

and then it is discharged by short circuit. Even after the discharge, a small amount of

voltage remained stored in the capacitor for about 15minutes. This effect is called

dielectric absorption and it depends on the capacitance value and the dielectric thickness.

Film Absorption

PP 0.05%

PET 0.2%

For explanation, how to calculate the dielectric absorption refer section 10.4 in annexure

II

1.6 Time constant τ and Insulation Resistance:

Time constant is an important parameter which is related to the insulation resistance.

When the capacitance C<= 0.33μf, then an approximate insulation resistance is given

If C>0.33μf, then the time constant τ will be given τ=R*C,

Where R is the insulation resistance and it varies according to the capacitance value.

So higher the capacitance, lower is the insulation resistance. Theoretically, time constant

should be high for high insulation resistance.

For detailed explanation of insulation resistance and time constant on each type of

capacitor, refer section 10.5 in annexure II





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All the capacitors should satisfy the insulation resistance as per standard given in section

10.5

1.7 Tanδ:

If the customer specifies the tanδ, it can be checked to ensure the chosen film is correct

For PP film,

Tanδ<0.0008 at 1 kHz,


Tanδ<0.003 at 100 kHz.

For PET film,

If tanδ<0.008 at 1 kHz,


Tanδ<0.030 at 100 kHz.

For mixed dielectric of PP & PET, tanδ<0.004 at 1 kHz

PEN film is nearly equal to PET and PPS is nearly equal to PP film.

1.8 Finished Capacitor dimensions:

This is the very important parameter. Though the film is designed appropriately, if the

dimension of finished capacitor is more than the customer specified dimensions, then the

capacitor cannot be fitted into the PCB and it will be rejected.

Generally, the dimensions are big, if PP film is used rather than PET film.

Using this rule, the dimensions are matched accordingly by changing the films.

Comparison of dielectric films

Type Density

g/cm2

Melting

pt. oC

Operating

temp oC

BDV Tanδ

(1kHz)

%

Dielectric

constant

IR

MΩ/μF

PET 1.4 263 -40 to

120

200 to

300

0.5 3.2 30000

PPS 1.35 285 -40 to

175

200 to

300

0.06 3 100000

PP 0.91 160 to

170

-40 to

105

300 to

400

0.02 2.2 300000

PEN 1.36 266 -40 to

150

240 0.4 3 30000





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2 Guide lines for selection of construction type of capacitor:

2.1 Film/foil type

2.1.1 Inductive

2.1.2 Non-inductive

2.2 Metallized type

2.2.1 Normal single section

2.2.2 2-section single side metallized film



2.2.5 PSH/TSH

2.2.6 MMPP

2.2.7 PP-MPP

2.1 Film/foil type

Film/foil type of construction is chosen when the capacitor should be capable of handling

high currents (i.e. high dV/dt) and when the low capacitance value is needed.

High FIT (Failure In Time) rates accepted and generally film/foil type is recommended

for only less than 0.1μf and above 0.1μf it is not cost effective because of more raw

material consumption.

FIT rate can be represented as <10 fit (at 0.5Ur and 40 oC) for film/foil type. FIT

is represented in 10-9

/hour.

2.1.1 Inductive type

Inductive film/foil type is chosen on the following criteria,

Pitch < 10mm

Ip of signal upto 4A

High dV/dt

Cheaper price

But this type is not recommended for pulse applications and spikes because the chances

of failure is more due to the absence of self-healing. So it is not considered as reliable.

2.1.2 Non-inductive type

Non-inductive film/foil type is chosen on the following criteria,

Pitch >= 10mm





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Ip of signal > 4A

High dV/dt

This type, capable of handling more current than inductive type but this also not

considered as reliable because of the chances of failure at high spikes. It is better than

inductive type but with the compromise of cost.

2.2 Metallized type

This type of capacitors is chosen when,

A high capacitance value required in small size

A highly reliable capacitor required and failure is not allowed in its life time

because of its self-healing property.

Very low FIT rates

FIT rate can be represented as <5 fit (at 0.5Ur and 40 oC) for metallized type.

But the limitation is the low current carrying capability compared to the film/foil type.

That can be overcome by choosing appropriate type of metallized film capacitor.

2.2.1 Normal single section metallized capacitor

This type of construction is chosen for any kind of DC applications upto 1000Vdc of

rated voltage but for AC applications it should be chosen only when the signal voltage is

less than 275Vac.

Above 275Vac this construction is generally not recommended because of the more

chances for failure due to internal stress in the films.

But in some cases, single section metallized film is used for the capacitor with rated

voltage more than 275Vac. For example, rated voltage of 440Vac, single section is used

but the fact is that, the capacitor is subjected to maximum of 250Vac during its life. But

during testing, it must be tested at 440Vac for 4 to 8 hours. So, on such cases single

section is possible.





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But if the application is like, upto 440Vac applied continuously during its entire life then

the only possibility is series construction.

Check the continuous Vrms applied at each frequency and compare with the voltage

derating curve of the capacitor and make sure that the applied Vrms is less than the

Voltage in the derating curve at each frequencies.

If not, then this construction is not recommended. In order to handle more current double

side metallized construction should be chosen as in section 2.2.7.1.


This type of construction is chosen when the voltage of greater than 275Vac (and less

than 500Vac) is applied continuously during its life time.

In dc applications, above 1000Vdc and upto 1600Vdc is possible.



Voltage in the derating curve at each frequencies

If not satisfied, then this construction is not recommended. On such case, double side

metallized 2-section is recommended with compromise in the dimension.





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This type of construction is chosen when the voltage of 700Vac is applied continuously

during its life time. With reference to DC voltage it can be two types 1600Vdc or

2000Vdc, for that the thickness of dielectric is varied.




If not satisfied, then this construction is not recommended. On such case, double side

metallized 3-section is recommended with compromise in the dimension.

2.2.4 4-section single side metallized film.





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This type of construction is chosen when the voltage of 900 to 1000Vac is applied

continuously during its life time. With reference to DC voltage it is 2200 to 2500Vdc, for

that the thickness of dielectric is varied.



Voltage in the derating curve at each frequencies.

If not satisfied, then any other construction is not possible because of non availability

films in double side metallized film.

2.2.5 PSH/TSH

PSH: Polypropylene Self-Healing

TSH: Polyethylene Terephthalate Self-Healing

This type of construction is chosen for the following reasons,

When pitch<10mm

When the applied voltage is more than 400Vac and is possible upto 700Vac. (or

1250Vdc to 2000Vdc)

When high current handling is required, i.e. high peak current, high dV/dt.

High reliability for the capacitor is required where no failure is allowed in its life

time.

Due to the self-healing property of metallized film, it is highly reliable and available at

lower price





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When the operating temperature is less than 110 degrees and tanδ requirement is very

less then PP film is chosen. (i.e. PSH)

When the operating temperature is greater than 110 degrees and tanδ is not a concern

then PET film is chosen. (i.e.TSH)




If not satisfied, then other constructions are not possible because of the pitch dimension

problem.

2.2.6 MMPP

2.2.6.1 Single section

This construction is chosen for upto 1000Vdc for DC applications and upto 275Vac for

AC applications.

This construction is chosen, when a high current handling capability required compared

to the construction in section 2.2.1 with some compromise in the dimensions. This

construction has good current handling capability due to the double side metallized film.





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2.2.6.2 2-section and 3-section

This type of construction is made with double side metallized PET film and plain PP

films. In PP film double side metallization is not possible and the PET film is not

involved in tanδ.

This construction is chosen when the higher current carrying capability is required more

than the single side metallized series construction. The voltage possible is 500Vac or

700Vac (or 1250Vdc to 2000Vdc).

MMPP current capability is less than the PP-MPP type but if the size and material

consumption is a criterion then MMPP is the choice and it is the stable capacitor with

respect to temperature compared to single side metallized series construction.




If not satisfied, then this construction is not recommended. On such a case, the possible

construction is PP-MPP with compromise in dimensions.

2.2.7 PP-MPP





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This construction is made of film/foil and double side metallized film. This kind of

construction is chosen for the following reasons,

When pitch>10mm

When the applied voltage is more than 400Vac and is possible upto 500Vac. (or

1000Vdc to 2000Vdc)

When high current handling is required, i.e. high peak current, high dV/dt.

High reliability for the capacitor is required where no failure is allowed in its life

time.

But in this type, size is the main criteria. For high capacitance values, it is not cost

effective because of more raw material consumption.




If not satisfied, then other constructions are not possible and it cannot be designed.

3 Guidelines for selection of film type

This is about the film characteristics, they are

3.1 Edge characteristics

3.2 Wave cut film

3.3 Segmented film

3.4 Type of metallization on film

3.5 Resistance of the metallization

3.1 Edge characteristics

Normal flat film

Heavy edge film

Heavy edge film is having more current handling capability (i.e. more dV/dt) compared

to normal flat film. But the price of HE film is 5 to 6% more than the normal film. So it is

recommended for high peak currents if the price is not a concern. HE is also referred as

Reinforced edge.





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3.2 Wave cut film

Cutting the edge of the film like sine wave is the wave cut film. Wave cut film minimizes

tanδ and ESR due to good end spraying and hence better welding contact. It can handle

high pulse currents, more dV/dt and more reliable. In a way, it minimizes the stagger so

gains more active area and reduces size. Due to good spray, it is more rigid at the ends.

Two types,

3.2.1 Wave cut at free margin

3.2.2 Wave cut at metallized portion

3.2.1 Wave cut at free margin

The wave cut at the free margin distributes tensions between the film layers more evenly.

Less mechanical stress at pressing and heat treatments. But the limitation is, due to

minimal free margin the insulation resistance and break down voltage is reduced.

3.2.2 Wave cut at metallized portion





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This type overcomes the disadvantage of previous type. i.e. It has no effect on insulation

resistance and breakdown voltage. This type is recommended for small free margins.

Care must be taken while setting the offset (stagger) in the winding machine while using

wave cut film. The rule is; winding offset is equal to the amplitude of the wave.

Generally the wave cut film is about 15% higher price than the normal films, but if the

above requirements are necessary then the price should not be a concern.

Generally for voltages above 1200Vdc, wave cut film is very necessary.

3.2 Segmented film

Top view of the T-segmented film Diamond Pattern

In this construction, one film is segmented and other is normal film. This type of film is

chosen on the following requirements,

When the internal stress is very high, due to high pulse and current

Explosion of capacitor is not allowed due to self-heating of capacitor

More breakdown voltage required.

Safety is of major concern rather than material consumption and cost

Segmented film is expensive. T-segment is about 15% higher costs than the normal film

and consumes slightly more material.

Diamond pattern is very high cost and consumes 10% more material and increases size

abruptly.

During the working, when any short circuit in film the fuse of particular section opens

thereby reducing the stress but compromises the capacitance value. i.e. capacitance value

decreases.





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In T-segment, each T-segment will be disconnected gradually so the capacitance change

(decrease) will be more. In diamond pattern, due to smaller sections capacitance change

will be less, hence more reliable.

3.4 Type of Metallization on film

This is the type of metal layer on the dielectric film, generally two types of metal used,

3.4.1 Aluminium

3.4.2 Zinc

3.4.1 Aluminium

Aluminium metallization is chosen for the following reasons,

DC Application

Less leakage current

Capacitance accuracy i.e. capacitance tolerance is very less

Self life is about 2years

Since the self life is more there is no oxidation problem but the price is 15% higher than

zinc. It is not recommended for AC applications because self-healing is not as good as

zinc.

3.4.2 Zinc

Zinc metallization is chosen for the following reasons,

For AC Applications due to good self-healing

Extreme low capacitor loss

Loss independent of resistance of metal layer

That is, loss does not increase for neither thick coated nor thin coated films. But the

limitation is the self life (only 6 months); zinc oxidizes faster so once the film packing is

opened then within 48 hours upto spray section of manufacturing process should be

completed after that, it is difficult to make a good contact with the film.

But there is an exceptional case where zinc film cannot be used in AC applications at

high humidity & temperature conditions. Because the moistures air will penetrate the

epoxy and oxidize the zinc then the capacitor fails. So at high humidity & temperature

conditions, aluminium film is the only choice provided it is used at low voltage and low

frequency applications.





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At high voltage applications, aluminium film fails because of poor self-healing. So in

order to reduce the stress in the film series construction can be made using PET film to

match the dimensions. For PP films, the size will be bigger.

3.5 Resistance of the Metallization

The resistance of the metal layer on the film is related to the thickness of metal layer on

the film. When the thickness is more, resistance is low and vice versa.

Thick coating (Low resistance) Thin coating (High resistance)

Good face contacts & good pulse current

handling

Highly stressed face contact, heavy edge

necessary

Poor self-healing Good self-healing

Deterioration of insulation resistance Good insulation resistance and dielectric

strength

No damage to metal layers during

production

Possibility for damage is more. Careful

handling is required

Not possible on thin and heat sensitive

films

Suitable for thin & heat sensitive films

BDV is less BDV is high

For high voltage DC Applications 1.5Ω aluminium is used and 3Ω is used for normal

applications.

For zinc, generally heavy edge type with 2.5 Ω at edge and 7.5 Ω at the active area is

used.

7.5 Ω of zinc is equivalent to 2.5 Ω of aluminium in metal thickness. So the resistance of

the film is chosen accordingly and the resistance does not affect the price of film.

For high resistance films, heavy edge is compulsory otherwise spray contact with the film

is not possible.

Maximum resistance for aluminium flat film is 1.5 Ω and

Maximum resistance for Zinc flat film is 2.5 Ω





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So for PP film with zinc metallization, if the operating temperature is 105oC then it needs

15 Ω for 4μ film and it is 10Ω for 6μ film in order to have good self-healing property.

Similarly for PP film with aluminium metallization, if the operating temperature is 105oC

then it needs 7Ω for 4μ film and it is 5Ω for 6μ film in order to have good self-healing

property.

For film thickness more than 7μ, it is obvious that the resistance is constant.





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Similarly for PET aluminium metallized film, the resistance with respect to temperature

and film thickness can be known from the above shown graph.

For the best operation of a capacitor, one film resistance should be very high to have

good self-healing and another film resistance should be very low to handle high current.

4 Design sheet verification

Feed the details concluded from the above guidelines into the design sheet.

Capacitance value

Rated voltage

Film dielectric constant

Type of construction

Choose the dielectric constant of 2.2 for PP, 3.2 for PET and 3 for PEN & PPS. Winding

offset depends on the capability of the winding machine. Winding offset of 0.3mm is

enough to make a good spray contact but according to machine accuracy it is set. Some

machines possibility is only 0.5mm and some other standard machine can go upto

0.2mm.

Winding offset is not an important parameter in normal films but care must be taken that

it should not affect the pitch. If the wave cut film is chosen, then winding offset plays an

important role since it affects the electrical parameters as discussed.





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4.1 Guidelines for choosing film thickness

Choosing the dielectric film thickness according to the rated voltage and construction

type is an important work in the design sheet.

Film thickness is based on

PP film 70Vdc/micron or 50Vac/micron

PET film 70Vdc/micron or 50Vac/micron

But there are two factors affecting the above rules,

Machine capability to handle lower micron films

Machine capability to handle smaller wound elements

That is, machine requires a minimum outer diameter of wound element so that it can be

processed further.

So, the film thickness is increased till the required dimension is achieved. It is always

good to have higher film thickness for better dielectric strength if the dimensions are

matched with the customer specified dimensions.

For example: Fan regulator type PET film capacitor with rated voltage 250Vac uses

minimum of 4.3μ, if good reliability is required, then 4.6, 5.6μ films can be used but the

size will be bigger.

For series construction, actual voltage = rated voltage/number of sections.

Then the film thickness is decided from the actual voltage.

For example: 1600Vdc/500Vac 2 section PP film capacitor. In this series type capacitor,

the rated voltage gets divided in the internal series construction.

So actual voltage is 1600/2= 800Vdc or 250Vac

For PP film, 70V/micron; so for 800Vdc 12μ is the reliable film.

But in order to reduce the dimension, 80V/micron rule used then 10μ is sufficient

Even 90V/micron can be used, it takes only 9μ film.

This is the way film thickness is chosen to match the dimension.

If the film thickness goes beyond 20micron then divide it into two films. Since, film is

not available above 25μ.

In some cases, if the size of the wound element is big then divide it into two films and

reduce thickness of each film.

For eg: Two 8μ films can be replaced for a 20μ film.

This has the advantage of higher film strength and higher breakdown voltage but the

electrical properties (tanδ, IR) must be ensured that it is same.





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4.1.1 Criteria for Inductive type:

Choose Aluminium foil width 3 to 4mm less than the film width to avoid breakdown at

high voltages due to charge jump from one layer to another.

Choose any one of the foil width 1mm greater than another foil width in order to avoid

tolerance rejection due to slip in the winding.

Thickness of foil is not considered; generally 5 or 6μ is used. Though it is related to

current carrying capability above 6μ will affect the dielectric strength and reduce the

BDV. Below 5 μ is not compatible with winding machine.

4.1.2 Criteria for metallized non-inductive type:

In some cases, if the dimension is needed to be reduced for the PP film type capacitor,

voltage is increased from 70Vdc to 90Vdc per micron even sometimes 100Vdc per

micron in order to meet the dimension but the dielectric strength reduces.

4.1.3 Guidelines for choosing free margin of the film

The free margin of the film is directly related to the breakdown voltage of the capacitor.

1mm free margin corresponds to a BDV of 1000Vdc. It also affects the active area of the

capacitor; thus leads to bigger size of the capacitor.

So the free margin is selected by considering the above parameters.

4.1.4 Guidelines for choosing width of the film

Width of the film has a big effect in the dimension and it mainly affects the pitch of the

capacitor.

Inductive type:

For inductive type capacitor, the width of the film does not affect the pitch rather it

affects the height of the capacitor. So for inductive type capacitor, the film width is

chosen slightly 2 to 3mm less than the height of the capacitor.

Non-inductive type:

For non-inductive type capacitor, there is a rule

Film width + stagger <= pitch

But the MMPP 2-section and PP-MPP capacitors will deviate from this rule due to its

construction





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For MMPP 2-section, the double side metallized PET film is chosen 1mm less than the

pitch and for PP film,

PP film width = MMPET film width - 2*slip of MMPET film.

For PP-MPP, the PP film is chosen 1mm less than the pitch and for MMPET,

MMPET film width= PP film width -2*free margin (here empty space or gap).

For series construction capacitors, make sure that the centre margin of the film is nearly

two times the free margin to reduce the stress on the film.

Compare the obtained dimensions from the design sheet with the customer requirement.

If the dimensions are matched, it is accomplished otherwise adjust the mandril diameter

or increase/decrease the film thickness to match the dimensions without deviating from

the V/micron rules.

5 Guidelines for selection of spray coating material

Possible types of coatings are,

Pure zinc 99.9%

Pure tin 99.9%

Pure aluminium 99.9%

Zinc-Aluminium 85:15

Tin-zinc 70:30

Generally for a capacitor, two types of the above coating should be done to make easier

for next process called welding.

Base coat

Top coat

5.1 Top coat

It is always recommended to make the top coat/outer coat with tin-zinc 70:30 because tin

is very resistant to oxidation and it makes the capacitor capable to be handled in high

speed spot welding machines to make the welding effective.

5.2 Base coat

If the capacitor is made with zinc metallized film or Zinc HE Al flat metallized film, the

only preferred coating is pure zinc in order to make good contact with the film.





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If it is made of pure aluminium metallized film, then first preferred coating is Zn-Al

85:15 to make good contact but it is not cost effective and difficult to make thin coating

Second choice is Tin-Zn 70:30 but if that also not cost effective then the third choice is

pure zinc, it is cost effective but only if the electrical parameters like ESR, tanδ are

satisfied.

Pure aluminium and pure tin is not used because of very expensive nature.

The thickness of total spray coating affects the parameters like ESR, Dv/dt. For less ESR

and high Dv/dt, the spray thickness should be more.

Note: For the case of high speed spot welding process only these two kinds of coating is

needed. If it is manual welding process, pure zinc base coat alone is enough to make

contact.

6 Guidelines for selection of lead wire:

Two types of lead wires used to make contacts,

Steel wire

Copper wire

Copper wire is of low loss factor and low ESR but is three times more in price than steel

wire. Strength is more for the steel wire but loss factor and ESR is very high.

Generally for low loss applications and AC applications, copper is the ideal choice.

For inductive type capacitors, generally 0.5mm thick wire is recommended for better

strength since more thick wire will damage the foil.

For Non-inductive type capacitors, generally 0.6mm for pitch <=10mm and 0.8mm for

pitch >10mm is used.

The lead wire length is having effect on ESR and ESL. When lead length is more ESR

and ESL will be more but sufficient lead length is required to fit in the PCB. So minimum

of 17mm lead length is recommended.

7 Guidelines for selection of outer encapsulation:

Two types of outer coatings are given to the capacitors,

Powder coating

Box type

In the powder coating type, two types of epoxy resin are used for capacitors.





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Normal grade epoxy and flame retardant grade epoxy. Though flame retardant grade

epoxy is higher price, it is the only recommended resin because safety is the main

concern. The disadvantage of powder coating is that, it is not resistant to humidity.

When the environment humidity is high, chances for failure is more.

Box type overcomes the disadvantage of powder coating. Box is resistant to humidity and

it is recommended when there is more possibility of explosion risk in its life. Generally

flame retardant grade PBT type UL 94V-0 type box filled with flame retardant grade

epoxy resin like SE770/MH770 is used. It is clear that box type is expensive than the

powder coating type.

To reduce the cost, Non flame retardant PPR CAN can be used and also non flame

retardant grade resin like SE01/MH01 can be used.





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Annexure II

Reference:

10.1 dV/dt:

It is the rate of change of voltage with respect to time.

dV/dt = Vpp/(t2-t1)

The above formula gives the dV/dt in volts per micro seconds during the start phase.

10.2 Temperature co-efficient α:

α = (c2-c1)/c3*(t2-t1)

Where c1 is the capacitance at lower temperature t1

c2 is the capacitance at higher temperature t2

c3 is the capacitance at reference temperature 20+-2 oC





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10.3 Humidity co-efficient β:

β = 2*(c2-c1)/(c2+c1)(h2-h1)

c1 is the capacitance at relative humidity h1

c2 is the capacitance at relative humidity h2

Note: the above formula is valid only when RH is more than 1% and at constant

temperature

10.4 Dielectric absorption:

Capacitor is charged at the rated voltage for 60minutes and the initial current should not

exceed 50mA. Then the capacitor is discharged through a 5Ω resistor for 10seconds and

the voltage remaining after discharge is measured

Absorption % = (u1/u2)*(c1+c0)*100/c1

Where u1 is the remaining voltage after discharge

u2 is the charging voltage

c1 is the capacitance value

c0 is the capacitance of voltage measurement unit (input capacitance)

Note: Measurement unit should have a resistance of atleast 10000MΩ.

10.5 Time constant and Insulation Resistor:

Polypropylene Inductive type:

If C<=0.1μf, IR >= 100000MΩ

If C>0.1 μf, IR >= 10000MΩ.

PET Inductive type:

If C<=0.33μf, IR >= 30000 MΩ

If C>0.33 μf, τ=10000 S

Metallized Polypropylene non-inductive for AC applications:





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If C<=0.33 μf, IR >= 6000MΩ

If C>0.33μf, τ=2000 S

Metallized Polypropylene Non-inductive type for DC applications:

τ=R*C in S IR (MΩ)

If C>0.33 μf If C<=0.33μf

Ur>100 Ur<=100 Ur>100 Ur<=100

Grade 1 Grade 2 Grade 1 Grade 2 Grade 1 Grade 2 Grade 1 Grade 2

30000 7500 15000 3750 100 25 50 12.5

Metallized PET Non-inductive type:

IR (MΩ)

If C>0.33 μf If C<=0.33μf

Ur>100 Ur<=100 Ur>100 Ur<=100

Grade 1 Grade 2 Grade 1 Grade 2 Grade 1 Grade 2 Grade 1 Grade 2

10000 2500 5000 1250 30000 7500 15000 3750

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