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Design Guidelines for Film Capacitors
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Department
Prepared by:
Mr. Vishnu
Checked & Approved
by:
Mr. Sankar
History
Revision
Signed
R&D and Engg.
R&D and Engg.
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
2
3
4
5
6
7
Guidelines for selection of dielectric film
Guidelines for selection of construction type of capacitor
Guidelines for selection of film type
Design sheet verification
Guidelines for selection of spray coating material
Guidelines for selection of lead wire
Guidelines for selection of outer encapsulation
Annexure II:
10.1
10.2
10.3
10.4
10.5
Dv/dt
Temperature co-efficient α
Humidity co-efficient β
Dielectric absorption
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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Tanδ Vs Temperature
Insulation resistance Vs Temperature
Tanδ Vs Frequency in Hz
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For PEN & PPS film:
Capacitance change ∆C/C Vs Temperature
Tanδ 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
o
C, 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.
Temperature Co-efficient α:
1.3
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
PP
PET
Temp co-eff. Α
(*10-6/Kelvin)
-250
+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
PP
PET
β (*10-6 per % of RH)
40 to 100
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<0.001 at 10 kHz,
Tanδ<0.003 at 100 kHz.
For PET film,
If tanδ<0.008 at 1 kHz,
Tanδ<0.015 at 10 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.
Type
Density
g/cm2
PET
1.4
PPS
1.35
PP
0.91
PEN
1.36
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Comparison of dielectric films
Melting Operating BDV
Tanδ
o
o
pt. C
temp C
(1kHz)
%
263
-40 to
200 to
0.5
120
300
285
-40 to
200 to
0.06
175
300
160 to
-40 to
300 to
0.02
170
105
400
266
-40 to
240
0.4
150
Approved by
Dielectric IR
constant
MΩ/μF
3.2
30000
3
100000
2.2
300000
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
2.2.3 3-section single side metallized film
2.2.4 4-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.
2.2.2
2-section single side metallized film
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.
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 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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2.2.3 3-section single side metallized film
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.
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 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.
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 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)
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 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.
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 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.
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 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)
Good face contacts & good pulse current
handling
Poor self-healing
Deterioration of insulation resistance
No damage to metal layers during
production
Not possible on thin and heat sensitive
films
BDV is less
Thin coating (High resistance)
Highly stressed face contact, heavy edge
necessary
Good self-healing
Good insulation resistance and dielectric
strength
Possibility for damage is more. Careful
handling is required
Suitable for thin & heat sensitive films
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:
IR (MΩ)
If C<=0.33μf
Ur>100
Grade 1 Grade 2
100
25
Ur<=100
Grade 1 Grade 2
50
12.5
IR (MΩ)
If C<=0.33μf
Ur<=100
Ur>100
Grade 1 Grade 2 Grade 1 Grade 2
5000
1250
30000
7500
Ur<=100
Grade 1 Grade 2
15000
3750
τ=R*C in S
If C>0.33 μf
Ur>100
Grade 1 Grade 2
30000
7500
Ur<=100
Grade 1 Grade 2
15000
3750
Metallized PET Non-inductive type:
If C>0.33 μf
Ur>100
Grade 1 Grade 2
10000
2500
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