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Film capacitors - general information

There are two basic groups of capacitors:

Film-foil capacitors, which ensure very high pulse load and current handling capability, very good capacitance stability and reliability, high insulation resistance and very low dielectric losses. Special construction of this capacitors allow also the self healing property.

Metallized film capacitors with excellent self healing property, very small dimensions. The contacting is made by spraying of metal alloys onto windings face ends. The leads are then welded on these sprayed faces. By the spraying are the windings short circuited. This technology ensures very low self inductance and high resonance frequency of the capacitors.

The dielectric materials used in capacitors are:
Polyester film, or metallized polyester film
- MKT capacitors
Polypropylene film or metallized polypropylene film.
- MKP, MKPI and KPI pulse capacitors

A comparison of the characteristics of the capacitors is shown in the following table:

 

Dielectric properties of capacitors

Type	MKT	MKP
Relative dielectric constant e	3,2	2,2
DF at 1 kHz, tan d	0,005	0,0005
Ris [G W x mF]	25	100
Dielectric absorption [%]	0,2	0,05
Capacitance Drift D C/C [%]	1,5	0,5
Moisture absorption [%]	0,4	0,01
Maximum temperature [°C]	100 - 125	85 - 100
Tc [ppm/°C], [10-6/°C]	+400, ± 200	-200, ± 100

MKT capacitors have high dielectric constant, high dielectric strength, excellent self-healing properties, good stability, positive temperature coefficient (+400ppm/°C). MKT capacitors are regarded as general purpose capacitors and are preferably used for DC applications as decoupling, blocking, bypassing and noise suppressing capacitors.

MKP, MKPI and KPI capacitors have superior electrical parameters, very low dielectric losses, very high insulation resistance, very low dielectric absorption and very high dielectric strength, excellent moisture resistance and very good long term stability. Temperature coefficient is negative (-200ppm/°C). 

Typical applications:

• AC and high pulse applications at high frequencies 
• switch mode power supplies and snubber applications
• filter circuits
• sample and hold applications and other applications where are the excellent features necessary.

Rated capacitance
Rated capacitance of a capacitor is measured at 1 or 10kHz and at +20°C, 1kHz is reference frequency. Detailed description of measuring methods and measuring conditions you find in generic standard IEC 60384-1 and in corresponding sectional specifications.

Dissipation factor
express the losses of the dielectric material, contact-resistance and insulation resistance. Dissipation factor is the ratio between the resistive and the reactive part of the impedance of the capacitor, expressed by tgd.

 

Dissipation factor change as a function of temperature at 1 kHzz

Dissipation factor change as a function of frequency (room temperature)

 

The dissipation factor is especially important when the capacitor operates on AC. The losses in capacitor causes heating of the capacitor. The heating may lead to a destructive breakdown at high frequencies if the losses are high.

Equivalent series resistance, ESR
is the resistive part of equivalent series circuit. It depends on the resistivity of electrodes, internal connections contacts and dielectric losses. It is frequency and temperature dependent.

 

Ls - Parasitic series inductance
Rs - Series resistance of leads and internal contacts
Rp - Parallel insulation resistance
C - Capacitance of capacitors

Insulation resistance
is measured at reference-voltage, usually 100V, possible 10VDC or up to 1000VDC, after 1min. charging and at +20°C and relative humidity RH=50%. Basically 100VDC measuring voltage is used. For capacitor values C>0,33mF the RIS is expressed as time constant t = R x C [MW; mF] The RIS decreases with increasing temperature.

 

 

 

Rated voltage
The rated voltage is the max. direct voltage or the max. RMS alternating voltage, or the peak value of a pulse voltage, which may be applied continuously to the capacitor at any temperature between the lower category temperature and up to a rated temperature of +85°C.
For temperatures higher than the rated temperature +85°C the rated voltage has to be derated in dependence upon the type of used dielectric, to prevent or exclude any damages of the capacitors.

 

 

Superimposed AC voltage 
when alternating voltage are present, eventually in addition to direct voltage, the sum of both, U_DC and the peak value of U_AC must not exceed the rated voltage U_R of the capacitor. 

AC applications 
Wwhen the capacitor is used in any AC applications internal heating may arise, which is caused by the heating effect of the
current flowing through the internal resistance of the capacitor. The dissipated power by the capacitor is
P_D = (U_RMS)² x 2pfC x tgd
The max. permissible P_D must not cause the increasing of the surface temperature of the capacitor under working conditions higher as DT_S < 10°C

For the operations in the higher frequency range, the permissible AC voltage has to be derated. The permissible AC voltages are shown in the graphs Permissible AC voltage versus Frequency for each Type of capacitor.

 

 

When using the capacitors in a high frequency circuit, pleas apply the max. voltage which does not exceed the value shown in the corresponding diagram.

Climatic category
appoint the climatic conditions in which the capacitor may be operated continuously. According to IEC 60068-1 the climatic category is expressed by three groups of numbers 55/100/56.

First group define the lower category temperature T_MIN (-55°C), which is also the test temperature for test Aa cold, IEC 60068-2-1.

Second group define the upper category temperature T_MAX (+100°C), which is the test temperature for test Ba-dry heat in accordance with IEC 60068-2-2.

The third group specify the number of days of exposure by the Damp heat steady state-Ca Test at 95% relative humidity and +40°C in accordance with IEC 60068-2-3

Maximum Temperature
or upper category temperature is the highest temperature at which a capacitor may still be working. At pulse or AC operation, the sum of the max. ambient temperature and the temperature increase by the load conditions ( 10°C), must not exceed the upper category temperature.

Pulse load and current handling capability Permissible pulse rise time is defined by dU/dt [V/msec]

 

 

Voltage pulses with rapid voltage changes dU/dt will lead to strong current I_P (peak current) in the capacitor 
Pulse current I_P = C x dU/dt

flowing through the capacitor, causes a local heating of the contact area of the capacitor, between the sprayed-on metal 
terminations and the metal or metallized layers. To prevent the risk of damage, minimal resistance in series with the capacitor, which is connected to the source with very low R_IN is necessary
R_S = UR/(C x dU/dt)

The dU/dt is referred to the rated voltage U_R.

If the applied pulse U_OP voltage is lower than U_R voltage, higher dU/dt value can be permitted
dU_OP/dt = dU_R/dt x U_R/U_OP

Capacitance changes up and down with temperature due to the dielectric constant and the temperature coefficient of the dielectric material.

The temperature coefficient is expressed by ac = C₂ - C₁/C₁(T₂-T₁)

C1: capacitance at the temperature T1
C2: capacitance at the temperature T2
reference temperature is T1 
ac is expressed in ppm/°C.

Depending upon the dielectric material the aC can either be positive +400 ppm - MKT capacitors, or negative
200 ppm - MKP capacitors.

 

Capacitance change versus temperature

 

Equivalent series inductance: ESL

 

The real capacitor has a certain self inductance due to the length of connections and the construction of capacitive element. This inductances represents the Equivalent Series Inductance. 
The value of this inductance is indicated by resonant frequency of the capacitor at +20°C ±5°C. At resonant frequency the capacitive reactance equals the inductive reactance
1/(2pf x C) = 2pf x ESL

The ESL affects the impedance Z(f) of the capacitor.

 

The construction of the capacitors enables to achieve very low ESL and high resonance frequency of the capacitors. At resonance frequency the impedance of the capacitor is minimal and is equal to ESR of the capacitors. Self inductance of the capacitors depends on the length of leads and on the length of the body of capacitor. The capacitors in prismatic cases end leads with the length 2mm have the typical ESL.

 

 

The round or oval capacitors with axial leads have ESL approximately 1nH per 1mm leads and body length.

Rated RMS Current I_RMS is the highest permissible RMS value of the continuous current flowing through the capacitor at the max. case temperature of +70°C.

Operating at the rated RMS current, the capacitor produces a case temperature rise of about +10°C over the ambient due to the resistive losses. The RMS current I_RMS must be derated taking into account the ambient temperature.

The max power which may be dissipated by the capacitor is P_Dlim = D T_MAX / J _R

J_R - thermal resistance of the capacitor [°C/W]
DT_MAX - permissible temperature increasing of the capacitor-surface 
Informative thermal resistance of the capacitor-cases are in following table.
Simply you can calculate the max permissible dissipated power of the capacitor by using following formula:

P_D = K x S x DT_MAX

S - surface of the capacitors-body [cm2]
DT_MAX : 10°C
K - coefficient 1,0 - 2,5
lower K values are to be used for the general purpose metallized film capacitors higher K= 2 - 2,5 values for the 
pulse capacitors, IGBT, GTO and special construction.

 

Technical parameters of the capacitors - cases

Rozměry pouzdra [mm]			Rozteč	Povrch [cm2]	Tepelný odpor	P Dlim
B	H	L	P	S	J R [°C/W]	[W]
6	15	26,5	22,5	12,93	43	0,32
7	16	26,5	22,5	14,43	41	0,36
8,5	17	26,5	22,5	16,405	38	0,41
10	18,5	26,5	22,5	18,805	36	0,47
11	20	26,5	22,5	20,83	34	0,52
9	17	32	27,5	19,7	35	0,49
10	20	32	27,5	23,2	32	0,58
11	20	32	27,5	24,24	31	0,61
13	22	32	27,5	28,12	29	0,70
14	28	32	27,5	34,72	26	0,87
15	24,5	32	27,5	32,63	27	0,82
18	33	32	27,5	44,52	23	1,11
22	37	32	27,5	54,04	21	1,35
10	20	42,5	37,5	29,50	28	0,74
11	22	42,5	37,5	32,89	27	0,82
13	24	42,5	37,5	37,69	25	0,94
16	28,5	42,5	37,5	46,945	23	1,17
19	32	42,5	37,5	55,51	21	1,39
20	40	42,5	37,5	67	19	1,68
24	44	42,5	37,5	78,92	17	1,97

 

During the operation the capacitor-temperature has always to be lower as the max. operating temperature. Therefore the P_D always has to be lower as the P_Dlim, which the case of the capacitor can radiate. The dissipated power of the capacitor, working with AC, is determined by the working U_RMS or I_RMS, working frequency and tgd(f) of the capacitor.

P_D = U² _RMS x tgd x 2pfC = I² _RMS x tgd / 2pfC

The max. value of the U_RMS under which the capacitor can continuously works is also limited by the ionization-effect, or corona-effect. The ionization effect appears, if the working voltage is to high and can cause a destructive process in the capacitor.

The limited U_RMS is defined in the range till the working frequency f1. Between f1 and f2 the U_RMS have to be reduced, not exceed the max permissible dissipation Power in the capacitor according to previous formulas. Between f2 and f3 the I_RMS has to be limited by the construction of leads of the capacitor.

For example, if the leads are tinned copper wire

0,8mm = 0,5mm2	IMAX = 7,0A
1,0mm = 0,785mm2	9,0A
Lugs	16 - 20A
Fastons	16 - 24A
Screws M4	20A
M6	40A
M8	63A
M10	100A
M12	160A

These statements is to be taken as informative, the practical test in particular applications should always be made in order to verify the correctness of the theoretical assumptions.

 

Alternating voltage and current versus frequency

 

Capacitance change as a function of frequency (room temperature)

 

Quality assurance
The Quality of the electronic components, which the firm ELECTRONIC COMPONENTS CZ, a.s. manufactures and supplies, is based on the continuous care of the procedures in the manufacturing process. In order to assure the highest quality and reliability of all components, the parameters has been continuously monitored during all production process and also in the firm testing-laboratory. The manufacturing process has been certified by ISO 9000 Standards and the capacitors have been certified in accordance to IECEE-CB system. The IECEE-CB system incorporate about forty international testing laboratories, which are obliged to acknowledge these and guarantee national CB-certificates.

Reliability of electronic components depends on:

• the construction of the components
• manufacturing process 
• the application-conditions, electrical stress and temperature
  

Regarding the first and second condition, the manufacturer guarantees the best care and supplies 100% tested components exclusively. To help the customer to rich optimal conditions and life expectancy of the electronic components we recommend taking into account following legality, which we describe below.

The expected life of the component is the time required to rich the failure. There are the critical failures, as short or open circuit and failures caused by exceeding limiting values of the 
critical parameters, for example:
DC/C 10% for MKT capacitors
DC/C 5% for MKP, KPI  capacitors
increasing of tgd > 2 x initial limit for MKT
increasing of tgd > 3 x initial limit for MKP, KPI

Failure rate l is the fraction failure divided by the operating time and it is expressed in FIT, (failure in time
1FIT = 1 x 10^-9/h failure per 10⁹ hour - components. The failure rate is referred to the failure rate criteria

l = n/(n x t_t)

n : Number of failures
N : Number of tested components
t_t : test time in hours
The 1 / l = mean time between failures (MTBF)

The operation time comprises operational time and interruption time as well as the time for storage and transportation. It is referred to +40°C and working voltage U_W = 0,5 x U_R and upper confidence level of 60%.

The life expectancy of electronic components subjected to a different voltage from the reference-voltage, can be approximately calculated by following formula

L_W = L_R x (U_REF / U_W)⁸

The life expectancy if the working temperature is different, can be calculated by formula

L_OP = L_TO x 2 ^(To-Tw)/Ac

To : reference temperature
Tw : working temperature
Ac : Arrhenius coefficient, typical AC = 7

The above formula is derived from Arrhenius equation, describing the ageing of dielectric films in function of the temperature and gives good results if the temperature range taken in consideration, is not to wide. The FIT ratings for other working voltage and other temperature conditions can beset up as follow

l = l _REF x C_U x C_T

C_U : Voltage conversion factor
C_T : Temperature conversion factor

For good estimation of influence of temperature and working voltage on the service life of the capacitors, which work by AC, you can use following diagrams.

 

 

Voltage conversion factor

Load ratio UW/UR [%]	Type, Typ MKT	Type, Typ MKPI, KPI
100	6	11
75	2,5	3
50	1	1
25	0,4	0,4
10	0,2	0,2

Temperature conversion factor

Temperature [°C]	CT
40	1
55	2
70	5
85	12
100	33
125	350

 

Further factors, which affect the failure ratings in practical application are

• working environment C_E
• influence of the capacity value C_C
  
  C_C = 1,4 x C^0,12

Working environment

Enviroment	CE
Laboratory	1
Consumer electronic	1,2 - 1,5
Industrial electronic	2,0 - 3,0
Automotive electronics	3,0 - 4,0

 

 Influence of the capacity value C_C

 

The failure rate is, of course, a function of time also. Typical components failure rate curve is shown below.

Typical components failure rate curve

 

The Bath tube curve represents the characteristic shape of the failure rate ower the operation time. It has three periods.

1. Period, in which early failures occur
2. Period, in which the failure rate is practically constant
3. Period, in which the failures due to wear of the components increase

The failure rates data are referred to the second part of the curve.

To minimalize the influence of early failure rates, the burn-in procedure, or artifical ageing procedure of the components is 
used. On request the components are subjected to a defined electrical and thermal stress and thereafter 100% tested and sort.