Power Film Capacitor Application Guide
PAGE
CONTENTS
DC Capacitor Overview
153
Construction
153
Metallized Capacitors
153
Film/Foil Capacitors
153
Hybrid Capacitors
153
Custom Designed Film Capacitors
154
Applications for Power Film Capacitors
154
DC Link for Inverter Applications
154
Advantages of Film vs. Aluminum Electrolytics for DC Link Apps 154
DC Output Filtering
154
IGBT Snubber
154
Definitions
154
the dielectric system vaporizes the metal deposit in the area of
the fault, a process known as clearing. The result of “clearing” is a
tiny amount of capacitance loss while allowing the capacitor to
continue to operate without any adverse effects. If a condition
arises that causes multiple clearings, such as overvoltage, or
dielectric aging at end of life, the capacitor will continue to self
heal and lose capacitance. The capacitor is considered to have
failed when it loses 3% or more of its capacitance.
Changing the metallized electrode thickness alters the
properties of the capacitor. Lighter metallization, higher ohms
per square, result in higher energy density designs. While light
metallization improves the voltage capabilities, it compromises
the rms and peak current carrying capabilities of the capacitor.
Patterned metallized layers add another dimension to the mix
of options. Metallized patterns have built in fuses that further
enhance the self healing capabilities and dielectric voltage
withstanding properties of the system.
Film / Foil Capacitors
--- Foil
DC FILM CAPACITORS FOR POWER ELECTRONICS
AN OVERVIEW
--- Film
Film capacitors are widely used in power electronics applications
--- Film
including but not limited to DC Link, DC output filtering, and as
IGBT snubbers. The dielectric most often used is polypropylene
because it has low dissipation factor (DF) that permits high
AC currents with low self heating, and it performs well over
the temperature range and frequencies in power electronics
applications. Other materials such as polyester (PET) may be
used for light duty filtering but its high dissipation factor makes
it a poor selection where high AC current or high peak current at
high rep rates are encountered. Materials such as polycarbonate
and PPS have desirable characteristics for power electronics
applications but they are in scarce supply and are therefore
relatively expensive and exotic materials.
--- Foil
Aluminum foil electrodes are used where very high peak
and rms currents are required. IGBT snubbers, for example,
are designed to handle the high peak currents encountered
during IGBT switching. Typical IGBT applications, such as those
encountered in high power inverters, have voltage rise times
exceeding 1000 V/µs with switching rates of 10 kHz or more. The
end connections of a capacitor employing a simple metallized
electrode system would deteriorate with repeated exposure to
these conditions. Foil capacitors use electrodes that are about 5
microns thick to handle the high current pulses.
The capacitor’s electrode system is an important design
consideration. There are three basic options for electrodes used
with polypropylene capacitors. A description of each follows:
Foil electrodes are also used where the capacitor will see high
rms current, especially where the capacitor size is small. As an
example, tank circuits for induction heating devices typically
require capacitors less than 100 µF that must handle hundreds of
amperes. The main benefit of the foil electrodes is to reduce the
heat rise by reducing ESR. Cooler operation prevents thermal
runaway and dielectric failure from self heating. The main
disadvantages of foil electrode capacitors are their inability to
self heal and low energy density relative to metallized types.
Metallized Capacitors
Hybrid Capacitors
This catalog features only polypropylene types which cover the
vast majority of power electronics applications.
CONSTRUCTION
--- Metallized
Film
--- Foil
--- Metallized
Film
--- Metallized
Film
Metallized capacitors use a thin layer of vapor deposited
aluminum, zinc or alloy (aluminum/zinc) blend as the electrode
system. The metallized layer is only hundreds of angstroms
thick, so it takes up little space in the capacitor winding relative
to the dielectric thickness, measured in microns. Metallized
capacitors offer the highest energy density of all of the available
film constructions. Metallized capacitors also self heal. A fault in
--- Film
One scheme that combines the benefits of metallized and
foil electrode types, is the hybrid series capacitor. It has foil
electrodes that connect to the external leads of the capacitor
and afree-floating metallized electrode wound in a series
configuration. The result is a self healing capacitor that handles
high current pulses.
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With all of these variables at play, the choice of dielectric,
electrode metals, electrode thickness and metallized pattern
must be considered to optimize the capacitor’s performance for
a specific application.
CUSTOM DESIGNED FILM CAPACITORS
This catalog offers standard capacitor products for use in
common power electronics applications. Inverter design
engineers are often challenged with finding a capacitor
that takes them out of the realm of standard catalog items.
Packaging a capacitor to accommodate a physical bus structure
is a common design challenge. Designing for a specific
ripple current rating and life expectancy at a given ambient
temperature is another consideration that usually requires
custom solution. CDE is highly experienced in custom capacitor
design and manufacturing. Where possible, we “repackage”
standard materials to meet specific customer requirements.
APPLICATIONS for POWER FILM CAPACITORS
The most common applications for DC film capacitors in power
electronics are DC Link, DC Filtering and snubbers for IGBT
modules. A brief description of each application follows:
DC Link for Inverter Applications
Large value capacitors are used as the energy storage element
or DC-Link at the DC input to the inverter. The size of the DC Link
depends on the amount of AC energy it must absorb to maintain
required ripple current at the DC line and the level of rms current
it can handle because of ESR heating.
DC Link Capacitor
Aluminum Electrolytic capacitors offer greater capacitance per
unit volume and higher energy densities compared with film. The
trade-off is that the much higher ESR of aluminum electrolytic
capacitors often results in capacitor banks that are oversized
to handle the ripple current requirements. Polypropylene film
capacitors have much lower ESR to handle the AC ripple without
overheating. Film technology advantages over electrolytics are
listed below.
Advantages of Film Capacitors versus Aluminum
Electrolytics for DC Link Applications
IGBT Snubber
As with all switching devices, IGBTs are subjected to voltage
transients during turn-off operation. Voltage transients result
from energy trapped in the circuit’s stray inductance. The
amount of voltage overshoot is dependent on the amount of
stray inductance and the switching speed (dV/dt) of the IGBT.
Fortunately, there are ways to protect IGBTs and other switching
devices from over voltage; the most common is to use a snubber
capacitor across the switch to divert the inductive current. IGBT
snubbers may be terminated with lugs for direct mount across
the IGBT to minimize added inductance.
DEFINITIONS
Capacitance (C)
Nominal capacitance typically given at 25 ºC and 1 kHz in units
of microfarad (μF)
Capacitance Tolerance
Range in percent for which the capacitance may differ from rated
capacitance as measured at 25 ºC and 1 kHz. This range results
from variances in materials and manufacturing processes rather
than from temperature and or frequency characteristics. The
standard manufacturing tolerance for polypropylene capacitors
is ± 10% or “K” tolerance. Tighter tolerance of ± 5%, “J” tolerance,
can be achieved for polypropylene, usually at a slightly higher
cost.
Temperature Coefficient
The temperature coefficient is the average capacitance change
per ºC over a specified temperature range.
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•
•
•
•
Two times the voltage capability frees you from series
capacitors and voltage balancing resistors.
Three times the ripple current capability frees you from
needing excess cap to handle ripple.
Dry construction frees you from the explosive failures with
liquid electrolyte.
Solid encapsulation delivers higher shock and vibration
withstanding.
Non-polar dielectric delivers reverse-proof mounting and
AC withstanding.
DC Output Filtering
Film capacitors are widely used for DC filtering in power supplies.
Their function is to smooth out the DC voltage waveform after
rectification.
2
1
Cap Change (%)
•
Capacitance Change vs Temperature
0
-1
-2
-3
-55
-25
0
25
Temperature (ºC)
50
75
100
Capacitive Reactance (Xc)
The reactance is the capacitor’s opposition to passing AC current.
It is inversely proportional to frequency and capacitance.
1
X=
c . . .
2 πfC
Equivalent Series Resistance (ESR)
The total ohmic resistance that contributes to power loss,
represented by a single resistance in series with the ideal
capacitor. Typically given at 25 ºC at 10 kHz and 100 kHz in units
of milliohms (mΩ)
Equivalent Series Inductance (ESL)
The total series inductance of the capacitor winding including
any internal connections, typically low and given at 25 ºC in
units of nanohenries (nH)
Insulation Resistance (IR)
The ratio of the applied voltage to leakage current (DCL),
typically given in megohms (MΩ) or in the discharge time
constant format MΩ x μF. The formula for insulation resistance:
Vdc
IR=
DCL
Insulation Resistance vs Temperature
IR x C (MΩ x µF)
1000000
Rated DC Voltage (Vdc)
The maximum operating peak voltage for which the capacitor has
been designed for continuous operation at rated temperature.
Rated AC Voltage (Vrms)
The maximum operating AC rms voltage for which the
capacitor has been designed for continuous operating at rated
temperature, typically given at 60 Hz.
Peak Current (Ipk)
The peak current amplitude for which the capacitor is designed,
given in units of amperes (A). The Peak Current is related to dV/
dt by the formula:
Ipk=C. dV / dt
Where C is rated capacitance.
RMS Current / Ripple Current (Irms)
The maximum operating rms current, typically given at a specific
reference frequency and temperature in units of amperes rms
(Arms)
Thermal Resistance (θcc , θca)
The total thermal resistance from core to case (θcc) and case to
ambient (θca) defined as the temperature change per dissipated
power (ºC/W)
100000
10000
The formula for thermal resistance:
1000
0
25
50
Temperature (ºC)
75
100
Dissipation Factor (DF, tanδ)
The ratio of the capacitor’s ESR to capacitive reactance Xc , the
DF of a capacitor is frequency and temperature dependent and
is usually specified at 25 ºC and 1 kHz.
DF=
ESR
Xc
θca+ θcc=
ΔT
Irms . ESR
2
Where ΔT is temperature rise in ºC.
Life Expectancy
The life expectancy formula for the power film capacitors in this
catalog* is given in terms of applied voltage and temperature.
DF change with temperature and frequency are given for
polypropylene in the curves below.
Dissipation Factor vs Temperature
Dissipation Factor (tan δ x 10-5)
30
LifeS
LifeR
VR
VA
F
TR
TA
25
20
15
= Service Life
= Rated Life
= Rated Voltage
= Applied Voltage
= Voltage Acceleration Factor
= Rated Temperature
= Ambient Temperature
Expected Lifetime vs Core Temperature and
Applied DC Voltage
10
-75
-25
0
25
50
Temperature (ºC)
75
100
1.7
-4
Applied / Rated Voltage Ratio (V A/VR)
Dissipation Factor vs Frequency
100
Dissipation Factor (tan δ x 10 )
-50
10
1.6
1.5
1.4
50ºC
1.3
60ºC
1.2
70ºC
1.1
85ºC
1
0.9
0.8
100
1000
10000
100000
1000000
Expected Lifetime (h)
1
1
10
Frequency (kHz)
100
1000
* Life Expectancy curves for the DC Link types 944U and 947C
are given on their datasheets.
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