IEEE Power Electronics Magazine - March 2018 - 46

Film Material, Abbreviated Codes
Polyphenylene
Sulfide

Polyethylene
Naphthalate

Polyester

Film Characteristics

Polypropylene

Relative Permittivity at 1 kHz

3.3

2.2

Minimum Film Thickness (µm)

0.7 ... 0.9

0.9 ... 1.4

1.2

1.9 ... 3.0

Moisture Absorption (%)

Low

0.4

0.05

<0.1

Dielectric Strength (V/µm)

580

500 (?)

470

650

dc Voltage Range (V)

50-1,000

16-250

16-100

40-2,000

Application Temperature Range (°C)

-55-+125/+150

-55-+150

-55-+105

∆C/C Versus Temperature Range (%)

±5

±1.5

±2.5

at 1 kHz

50-200

42-80

2-15

0.5-5

at 10 kHz

110-150

54-150

2.5-25

2-8

at 100 kHz

170-300

120-300

12-60

2-25

at 1 MHz

200-350

18-70

4-40

at 25 °C

≥10,000

≥100,000

at 85 °C

DF (. 10-4)

Time Constant
(RISO . C (s)

Dielectric Absorption (%)

0.2-0.5

1-1.2

0.05-0.1

0.01-0.1

Specific Capacitance (nF . V/mm3)

400

250

140

Self-Healing

Medium

Medium Low

Low

Good

FIG 1 The capacitor film characteristics. (Data taken from [4].) DF: dissipation factor.

DF Versus Temperature

DF (tan δ × 10-5)

30
25
20
15
10
-75

-50

-25
0
25
50
Temperature (°C)

100

FIG 2 The variation of the DF with the temperature for
polypropylene film [2].

after stress, leading to better system reliability and lifetime.
However, the ability to self-heal depends on the stress level,
peak values, and repetition rate. Additionally, eventual catastrophic failure is still possible due to carbon deposition and
collateral damage from the plasma arc generated during
fault clearing. These characteristics match the modern
applications of power conversion in electric vehicles and
alternative energy systems where there is no hold up
required with outages or between line-frequency ripple
peaks. The main requirement is the ability to source and
sink high-frequency ripple currents that might reach hundreds if not thousands of amps while maintaining tolerable
losses and high reliability. There is also a movement to
higher bus voltages to reduce ohmic losses at given power

IEEE PowEr ElEctronIcs MagazInE

z March 2018

levels. This would mean a series connection of Al electrolytics with their inherent maximum voltage rating of approximately 550 V. To avoid a voltage imbalance, it may be
necessary to choose the expensive capacitors with matched
values and use voltage balancing resistors with their associated losses and cost.
The reliability issue is not straightforward, although,
under controlled conditions, electrolytics are comparable
with power film, meaning that they will typically withstand
only 20% of overvoltage before damage occurs. In contrast,
film capacitors can withstand perhaps 100% of overvoltage
for limited periods. Upon failure, electrolytics can short-circuit and explode, taking down a whole bank of series/parallel components with a dangerous electrolyte discharge. Film
capacitors can also self-heal, but system reliability under
authentic conditions of occasional stress can be very different between the two types. As with all components, high
humidity levels can degrade film capacitor performance,
and, for best reliability, this should be well controlled.
Another practical differentiator is the ease of mounting
film capacitors-they are available in insulated, volumetrically efficient rectangular box enclosures with a variety of
electrical connection options, from screw terminals to lugs,
fastons, and bus bars, compared with the typical round
metal cans of electrolytics. The nonpolar dielectric film
gives reverse-proof mounting and allows use in applications
where ac is applied, such as in inverter-output filtering.
Of course, there are many film capacitor dielectric types
available, and Figure 1 gives a summary of their comparative

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2018