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THE DIELECTRIC

The film does the real work

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Insulates

Keeps the plates from shorting, even at thousands of volts

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Polarizes

Its molecules stretch in the field, raising capacitance by Īµr

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Sets the limit

Its breakdown strength sets how much voltage fits in a given gap

The dielectric decides how much field, and so how much energy, each cubic centimetre can hold.

THE FILM MENU

Polypropylene and friends

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Polypropylene (PP) Īµr ā‰ˆ 2.2, very low loss, high breakdown strength: the workhorse for pulse, AC and high-voltage DC
Polyester (PET) Īµr ā‰ˆ 3.3: smaller parts, but higher loss, so mostly DC and low-frequency use
Kraft paper Porous, so it soaks up oil; used in older and mixed paper-film designs
Paper + film "Mixed dielectric": paper wicks oil between the films
Polypropylene wins most high-voltage jobs because it wastes very little energy as heat.

BREAKDOWN

How much field before it fails?

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Dielectric strength The field a film can take before it punctures, in V/Āµm
In the lab Good polypropylene film can survive several hundred V/Āµm for a short time
In a real part Designers run at a fraction of that so the part lasts its rated life
Weak spots Pinholes, thin spots and trapped particles break down first, long before the average film
A film is only as strong as its weakest spot, and a capacitor contains square metres of film.

ENERGY DENSITY

Field squared, again

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u = Ā½Īµ0ĪµrE2 Energy per volume in the dielectric, for a field E
Double the field Four times the energy in the same film
Packaged parts Roughly 0.1 to 2 J/cmĀ³ for high-voltage film capacitors, once foil, margins, oil and case are counted
The catch Higher field means shorter life, the big trade-off of Part 4
Energy density is mostly a question of how hard you are willing to stress the film.

LOSS AND HEAT

Not all the energy comes back

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Dissipation factor (DF) The fraction of energy lost as heat each cycle, also called tan Ī“
ESR Equivalent series resistance: the foil, end connections and leads
Why it matters At high repetition rates, small losses add up to real heating
Polypropylene DF around 10āˆ’4, one reason it dominates pulse and AC work
Low loss keeps a capacitor cool, and a cool capacitor lives longer.

UNIT 2 STUDY COMPLETE

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Ready for the Fab Challenge?

You've seen what the dielectric does, which films are used, where breakdown starts, and why energy density goes as field squared.