THE BIG TRADE

Small, or long-lived?

Store the same energy at a higher field and the part shrinks, but its life collapses.

Every high-voltage capacitor design is a bargain between size and life.

VOLTAGE STRESS

A steep power law

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Life ∝ (V/V_rated)⁻ⁿ A common rule of thumb for film capacitors

n ≈ 7 to 9 Typical exponents; the real value depends on the design and duty

10% lower voltage About two to two and a half times the life

10% higher voltage Roughly half the life

Small changes in voltage make big changes in life, which is why derating works so well.

VOLTAGE REVERSAL

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Where it comes from An underdamped discharge rings, swinging the voltage past zero

Why it hurts The film sees a full swing of stress, and partial discharge grows

Typical rating Around 20% reversal is common; more needs a sturdier, bigger design

The fix Damping in the circuit, or a capacitor rated for the reversal

Two capacitors at the same voltage can have very different lives if one sees heavy reversal.

HEAT AND CURRENT

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ESR heating Each shot or cycle warms the part; repetition rate sets the average heat

Hot spot The hottest point inside, often near the middle of a section, sets life

Peak current Stresses the end connections and foil edges

Ambient Typical ratings run from about −40 °C to +60 °C or so

Voltage, reversal, temperature and current all spend the same life budget.

THE TRADE IN NUMBERS

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Big and gentle A part twice the volume, running at a lower field, rated for tens of millions of shots

Small and hard-driven The same joules in half the volume, rated for only thousands of shots

Who wants which An accelerator firing all day needs life; a one-off experiment or tight enclosure may want size

The spec This is why customers specify voltage, reversal, current, repetition rate and life together

There's no best capacitor, only the best one for a given duty.

UNIT 12 STUDY COMPLETE

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You've seen how voltage, reversal, heat and current trade against size and life.