■ CAPACITOR FUNDAMENTALS
Squeezed together
Metal end caps
Metal layer
Ceramic
Leads
FIGURE 4. Stack capacitors are made from an alternating stack
of insulating and metallic layers.
withstand substantial voltages. Electrolytics are polarized, meaning that voltage can only be applied in one
way due to the chemical electrolyte. They generally
have very broad tolerances of ±20%.
TANTALUM
Tantalum capacitors are a special type of capacitor.
Instead of a roll of foil, an extremely porous “slug” of tantalum makes one electrode and an outer metal capsule
the other. The dielectric is a chemical solution that forms
an oxide coating on the tantalum slug for insulation. The
slug has a tremendous amount of area, so capacitance
is high, but RS is also high (somewhat lossy). The short
leads and small size of the capacitor means that tantalums have low LS. The maximum applied voltage for tantalums is under 100 V. Like electrolytics, they are polarized and have broad tolerances of ±20%.
Figure 4 shows a stack capacitor. The most common
dielectric (as shown) is ceramic. Each piece of ceramic is
coated with a thin metal layer on one side. The stack is
oriented so that the metal layers only touch one side,
alternating in each layer. The stack is then placed under
pressure and heated (called sintering) to make a single
solid piece of material. Metal end caps with leads are
attached to each side of the stack, contacting the metal
layers. The whole capacitor is then coated with an epoxy
resin. The LS of stack capacitors is very low.
ELECTROLYTIC
The most common type of roll capacitor is the aluminum electrolytic. The dielectric is a porous layer of
paper-like fiber, impregnated with a chemical gel that acts
as a dielectric. Electrolytics have very high capacitance
for their volume, but also have high LP and RS (lossy),
and are relatively leaky (low RP). They can be made to
FILM
Film capacitors have a plastic film dielectric; polyethylene and polycarbonate are the most common. Most
film capacitors are of roll construction, so LS is moderate
although stack types are not unknown. Film capacitors
are non-polarized. RP is high (low leakage) and RS is low
(low loss). Special types of film are used for highly stable
capacitance values or extremely low leakage. Precision
film capacitors of 5% tolerance or better are available. See
www.filmcapacitors.com/specsum.htm for a good
table summarizing the different types of film capacitors.
CERAMIC
By far the most common form of capacitor used,
ceramic capacitors are used in high-frequency applications. Stack construction keeps LS extremely low so they
are useful at frequencies of hundreds of MHz. They are
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Choosing Capacitors
Now that you know more about the types of capacitors,
here is a list of common applications and the types of
capacitors used in them. The reason that type is chosen is
shown in parentheses.
● Timers and oscillators — polystyrene film or NP0
ceramic (stability and precision)
● Audio filters and amplifiers — film (low-loss and small)
● Power supply filtering ( 60 Hz) — electrolytic (high
value, low cost)
● Analog-to-digital conversion — low-leakage film (
stability and precision)
● Switching power supply filtering — electrolytics with low
ESR and low ESL, film (high frequency current handling)
● Power bus filtering — tantalum and electrolytic (good
medium and high-frequency characteristics)
● Digital IC bypassing — ceramic (excellent high frequency characteristics)
● RF filters — silvered-mica, air variable, or transmitting
ceramic (low-loss and stable)
The important thing to remember in order to
make the correct capacitor choice is to think about
what is important for your circuit — value, stability, cost,
loss — and choose a capacitor type that meets those
requirements.
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SEPTEMBER 2005
