08-11-2012, 02:42 PM
Capacitor Data
decoupling capacitor.pdf (Size: 79.9 KB / Downloads: 23)
Dielectric Types
PAPER Dielectric Capacitors are used in High voltage applications. Paper devices should not be used in a high moisture
environment unless the device is in a hermetic case, the capacitance will increase, the power factor - insulation resistance -
dielectric strength - and life will degrade. Paper capacitors have near 10% tolerances.
Plastic FILM
Mylar: has a wide temperature change of 20% over -55 to +125C. Moisture absorption is half that of paper.
Mylar is the same as polyester. The term Mylar is a trade name.
Polycarbonate: Out performs Mylar in all areas. Low Power factor, works best with AC. Polycarbonate has a low
temperature drift (graph below), dissipation factor, and dielectric absorption. Use polyphenylene sulfide as a possible
replacement.
Polyester: Replaces paper for many applications, and smaller in size. Polyester does not have the moisture problem that
paper does, but does have a worse tolerance, at 20% ~ over -55 to +125C. May also be called PETE, or PETP. The
Polyester Dielectric has an Absorption of around 0.20%.
Polypropylene: It has negative temperature coefficient.
Polysulfone: Has a very high operating temperature of +170C.
Polystyrene: Will only work to +65C. It has a flat to negative temperature coefficient.
Teflon: Has a very high operating temperature of +170C.
MICA High Precession. The material is inert it will not change over time or temperature.
Value x Temperature
Normally capacitors have a stated value at some frequency [1KHz], and some temperature [room temp]. However
that capacitance value will change by some percentage [depending on dielectric] over temperature. A chart
depicting the percentage change in capacitance depending on dielectric type versus temperature change is listed
on; Cap Value Change Over Temperature. The chart provides data for Plastic Film capacitor dielectrics.
Capacitor placement: on the PWB
Place the capacitors near the body of the device, leaving the shortest [and widest] possible trace. Additional larger
tantalum capacitors may be placed around the board. The tantalum help to reduce the over-all impedance of the
power plane. The larger value near-by tantalum also helps to re-charge the ceramic, instead of taking the charge
from the power supply source. Placing Power planes adjacent to ground planes in the board stack-up also helps. It
doesn't hurt to embed signal layers between a ground layer in the stack-up either, assuming you can add board
layers.