CERAMIC CAPACITORS

126

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POWER CAPACITOR SOLUTIONS

ULTRA-LOW ESR, HIGH RF POWER

In the RF world, one trend that continues to gain momentum is the need for higher RF

output power in amplifier modules and systems. Associated to a growing demand for

reduced unit size, the task for the designers and the component manufacturers is chal-

lenging.

First of all, the systems have to deal with higher RF power. At the component level, this

means that a particular function which required only a single component previously

has now to evolve to a sub-system made of several components to handle the total

amount of power.

Example of capacitor Module

Example of X-Rays analysis

Moreover, the reduction of the unit size led to higher operating temperatures, adding se-

vere requirements on the components. They have to survive higher temperatures, be-

ing able to dissipate the generated heat – small packages produce much higher power

densities – maintain their performances among huge operating temperature variations

and offer mechanical flexibility to accept significant PCB thermal expansion.

Now, when coming to the capacitor world, these new needs will affect the “single-chip”

standard model. For instance, when one capacitor was enough to ensure the matching

of a 100 W RF transistor, the recent 1’000W transistors need “n” capacitors, even some-

times with an increased size.

In order to get a better understanding of these new requirements and to study the

“n-chip” model, we will first look at the key parameters of high RF power systems. Then,

depending on the key parameter(s) considered, we will see which Power Capacitor

Solution is best-tailored to the designer needs.

I. HIGH RF POWER

I.1. Voltage Rating

Maximum voltage ratings for ceramic capacitors (WVDC) are linked to two factors:

strength of the dielectric and Paschen’s law. The strength of the dielectric provides a

maximum voltage breakdown and the Paschen’s law provides another maximum volt-

age above which the air around the chip arcs.

Strength of Dielectric

Paschen’s Law

The voltage rating of the ceramic capacitor is then defined as the lowest value when

considering both limitations.

I.1.1. Dielectric Strength

The capacitor maximum voltage rating is determined predominantly by the dielectric

strength or voltage breakdown characteristics. For instance, porcelain dielectrics ex-

hibit a breakdown voltage that typically exceeds 1’000 kVDC/inch of dielectric thickness.

Material

Dielectric Strength (kV/inch)

Vacuum

20

Air

20 to 75

Porcelain

40 to 200

Glass

2'000 to 3'000

Mica

5'000

For multilayer capacitors for instance, this means that one particular layer of standard

dielectric – let’s consider a theoretical 5 mils thick layer – will not crack until the voltage

exceeds a value around 5’000 VDC. In order to achieve even higher voltage ratings, spe-

cific internal electrode designs are used to split the voltage.

I.1.2. Paschen’s Law

In 1889, F. Paschen published a paper (Wied. Ann., 37, 69) which set out what has be-

come known as Paschen’s Law. The law essentially states that the breakdown charac-

teristics of a gap are a function (generally not linear) of the product of the gas pressure

and the gap length, usually written as V= f(pd), where p is the pressure (in Torr) and d

is the gap distance (in cm):

V =

365 x p x d

1.18 + In(p x d) (1)

Note: 1 bar = 100’000 Pa = 750 Torr = 14.5 psi.

For instance, if we consider an E-type capacitor (CLE series with an EIA chip size of

4040), the length between the two terminations (“L” as shown below) is around

10.50 mm.

This means, using the Paschen’s law (p=750 Torr; d=1.05 cm), that if the voltage across

such equivalent air gap exceeds 36’600Vdc, an electric arc would be created. However,

when dealing with the gap between the two capacitor terminations, another parameter

has to be considered. Actually, as the dielectric material is charged, there is an ioniza-

tion of air which influences the Paschen’s law. Therefore, for the capacitor considered

in this example, a voltage around 10’000 VDC will probably create a short circuit on the

capacitor external surface (carbon residues from the arcing). Moreover, the electric arc

itself could damage nearby components.

For applications where very high voltages are needed, a specific coating would be ap-

plied on the capacitor, thus covering both terminations. In this case, the gap itself disap-

pears and no electric arc could occur.

I.2. Current Rating

The current rating assigned to a capacitor is stated in one of two ways: voltage limited or

power dissipation limited. The rating that applies depends on the capacitance value and

operating frequency. The voltage limited area is based on the voltage rating. The power

dissipation limited area is based on the ability of the capacitor to dissipate the heat. The

current rating of the ceramic capacitor is then the lowest value.

I.2.1. Voltage Limit

The maximum current for the voltage limited operating condition is directly proportional

to the capacitor voltage rating and the impedance:

IVm = WVDC x √2 / Z

(2)

(3)

Z =

√ESR2+(Lq –1/Cq)2

General Information