CERAMIC CAPACITORS

128

www.exxelia.com

info@exxelia.com

Taping : dimensions

Page revised 06/20

The thermal resistance Vc of the capacitor is:

1

=

2

V

c V

(11)

and

V

=

L/2

A x l

(12)

therefore

V

c =

L

4 x A x l

(13)

Where

l

is the coefficient of thermal conductivity of the ceramic body W.cm–1.C–1

A is the section surface (thickness x width) (cm2)

L is the length of the capacitor (cm)

The geometry of the capacitor (A/S) influences the thermal resistance. For example

0711 size (Exxelia reference=SHD) has a factor A/S more beneficial than 1111 size to

reduce thermal resistance of the capacitor.

Considering the non-stationary state, we must solve equation (10). For example if we

consider a capacitor an initial temperature Tamb and a final temperature Tmax, we may

use the following equation for the temperature evolution: [2]

T(t) = Tmax. – (Tmax. –Tamb.)exp (–

t

)

mCpVc

(14)

Where :

m is the mass of the capacitor

Cp is the thermal capacity

V

c is the capacitor thermal resistance

Example of the temperature evolution inside the capacitor

where Tamb= 25°C and Tmax= 28.3°C

I.4. Global Power Model

All the above parameters have to be kept in mind when designing a high RF power

function. The capacitors used in the application should be fine-tuned to make sure their

voltage rating, their current rating and their heat transfer capabilities are in line with the

required specifications. Moreover, the specifications do not only include the capacitor

by itself, but also the PCB properties and the environment where the complete system

operates.

Let’s consider for instance the Global Power Model of a single capacitor mounted on a

PCB studied at a working frequency of 50 MHz.

The component characteristics are as-follows:

Type: EXXELIA CLE series

Voltage rating: 7’000 VDC

Capacitance value: 22 pF

First, the size of the component will give the capacitor thermal resistance – its ability

to dissipate heat. Then, in the PCB specification, we will look for its thermal resistance

properties. The environment – how the system is working in normal/maximum opera-

tion – will tell us the theoretical ambient temperature. Finally, the capacitor electrical

parameters will be used – capacitance value, voltage rating and ESR.

All these data are compelled in a simulation program which calculates the maximum

current rating of the capacitor for the considered system, at one particular frequency:

As previously written, the current rating assigned to a capacitor is stated in one of two

ways: voltage limited or power dissipation limited. The software calculates both limita-

tions: Iv for the voltage and Ip for the power. Finally, the smallest value is taken as it

represents the first limitation the user will reach when using the system.

In the example above at 50MHz, the capacitor, according to its power dissipation limita-

tion, should handle around 52A (Ip) but the voltage limitation will actually not allow it to

handle more than 34A (Iv). If the capacitance function has to handle more current, then

the designer has to switch to the “n-chip” model and to use a combination of several

capacitors, a.k.a as Power Capacitor Solutions.

II. POWER CAPACITOR SOLUTIONS

More RF power means either a higher current or a higher voltage, sometimes both. As

the current and voltage laws are quite fixed for capacitors – physical limitations give

few options on dielectric thickness and number of electrodes which are key to handle

more power in a single component – the only way to handle more power, for a given

ultra-low ESR series, is to increase the number of capacitors.

This led to a new branch of capacitor knowledge dedicated to thermal and power anal-

ysis, mechanical assembly, high temperature PCB soldering and specific RF test pro-

cedures.

The Power Capacitor Solutions are especially dedicated to applications where high

reliability, high operating voltages, high operating currents, ultra-low ESR and tighter

tolerances are required. Most of these applications are found in the following markets:

Medical Electronics;

Broadcasting Equipment;

Semiconductor Manufacturing;

Inductive Heating;

LASER Power Supplies;

MRI High Magnetic Environments;

Military Systems.

II.1. Parallel Combinations

To deal with a higher operating current or to further reduce our ultra-low ESR, one can

use combinations of HiQ ceramic capacitors in parallel – current rating multiplied.

General Information