CERAMIC CAPACITORS

129

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General characteristics

Page revised 06/20

II.2. Series Combinations

To deal with a higher operating voltage, one can use combinations of HiQ ceramic ca-

pacitors in series, within the same dielectric die or using separate entities – voltage

rating multiplied.

II.3. Matched Sets

To achieve non-standard total capacitance values or ultra-tight tolerances, EXXELIA can

match capacitors using computer specific software. Another use of matched sets is

to reduce the overall purchasing costs; when several capacitors are used in parallel,

a given tight tolerance can still be obtained on the final assembly while using wider

tolerance single chips.

III. GUIDELINES

Several factors have to be considered when designing high RF power applications and

these factors are in fact all linked to the overall thermal management of the entire de-

sign.

III.1. Influence of ESR

The capacitors with ultra-low ESR provide a higher maximum current for power dissi-

pation limited operating conditions – as Ip = ƒ(

1

)

ESR

– allowing the overall design to

handle more RF power. Of course, the dissipation factor characteristics also have to be

compliant with this increased power.

In the example below, a 300W CW module at 350MHz is pushed above its limits to em-

phasize the importance of ESR.

In the same conditions, several capacitor types are monitored as DUT and the results

are shown below:

III.2. Influence of Magnetism

The choice of Power Capacitor Solutions in high magnetic field environment is critical.

EXXELIA has conducted several tests with both his final customers and external labo-

ratories to extend his knowledge and develop better solutions. These solutions play a

major role in reducing the overall system temperature. Please contact EXXELIA for any

further information.

III.3. Generic Comments

The main guidelines to lower the overall thermal load on the design are listed hereafter:

The thermal conductivity of all devices involved as well as board trace dimensions

and material thickness have to be evaluated;

The main part of the heat transfer is achieved by thermal conduction. Actually,

around 80% of the power is dissipated by conduction, 15% by convection and 5% by

radiation. Therefore, the greater part of heat transfer is through the terminations of

the capacitors. In order to further improve the thermal path of a porcelain capacitor,

one should use leads such as non-magnetic micro-strip silver ribbons. The leads

also offer another advantage: when the thermal expansion coefficients of the ca-

pacitor and the board are mismatched, they may act as a mechanical strain relief;

To avoid reducing drastically the thermal conductivity at some specific locations

within the circuit, one should avoid reducing the width of the board trace and using

wires;

Heat sinks and blown cool air will also help to reduce the additional sources of heat

generated by passive components, FETs and active gain blocks;

Paschen’s law defines the voltage rating for a given pressure. Therefore, depending

on the operating conditions, the pressure parameter has to be considered (coating,

voltage safety margin...);

Using Power Capacitor Solutions with parallel combinations will extend the RF pow-

er handling. For instance, N capacitors in parallel will led approximately to an ESR

which is N-times lower than the one of a single capacitor, thus increasing the maxi-

mum current handling capability by a factor of

dN.

IV. CONCLUSION

This article has described the major factors to consider while designing a Power Ca-

pacitor Solution for high RF power applications. The benefits of using Power Capacitor

Solutions are numerous: high RF power, enhanced reliability with pre-tests, ultra-low

ESR, reduced costs with matched sets, availability of specific capacitance values and

tolerances, fewer assembly stages, customized styles...

To ensure the highest level of reliability in high RF power designs, factors such as heat

transfer, maximum voltage and current ratings, thermal characteristics of the circuit

devices and ways to remove the heat should be taken into account.

EXXELIA designs Application-Specific Solutions based on parallel and series combina-

tions of designer-acclaimed capacitors. Customer requirements are addressed by com-

puter matching sets, a wide range of mechanical configurations, a protective coating

and adapted ribbons or wires which have enabled EXXELIA to extend overall perfor-

mance while decreasing the total cost of ownership.

EXXELIA - by knowing the ESR and power dissipation of its capacitors at the application

operating frequencies - helps the designers by simulating the thermal behavior of the

assembly and proposing the optimum Power Capacitor Solution.

Ref

[1] Thermal Conductivity wikipédia

[2] Cours de thermique P.ROUX (2016)

General Information