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  Film technology to replace electrolytic technology

ABSTRACT

Trend of industrial and traction market for power conversion is to replace electrolytic capacitors by film technology.

This trend is generated by many advantages that film technology is offering. Among these advantages, we have:

*High rms. current capabilities up to 1Arms. per F

*Overvoltage withstanding up to 2 times the rated voltage

*Handle a reversal voltage

*High peak current capabilities

*No acid inside

*Long lifetime

*No storage problem

However, this replacement wont be done <<can for can>>, but for the function. Indeed, despite the very big improvement of film technology, replacement solution wont be possible for each application, there will be several approaches to do this.In order to help the user to see clear, we will present some concrete figures where film gives a major benefit instead of electrolytic technology.

1) DC LINK FILTER: HIGH CURRENT DESIGN & CAPACITANCE VALUE DESIGN

1a   Energy supplied with batteries

Applications will be: electric car, electric fork lift truck

In that case, capacitor will be used as a decoupling capacitor. Film capacitor is particularly well adapted for this use, because main criteria for DC link capacitor will be rms. current withstanding.

It means that DC link capacitor can be designed on rms. current value.

If we take an electric car in account as example:

Requirement data:

Working voltage: 120Vdc

Ripple voltage allowed: 4Vrms

Rms. current: 80 Arms. @ 20kHz

Minimum capacitance value will be:

So, it will be easy to find a capacitance value close to these values.

Comparison with electrolytic capacitor.

If we take in account 20mA per F for example, in order to handle 80 Arms, capacitance value minimum would be:

2) OVERVOLTAGE DESIGN

We will consider light traction application, like metro, tramway, electric buses, ...

Due to the principle of carrying the power from the catenary to the train, some contact discontinuity appears between pantograph and catenary.

When contact is not done, energy come from DC link filter,with for effect, to decrease the voltage. So, as soon as the contact is re-established, an overvoltage appears.

Worse case would be V = catenary voltage, because overvoltage could almost reach 2 times the rated voltage.So, film capacitor can handle this kind of overvoltage.

Comparison with electrolytic technology:

Electrolytic handle 1.2 DC voltage max:

So minimum voltage that electrolytic should handle would be:

DC voltage of electrolytic technology: 21000V/1.2=1670V

4 capacitors 450 Volts in series would be needed. Volume occupied for 10mF with electrolytic would be: 26 I and Irms. max would be 220Arms.

With film, volume occupied would be 25 I, and rms. Current capability would be higher than 500Arms.

In other hand, link to these overvoltage, peak current appear through the capacitor,So, we have to calculate the energy generated by this overvoltage This energy calculation will be used for short circuit discharge between terminal as well. Such discharge will generate a very high peak current and some ringing that electrolytic could not handle.

3) VOLTAGE RATING

Function of the voltage rating needed, film solution will become more and more interesting. If high capacitance value is requested, film solution will be less competitive. Indeed, if there is no overvoltage, low rms. current, large capacitance value, it will be difficult for film technology to be competitive below 900 volts.

This energy calculation will be used for short circuit discharge between terminal as well. Such discharge will generate a very high peak current and some ringing that electrolytic could not handle.

4LIFETIME CALCULATION

Film technology allows a very long lifetime expectancy, depending on voltage load conditions (working voltage) and hot spot temperature.

For DC filtering, lifetime meets the curves shown in this catalog. End of life criteria is a decrease of capacitance value of 2%.

However, this is a theoretical end of life, because capacitor can be still used after this point. If application can allow 5% capacitance decrease, lifetime will be widely increased.

5) CONCLUSION

This document gives some ways for the engineer designer to do their choice. Of course, for each case a complete calculation will have to be done. Anyway, if the request is only capacitance value, low voltage, low rms. Current, no overvoltage, no reversal voltage, no peak current, film technology certainly wont be a good solution.

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