Selecting Film Capacitors

Sine, or Near Sine, Wave Applications

The two primary considerations when selecting a capacitor for steady state sine wave applications are the AC and DC rating. Orange Drop* capacitors are conservatively rated and require no DC voltage derating for capacitance value or temperature (as long as they are operated within the specified temperature range). The peak-to-peak voltage of the waveforms (including noise and transients) must be less than 2.828 times the AC voltage rating.

For steady state sine wave applications the allowable AC voltage vs. frequency performance curves listed for each polypropylene capacitor series/voltage are ideal for selecting the minimum size and cost capacitor for a given application. These curves are conservative in that they represent solutions in an +85°C still air environment with internal hot spot temperatures less than +105°C. The calculations used to create the curves assume worst case DF and include the losses due to skin effects in the lead wires as frequency is increased. To minimize the effect use the shortest lead wire and largest circuit board traces possible.

Pulse Applications

For pulse applications power dissipation must be seriously considered in addition to the AC and DC ratings. For most applications where a Film/Foil capacitor is used, dV/dt is more limited by circuit loop inductance and allowable peak-to-peak voltage than by catalog ratings.

Since capacitor losses are frequency sensitive, selecting the proper capacitor for pulse applications is more complex. Power dissipation depends on the harmonic content of the waveform. The only way to accurately determine the losses are to break down a complex waveform into its Fourier series components and sum the dissipation resulting from each harmonic.

In practice, the approximation of each fast voltage transition by a ½ cosine wave with the period equal to twice the rise time will be close enough. Because of the small duty ratio of the pulse, the allowable voltage swing for the calculated pulse frequency can be considerably higher than the performance curves indicate, as long as it is below the Maximum AC rating. In any case, it is the responsibility of the designer to verify that the capacitor is not operating above the maximum rated temperature.

For assistance with selecting capacitors for pulse applications, complete the Technical Assistance Questionnaire and contact the Application Engineering Department at SB Electronics.