Pulse Application Notes
The purpose of the information below is to highlight factors that should be understood and considered when evaluating metallized polyester film capacitors for pulse applications. We will start with some general information. First,
A “pulse application” will be defined as any application where very substantial and rapid voltage changes occur across the capacitor at a relatively low duty ratio.
It is also necessary to describe the construction (valid for other suppliers as well) of metallized film capacitors. To begin plain dielectric film is placed in a vacuum and a thin layer of metal alloy is deposited on it. The metallized film is wound into a cylinder so that a metallized edge extends on each end of the wound capacitor section. Both ends are then sprayed with small droplets of molten metal that adhere along, and make contact with, the metallization on the film. The wire lead is welded to this end spray (see metallized film capacitor illustrations). The end spray process does not result in a uniform connection to the metallization and there is some damage and connection loss at the edge of the film from the heat contained in the metal droplets.During application the capacitor current must be shared by all the minute connection points of the metal end spray to the metallization of the film. Since the actual contacts between the metal end spray and the metallization are relatively few and tiny, and the contact is made to a very thin layer of metallization, the current density at each contact is enormous. If the peak current rises past a certain point the metallization vaporizes adjacent to the connections with the highest current density. Since these connection points are now gone, the remaining connection points must carry more current. After a finite number of pulses the capacitor becomes an open circuit.
Therefore when metallized film capacitors are used in pulse applications the overriding concern is the peak current during a voltage transition.
Because it is very difficult to measure fast current pulses directly, maximum pulse current is usually specified by dV/dt of voltage waveforms across the capacitor.
I = C * dV/dt. There are several factors that determine pulse current capability. To begin with, as the diameter of a unit increases so do the number of connection points between the end spray and film metallization. Thus, for the same capacitance and voltage rating, a “short, fat” [rather than a "long, skinny"] capacitor will have better pulse current capability. The longer, thinner designs are also prone to more process damage explaining why in general very small diameter metallized capacitors have VERY low dV/dt ratings!In addition, if a proposed capacitor application falls within the maximum dV/dt specified then the application must be checked to ensure the power dissipation [temperature rise] is within the allowable limit. For this purpose it is useful to consider each voltage transition as ½ cycle of a cosine waveform. The period would be defined by “mentally fitting” the cosine wave to the actual rise/fall time. Determine the frequency by doubling the peak-to-peak time observed from the cosine approximation. Although appearing to be a loose method to use for analysis, it produces surprisingly useful results when compared to behavior in an application. This method of analysis is applicable to any kind of capacitor used for a pulse application. Contact us for the analysis details.To avoid reliability issues in any application the total voltage transition between the most positive and negative peak [including ringing] over one waveform period should not exceed 2.828 times the rated AC voltage. In addition, the highest voltage peak across the capacitor should not exceed the DC voltage rating.
Also of interest is the fact that metallized polyester film capacitors have higher dV/dt ratings than comparable metallized polypropylene because the film does not sustain as much damage by the hot metal end spray or the heat generated at lead welding. For pulse applications with very low duty ratios and slow repetition rates they may be the best choice. However, because polyester dielectric loss is so high they are not suitable for large voltage swings at high repetition rate applications.
If our standard capacitor dV/dt ratings do not meet the needs of your application please contact us. There are several option available to substantially improve the allowable dV/dt.
SBE is very conscious of maintaining metallized film capacitor pulse capability. Every lot is sample checked at four times the rated dV/dt for 1000 pulses [Exceptions do exist]. We encourage you to contact us to further discuss your pulse application to assure you specify the right part!