When the application requires an extremely long life (e.g. 100 million discharge cycles), the extended metal foil construction has an advantage since it is not prone to the eventual end connection deterioration from clearing. The foil design does not, however have the advantage of self healing when a dielectric fault occurs since the foil cannot be vaporized. Thus, foil designs generally use thicker dielectrics, sacrificing energy density.

The extended foil designs also improves the thermal conductivity of the capacitor, since the metal foils act as heat sinks and draw the heat out from the interior. The superior heat dissipation of the metal foil designs also reduces dielectric degradation due to thermal aging which has a major impact on capacitor life. The foils also allow the capacitor to operate at higher peak currents and operation at faster duty cycles. The thermal conductivity of the metallized design can be improved by filling or impregnating with a liquid dielectric such as silicone or mineral oil, but even this does not approach the heat dissipation efficiency of solid foil conductors.

Another type of end connection and dielectric configuration is the "soggy-foil" construction. This design attempts to marry the advantages of the metallized film and extended foil. "Soggy foil" is paper dielectric tissue which has been metallized on both sides. The double metallized paper is wound into the capacitor in the same manner as a metal foil and the capacitor is impregnated (Figure 5). The end connection is effectively doubled in comparison to the extended metallized film design, and the metallization can clear on the paper resulting in minimum damage to the dielectric film. The paper is not I the active capacitor layers or field and acts only as a carrier for the metallization and the impregnant.

Regardless of the termination technique and dielectric configuration selected, the inductance and series resistance of the energy discharge capacitor must be kept to a minimum. Both are affected by the final geometry, internal construction and external terminal placement of the final capacitor package. Why are these two factors important? They directly affect the parameters of the discharge pulse as wee shall see in the next section.

First, let’s consider the importance of the capacitor’s resistance. The heat generated within the capacitor during operation is equal to the product of the capacitor’s equivalent series resistance (R) and the square of the rms current (I).

Equation 1

H = I² R

If heating becomes too severe, it decreases capacitor life; but if the resistance is allowed to become too high, it will reduce the peak current value at discharge.

Very little can be done about the current since it is an application requirement, so resistance must be controlled by the designer. A reduction in film width is a design option when the contribution of the electrodes and termination resistance must be minimized. Again, this sacrifices energy density. An alternative approach to reducing the resistance of the capacitor to attach two separate windings in parallel. This parallel design also offers the added advantage of reduced inductance.

The current pulse duration (in seconds) at discharge is given by the product of pi and the square root of the product of the capacitor’s inductance (L in henries) and value (C in farads).

Equation 2

Typically, energy discharge capacitors release their stored energy into a load of less than two ohms, for example, Xenon flash tubes. In many applications an inductor is placed in series in the discharge circuit to shape the current pulse for specific requirements.

When the energy is released, two types of current pulses may occur which are dependent on the inductance of the discharge circuit containing the capacitor. They are oscillatory and non-oscillatory. The oscillatory discharge is characterized by several voltage peaks during the release of energy to the load (Figure 6). This type of discharge is the most severe form of duty for an energy discharge capacitor because of the oscillating frequency (ringing) and voltage reversal. (Please note that references to exp. In the following equations indicate e, the natural logarithm.)

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