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Electrocube Data Sheets
Electrocube Data Sheets

Metallized Capacitors

Technical Bulletin No. 08

A great deal of confusion still exists regarding metallized dielectric capacitors. This is unfortunate because this confusion has resulted in the exclu­sion of a most valuable tool from many circuit de­signs.

The electrodes in a non-metallized design are sep­arate sheets of metal foil wound with sheets of die­lectric material. These electrode sheets (foils) ex­tend alternately out each end of the capacitor roll beyond the dielectric. This provides a mass of me­tallic material to which leads are attached by weld­ing or soldering.

The metallized type of construction greatly reduces the physical size of the capacitor. The aluminum or tin foils are replaced with a thin layer of 99% pure aluminum vapor-deposited directly onto the dielec­tric. Provisions are made, by means of masking, to provide each dielectric with a margin. The two sheets of metallized dielectric are positioned prop­erly relative to a right and left margin and wound into a roll. Since the dielectric material itself is pres­ent at each end of the wound capacitor along with the metal electrode material, direct attachment of the leads is not done.

The end terminations of the metallized unit are ac­complished by application of a fine molten metal spray. The spray makes contact with the electrode material resulting in the plate contact. The lead wires are attached to this metal spray.

Figure 1
Figure 1
Figure 1
Figure 1

VOLUMETIC EFFICIENCY

The construction of the metallized design is quite similar to that of a non-metallized design except for the slight differences in the electrodes and end ter­minations. The volumetric advantage of the metallized design is self-evident due to the physical reduction of total electrode material. The advantage is more evident and becomes more pronounced as the capacitance value increases.

There is little volumetric advantage evident in smaller capacitance values because the end termi­nations, casings, and end seals, which are approxi­mately the same for both types of units, form a dis­proportionate part of the total volume. Table 1 com­pares the approximate volumetric ratio for various values of capacitance between the metallized and non-metallized designs. All the example capacitors are wrap and fill style, 200 VDC rated, polyester di­electric.

Table 1
Table 1
Table 1
Table 1

The volume of the metallized design is set as unity, and the volume of the non-metallized.design is then expressed as an equivalent ratio.

It should be noted that the metallized unit is not voltage stressed (volts per mil of dielectric) higher than the non-metallized version. The increasing na­ture of the percent volume difference with increas­ing microfarads is due to the difference in the total amount of electrode material wound into the units. The thickness of the electrodes in the metallized capacitor is approximately two-millionths of an inch, or about 100 times thinner than the non­metallized designs.

SELF HEALING PROPERTY

The self-healing feature of the metallized capacitor offers a distinct advantage over the non-metallized unit. This self-healing feature is a result of the ex­treme thinness of the metallized electrode material. Whenever a flaw or weak spot in the dielectric re­sults in a short condition, the stored electrons in the capacitor and the associated circuitry will imme­diately avalanche and cross at the shorted point. The electron density concentration results in an ex­tremely high current condition which in turn pro­vides sufficient energy in the form of heat to vapor­ize the thin metallic electrode. The vaporized elec­trode forms a fairly concentric pattern away from the point of the short. As a result of the vaporization, the short condition is removed and the capacitor is again operational. This is known as a "clearing" which is the self-healing process.

To illustrate this process Figure 2A represents a greatly enlarged section of a capacitor containing an extremely thin spot in one of the dielectric sheets. In this case, it is the result of two valleys on opposite sides of the dielectric surfaces coinciding at a point. This is an example of the distance be­tween the electrodes at a thin spot that is not ca­pable of withstanding the voltage stresses-thus a short develops. The "self-healing" or "clearing" ac­tion vaporizes the metal electrode sufficiently; therefore, the effective distance between the elec­trodes increases, the short is removed, and the ca­pacitor is again a "good" unit (shown in Figure 26).

There are two factors to be considered relative to a self-healing or clearing action. There must be suffi­cient energy present to accomplish this clearing action and during the process, the circuit will expe­rience a small, short-duration, transient voltage drop.

Figure 2
Figure 2
Figure 2
Figure 2

ENERGY REQUIRED TO CLEAR

The energy required for an average or "normal" clearing is approximately 1 O microwatt-seconds. This means that if energy is available only from the capacitor itself, there is a relationship between the capacitance rating and the magnitude of the charg­ing voltage below which clearings may be ques­tionable.

The relationship is illustrated below for both normal clearing (1 O microwatt seconds) and clearing re­quiring 10 times the energy (100 microwatt sec­onds).



The energy required to clear the capacitor will be the difference between the energy before and after the clearing:



CLEARING DURING SERVICE

In many applications, when the actual voltage ap­plied to the capacitor is less than the rated voltage, and the ambient temperature is less than the maxi­mum rated temperature, the unit should function for years without clearings during its service life. This is because both voltage and temperature derating will tend to minimize the short producing condition necessary for the initiation of the self clearing pro­cess.

The dielectric itself will have a strong controlling ef­fect on these self-healing occurrences. For in­stance, under accelerated life test conditions, a metallized polyester dielectric unit will exhibit con­siderably less clearings than a metallized polycarbonate device.

SUMMARY

Disadvantages of a Metallized Capacitor:

  1. The dissipation factor is slightly higher than that of a non-metallized unit.
  2. The insulation resistance is slightly lower than a non-metallized unit.
  3. The metallized unit maximum current limitation is lower than its non-metallized equivalent unit.
  4. The maximum AC voltage-frequency capabili­ty of a metallized unit is slightly less than its non-metallized equivalent unit.

Limitations of a Metallized Unit:

  1. Under a clearing operation situation, sufficient energy must be present to accomplish this function.
  2. During a clearing -operation, the resulting tran­sient drop in voltage must be considered in the application.

Advantages of a Metallized Unit:

  1. The self-healing feature of a metallized design enables the metallized unit to remain opera· tional after a short condition develops. In a non-metallized unit, the short condition results in a catastrophic permanent failure.
  2. The volumetric efficiency of a metallized unit is higher than its non-metallized equivalent unit.
  3. In direct proportion to the volumetric efficien­cy is the weight savings realized with a metallized unit.
  4. The cost of a metallized unit in the higher microfarad ratings (above 0.1 Mfd) is less than its nonmetallized equivalent unit.

 
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