A non-oscillating discharge pulse occurs when the R²/4L² factor in Equation 6 is equal to or greater than 1/LC. Non-oscillatory discharge pulses fall into categories, critically damped and overdamped. Representations of these two forms of discharge pulses are given in Figures 7 and 8.

A wave is critically damped when the discharge circuit satisfies Equation 8.

Equation 8

the peak discharge current, the time to peak current, and the discharge current at an instant in time of a critically damped discharge can be calculated using Equations 9, 10, and 11 respectively.

Equation 9

Equation 10

tp=2L/R

Equation 11

A discharge pulse is overdamped when R is greater than the term in Equation 8. The peak current of an overdamped discharge can be calculated using Equation 12.

Equation 12

The to which is the time to peak current of the overdamped pulse, is defined by Equation 13.

Equation 13

One of the simplest ways to minimize inductance is to configure the capacitor package with vertical, parallel terminals. The inductance is consequently reduced by cancellation of opposing fields.

When conventional steady state DC capacitors are evaluated, they are usually life tested at specified temperatures and voltages for lengthy durations (eg 250, 2,000 and 10,000 hours). After the test, measurements re typically made of capacitance, dissipation factor and insulation resistance, which are then compared to predetermined minimum or maximum values. The normal criterion used to evaluate energy discharge capacitors is the number of discharges until failure, typified by a short or open condition.

Normally the insulation resistance of the energy discharge capacitor is not used as an electrical end point requirement, since the capacitor is not called upon to store energy for long periods. Experience has shown however, that measurements of insulation resistance and capacitance change can be used to gauge the performance of most energy discharge capacitors especially those designed with metallized dielectrics. When the capacitor under test develops clearings resulting from faults in the dielectric, the electrode area is reduced, decreasing the capacitance. The greater the number of faults, the greater the capacitance change. The insulation resistance of the capacitors will also decrease as the number of clearings increases. By monitoring the dissipation factor, typically at one kilohertz, the integrity of the end connection can be assessed. Some actual individual test results are provided in Table II. The capacitors were tested at +25 degrees ºC, discharging into a load of one ohm at a duty cycle of one discharge every ten seconds at 110% of the rated DC voltage with no voltage reversal.

In most cases there is a minor decrease in capacitance, a small increase in dissipation factor, and an increase in insulation resistance. This indicates that low insulation resistance areas in the dielectric cleared and that the design is not overstressed. Unit number one is an example of a marginal energy discharge capacitor compared to the similar units two and three. Note that it experienced the largest capacitance change and a significant drop in insulation resistance in a smaller number of cycles performed. By plotting the product of capacitance and insulation resistance versus the number of discharges, performance can be evaluated. Graph 4 provides a life test comparison when energy discharge capacitors similar to those in Table II were tested at: rated voltage at +25 degrees ºC, 140% of rated voltage at +25 degrees ºC and at rated voltage at +85 degrees ºC.

http://www.dei2000.com
email: info@dei2000.com