We have demonstrated that the geometry of a capacitor can be modified to reduce ESR. The practical result of reduced ESR is reduced heating, and we have examined the effect of lower ESR on heating for the capacitor pairs described in Table 3.

Testing was performed in still air at room temperature. Test parts were subjected to incremental increases in 40 kHz sinusoidal current. The case (capacitor bodies) and lead wire temperatures were continuously monitored. Temperature increases as functions of rms current are shown in Figures 23, 24, and 25.

The differences in heat rise are dramatic. To demonstrate this the 40 kHz sinusoidal current values required to produce a 20șC rise in lead wire temperatures were extracted and tabulated in Table 11.

Table 11
Capacitance    (F)	Film Width    (inches)	40kHz RMS Current    (amperes)
1.5 1.5 3 3 10 10	0.5 1.0 0.5 2.0 1.0 2.0	16 11 15 11.5 21 16

By reducing capacitor length (and increasing capacitor diameter) we cut the heat rise by almost 50%. Conversely, we could increase the operating current by perhaps 50% by using a capacitor geometry with a lower ESR.

While the differences in heat rise between short, stubby capacitors and long, small diameter capacitors are significant, the stubby configuration should also provide better heat dissipation.

The capacitor lead wire or terminal should be large to increase heat dissipation from the capacitor body and to prevent the wire from becoming a heat generator at high current levels. This latter condition could produce a situation where heat would be pumped back into the capacitor. The heat rise graphs show the change in temperature for lead and body diverge as the rms current is increased. This is undoubtedly due to increased body heat being transmitted to the lead wire in addition to the lead wire heating due to its own ESR losses.

One frequently overlooked factor that can restrict the use of metallized film capacitors in high current a-c applications is the ability of the metallized-film electrode/metal end-spray interface to withstand high peak currents. If the situation arises where the current is limited for this reason, a simple solution is to change the geometry of the section, decreasing the capacitor length and increasing the diameter until the safe current value has been reached. If this is not possible with a single section, use of parallel sections would be considered.

There are a number of valid reasons why capacitor geometry may not always be fully optimized, including: capacitor manufacturing limitations, encapsulation problems, volume efficiency, mounting requirements and available space in chassis.

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