It’s also important to keep the metaphorical capacitor bucket refilled so that it’s ready to supply the next pulse. Due to their symmetrical design, the low ESR of pulse supercapacitors, which enables them to deliver charge quickly, also allows them to efficiently refill using the primary or secondary battery source. Renewable batteries are characterized by limited charge/discharge lifetimes and have much narrower temperature ranges than most capacitor families. So, in pulse applications, capacitors will significantly extend the batteries’ lifetime, as well as allow the circuit to operate over a much wider temperature range.
In AC applications (AC power or signal transmission), the physical mechanisms within the capacitor that give rise to the ESR are critically important. The incoming signal will cause the charges on the capacitor electrodes to oscillate, which causes changes in the electric field between the plates. Consequently, the dipoles within the dielectric will resonate in sympathy with the electric field changes. At low signal frequencies, the dipoles have no problem keeping pace; however, at increasing frequencies, the dipoles could start to lag (depending on their dielectric material characteristics).
For example, aluminum and tantalum based dielectrics, which exhibit good bulk capacitance characteristics, will typically see their dielectric performance decrease in the 100 kHz to 1 MHz range, while class II ceramic and plastic film dielectrics typically exhibit decreased performance in the 10MHz range. Class I dielectrics, including porcelain and glass, have the lowest bulk capacitance, as well as the highest frequency response — up to 10GHz and beyond.
In addition to DC behavior, it’s important to consider is how the internal connections from the dielectric to the terminals contribute to the overall ESR. In the case of solid tantalum technology, the positive electrode is tantalum metal; the negative electrode, known as the counter-electrode, is manganese dioxide, which is a semiconductor; and the bonding materials that connect the capacitor element to the lead frame and external contact are graphite and conductive silver epoxy.
Alternatively, ceramic chip capacitors feature a symmetrical design with internal electrodes — predominantly made of nickel in commercial Class II dielectrics and either copper or platinum/palladium for Class I dielectrics — that connect to a copper or silver sub-termination and are then over-plated with nickel and a tin external finish. There may also be an additional compliant conductive layer between the sub-termination and external layer that acts as a stress-absorbing barrier by improving resistance to mechanical and thermal stress.
Chip film capacitor design is also symmetrical, featuring aluminum internal electrodes contacted by a brass end-spray sub-termination and then coated with conductive silver, a nickel barrier, and an external tin plating finish.
These different material sets are a determining factor with regard to ESR characteristics over frequency and temperature, as well as to the AC ripple current handling capabilities.
In each of the aforementioned examples, the equivalent resistance (ESR) of the dielectric material, which is actually an insulator, has a negative temperature coefficient while the metallic components in the resistance path have a small but positive temperature coefficient.
In MLCC and film capacitor designs, multiple metal electrodes are interleaved with the dielectric material and have a common link to opposite terminals by a system of plated metallic layers. Some MLCC designs may also incorporate an additional flexible conductive layer to improve mechanical robustness and thermal cycling capability.