Advertisement

Blog

Parasitics & Capacitor Selection, Part 2C: Equivalent Series Resistance (ESR)

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.

Figure 3: MLCC & Film Capacitor Construction: Examples of Symmetrical Design

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.

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.

5 comments on “Parasitics & Capacitor Selection, Part 2C: Equivalent Series Resistance (ESR)

  1. etnapowers
    April 4, 2014

    The ESR is very important in all the applications requiring a very high precision of the output voltage, for example in DC-DC voltage regulators the output capacitor has to be a low ESR capacitor , otherwise the output voltage drop might affect the precision of the output voltage.

  2. RedDerek
    April 9, 2014

    I am finding that for some of my power supply designs I have been moving more towards the ceramic chip capacitors. With these I can stack parts, and parallel very easy. Plus the parallelling helps drop the overal ESR.

  3. etnapowers
    May 6, 2014

    @RedDerek: ceramic capacitors are very useful for the applications requiring a stable input voltage. The soldering has to be really accurate, otherwise there's the risk of excessive leakage current.

  4. Chris-R
    May 28, 2014

    That's correct – it's not advisable to do any hand solder assembly to ceramics on the pcb, such as stacking, for example, as thermal damage can result. There are, however, stacked ceramic series available just for these types of application where real estate is an issue. Most series feature horizontal stacking, but there are also series available where capacitors are stacked sideways in a stick format. This means that the electrode layers in each capacitor are vertical with respect to the pcb and at right angles to the ground plane, reducing ESL in the system.     

  5. etnapowers
    June 12, 2014

    @Chris, nice post, for PCB the customized solutions that you described generally work well. As concerns the capacitors that are stacked sideways in a stick format, you're right the ESL is reduced, there might be concern for the volume occupation of the PCB inside the assembled module of the customer, in case of an high capacitance value.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.