Electrolytic Capacitor ESR: 5 Things You Should Know

Power supplies for modern digital ICs have stringent requirements for performance, demanding high capacitance and compact capacitors that are also cost-effective. Electrolytic capacitors are the most obvious choice, but when specifying these devices, it’s important to consider how the equivalent series resistance (ESR) will affect performance.

Here are five things you should know about ESR when choosing an electrolytic capacitor for your IC power supply design.

1.) ESR affects a capacitor’s performance as well as wasting energy

All capacitors have an equivalent series resistance, even if that resistance is minimal. Just like any type of resistance, when current flows through the device, some of the energy will be dissipated in the form of heat. The amount of energy dissipated will depend on how large the ESR is, so capacitors with very low ESR can help improve the efficiency of a power supply, as less energy is wasted as heat.

However, aside from adding inefficiency, a capacitor’s equivalent series resistance (ESR) affects its performance in several ways. Firstly, energy lost as heat will heat up the device and its surroundings, which can rapidly shorten the lifetimes of some types of electrolytic capacitors.

Other performance metrics are affected too. The higher the capacitor’s ESR, the less able it is to quickly sink or source charge. Since our IC to be powered demands low ripple, electrolytic capacitors with sufficient output capacitance in combination with low ESR must be used since voltage ripple is directly affected by ESR.

2.) ESR for ‘wet’ aluminium capacitors is age-dependent, temperature-dependent and frequency-dependent

‘Wet’ aluminium capacitors, despite being a relatively old technology, are still designed-in because they can come in high capacitance values and are cost-effective. They have a liquid electrolyte, which forms the cathode; the liquid is better able to make contact with the oxidised foil of the anode, whose surface is roughened to create more surface area. ESR tends to be better for bigger case sizes and sometimes for longer/thinner case shapes too.

The resistance of the liquid cathode material is temperature-dependent, with capacitor ESR reducing with increased temperature. This effect can be significant even over the temperature range experienced by an application PCB in the field as certain components expel heat into the system and the capacitors warm up.

Meanwhile, the resistance of the oxidised anode is frequency-dependent; it decreases with increasing frequency.

A third effect to be taken into account is ageing. After some time has passed, the liquid that forms the cathode eventually begins to evaporate inside the can, and it also diffuses into other parts of the structure. Because the volume of liquid is lower, there isn’t as much in contact with the anode, so capacitance decreases and ESR increases. This effect is called ‘drying out’ and happens much more quickly in the presence of elevated temperatures.

3.) ESR for hybrid aluminium capacitors is lower and more stable overall

Hybrid aluminium capacitors have a solid electrolyte made of polymerised organic semiconductor material that replaces the liquid in a wet electrolytic capacitor.

Hybrids have a lower ESR than wet types, especially at low temperatures, and the ESR is much more stable over the whole operating temperature range. Their ESR is still frequency-dependent, but it is lower than wet aluminium capacitors at high frequencies, which makes them popular for switch-mode power supplies.

Like Panasonic, hybrid electrolytic capacitors outperform wet electrolytic capacitors on ESR, especially at low temperatures.

4.) ESR for tantalum capacitors is largely frequency-dependent

In a typical solid tantalum capacitor, the tantalum anode is covered by an insulating layer of tantalum oxide, then a layer of manganese dioxide, then usually a layer of carbon and a layer of a metal such as silver, which is soldered to the lead or case. This termination is the main culprit when it comes to ESR. Losses in the oxide layer dominate, which is highly frequency-dependent, with the most significant losses occurring for low-frequency signals. These losses decrease as the frequency increases and become much less significant at very high frequencies.

The ESR of a tantalum capacitor reduces at high frequency, as for this AVX capacitor.

5.) ESR is not specified in a consistent way

It’s sometimes more challenging than it looks to directly compare data from the manufacturers’ data sheets to specify a capacitor for an IC power supply application because the ESR can be quoted in more than one way. The ESR at 25°C and 100kHz is often quoted, but not always, and some specify at a different frequency entirely. Some give only the dissipation factor (tan d). In all these cases, a calculation is required to determine the ESR at the operating frequency of your particular application. Consulting an experienced distributor such as Avnet Abacus for advice can help ease the selection process.