TEChNOLOGY REVIEW


Understanding equivalent series resistance (ESR) in electrolytic capacitors


bernd heim of avnet abaCus explains how Component diversity has made seleCting the best CapaCitors for power-related appliCations more Complex than ever, and why taKing independent adviCe maKes sense.


Today’s microprocessor-based systems require power sources that deliver high current and ultra-fast transient performance, with tight regulation. This creates a need for cost-effective, compact capacitors with high capacitance values. Electrolytic capacitors are the solution of choice but it’s important to go beyond specifying capacitance and voltage to consider equivalent series resistance (ESR) as a figure of merit.


esr: easier to define than speCify The capacitor equivalent circuit comprises four elements (Figure 1): capacitance, equivalent series inductance (ESL) - the sum of inductive elements including leads, a high-resistance DC


path (Rp) in parallel with the capacitance, and equivalent series resistance (ESR) - the series resistive effects combined into a single element. ESR is frequency-dependent, temperature- dependent, and changes as components age. It is usually only an important consideration in selecting electrolytic capacitors.


ConstruCting the CapaCitor ‘Wet’ aluminium electrolytic capacitors have an anode plate comprising an electrochemically- etched aluminium foil, a dielectric formed as an oxide layer on this foil, a paper spacer to hold conductive fluid that forms the cathode, and a second foil that connects the electrolyte and


Figure 2: Solid aluminium electrolytics exhibit low and stable ESR across their operating temperature range (Panasonic)


Aluminium electrolytic cap. hybrid cap.


100


TEMPERATuRE VS. ESR @ 100khz 10.0 x 10.2 / 35V / 330 µF


6.3 x 5.8 / 35V / 27 µF Hybrid


AI.Electrolytic Rp


ESR


c


ESL


Figure 1: The equivalent circuit of a capacitor is made up of four apparently simple elements


the device’s terminal.


Fluid electrolyte penetrates the pores of the oxidised anode foil, so contact area, and hence capacitance, are maximised.


The electrolyte’s temperature-dependent conductivity is a major contributor to ESR. Ohmic losses produced by the aluminium oxide layer are frequency-dependent, reducing as frequency increases. Fluid electrolyte is lost over time by vaporisation and diffusion, causing a gradual reduction in the amount of conducting material, reducing the contact area, increasing the ESR and reducing capacitance. This “drying out” process is temperature dependent and accelerates in components used at higher temperatures or subjected to higher ripple currents, which dissipate more heat as part of their circuit function. In aluminium electrolytics, ESR falls as temperature increases - its effects reduce as assemblies warm up.


Drying out is irrelevant in solid aluminium electrolytics, or ‘hybrid’ capacitors, where a polymerised organic semiconductor material replaces liquid electrolyte. This technology exhibits a specific conductance of around 10,000 times that of a liquid electrolyte. As Figure 2 shows, overall ESR is reduced substantially, most significantly at low temperatures.


10 1


tantalum teChnology Tantalum electrolytics have tantalum anodes of either sintered powder, or plain or etched foil. The insulator is an oxide layer on the anode surface. In foil devices the second conductor is an electrolyte held in a spacer. A deposit of manganese dioxide covers the oxide layer in solid versions.


0.1 0.01 -60 -40 -20 0 20 Temp ˚C www.avnet-abacus.eu 12 40 60 80 100 120


The component’s terminations are substantial contributors to ESR. In a solid capacitor the manganese dioxide is commonly covered with carbon and then a metal such as silver, which is soldered to the negative lead or case. In the foil style, the positive connection is a welded nickel or steel wire connected to a tantalum wire on the anode. Such devices also contain a second tantalum foil in contact with the electrolyte.


At low frequencies, the oxide losses are most significant. But their contribution


focus magazine - issue 14


ESR (Ω)


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