# Impedance of electrolytic capacitor

Discussion in 'General Electronics Chat' started by Varkatzas, Feb 1, 2013.

1. ### Varkatzas Thread Starter New Member

Oct 22, 2012
15
0
Hi all,

I need a low impedance electrolytic cap for a gate driver configuration, the manufacturer recommends a cap with 1 ohm or less impedance.

I've been reading some caps datasheets, however i find the data presented very confusing and it is not always show the actual impedance.

I know impedance has to do with the capacitance and frecuency, but most datasheets show it with a test frecuency of 120hz (My switching frequency is 20KHz). Can I take that reported impedance value even though the frequency they use is a lot smaller?

Other parameter is tan(delta) or Dissipation Factor, where it is represented the quotient of ESR and Xc. Some datasheets only present this data, how could i estimate the impedance only by tan(delta)?

Hope you can help me understand these parameters.
Thank you

2. ### antonv Member

Nov 27, 2012
149
27
I believe you need to look for ESR (Equivalent Series Resistance). If you look for capacitors on vendors' websites like Digi-Key, there is a column for ESR or one of the specs that you can use to filter the capacitors is ESR or controlled ESR.

3. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
In DigiKey's parametric search, you can choose both ESR and impedance for values (@100kHz, IIRC) to sort to meet your needs.

4. ### gootee Senior Member

Apr 24, 2007
447
50
DF = Dissipation Factor

DF = tan(δ) = tan (delta) = tangent of loss angle

DF = 2∙π∙f∙C∙ESR

and

ESR = DF / (2∙π∙f∙C)

You can usually get a fairly-decent APPROXIMATE ESR, with

ESR = 0.02 / (C ∙ Cap's Voltage Rating)

Since you are probably dealing with time-varying signals, possibly with fast rise and fall times, the inductance might be much more significant than the resistance.

And the self-inductance of the conductors (PCB traces or wires) might be more-significant that the cap's inductance.

You can approximate the self-inductance of "average" PCB traces and wires with 1 nH (1e-09 H) per mm.

You can approximate the resistance of an "average" PCB trace or wire with 1 mOhm (0.001 Ohm) per 25.4 mm.

For most capacitors, the "inductance" is the same as a wire with length equal to the cap's lead-spacing, i.e. 1 nH per mm of lead-spacing.

You will need to sum the impedances for the capacitor network, i.e. including the connecting conductors:

Z = R_total + jwL_total - 1/(jwC_total)

Remember that the L_total estimate has to include the round-trip conductor length times 1 nH per mm, plus 1 nH per mm times the cap's lead spacing.

And the R_total estimate should include the ESR of the capacitor plus 1 mOhm per inch of conductor.

Electrolytic capacitor characteristics can vary singificantly with both frequency and temperature.

Here is a nice Cornell Dubilier Java Applet that lets you see the changes by moving sliders:

http://www.cde.com/applets/CDEspiceApplet/CDEspice.html

Just remember that they don't include the parasitic inductance or resistance of any connecting conductors.

(If anyone is into LT-Spice, that applet ALSO produces frequency-dependent, temperature-dependent Spice models for electrolytic capacitors!)

Last edited: Feb 1, 2013
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