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

 

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.

 

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

 

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!)