Electrical hobbyist here and have a question with capacitor selection when designing switch mode power supplies.

When on digikey I search for the capacitance I need, but none of the data sheets give the capacitors ESR value.

How do you guys go about selecting output capacitors, when needing to be under a certain ESR value, when it's not listed in the data sheets?

Thanks guys.

 

Get the lowest ESR you can

 

In datasheets the ESR is rarely quoted but instead they list "tanδ" (Tan delta)

This is a pretty good application note that shows you how to find ESR from Tan delta, capacitance, and frequency. The App Note gives a pretty good summary of its contents in the first line: "Dissipation factor (tan δ)or DF is defined as the ratio of the ESR and capacitive reactance."

Dissipation Factor/ ESR - Illinois Capacitor

 

Some the manufacturers will also highlight the capacitors in their line that are specifically meant for SMPS use. These will typically be the ones with the lowest ESR that they offer.

 

Thanks for the link and the help! Very helpful.

Can't wait until I finish my first pcb.

 

Guys, I really appreciate the help.

Sorry for bumping this post up again, but I want to make it complete in case someone stumbles upon this post google'ing like I did, by including my calculations. Also allowing you guys to double check my calculations to make sure I'm getting this correct.

ESR = DF / (2 x PI x F x C)

ESR = Equivalent Series Resistance
DF = Dissipation Factor (Percentage)
PI = 3.141
F = Frequency (Hz)
C = Capacitance (F)

DF = 3.5%
3.5 / 100 = .035 DF in decimal form
.035 / (2 x 3.141 x 1,000,000 x .000,000,28)
.035 / 1.75896
ESR = 0.019,898