Buck output capacitor selection - Page 1
Forum section: electronics / general

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: May 17, 2020, 12:16:57 pm
content: If I want to plan a buck circuit with a 100uF output/bypass cap,what type of capacitor would be good for a buck?The reason I ask, is because the ONLY cap's I have on my shelf >=100uF is electrolytic capacitors.I wanted to see if those types would be ok in a buck circuit...if not, I welcome recommendations.

author: Styno
author_rank: Regular Contributor
timestamp: May 18, 2020, 07:25:00 am
content: What does the datasheet of the buck say about the requirements for the output cap?

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: May 18, 2020, 09:49:25 pm
content: Quote from: Styno on May 18, 2020, 07:25:00 amIn my case, I hope to begin building some buck circuits FROM SCRATCH for learning and testing.So for example, I thought of starting with a 12V input, and a 5V output (which might eventually serve as a usb charger).I realize newer devices can pull >2A,but for my first buck circuit/test, I will just us a simple a resistor as a load, and would plan to initially draw <500 mA.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: May 18, 2020, 11:50:40 pm
content: Hi,I'm glad your interested in getting into power electronics. I suggest doing some reading on the subject. I suggest:Switchmode Power Supply Handbook by Keith Billings. I have the second edition but there may be a newer version. There are other books on the subject.Also Texas Instruments has man ap notes that are very useful.Andy

author: David Hess
author_rank: Super Contributor
timestamp: May 19, 2020, 01:19:58 am
content: Quote from: mikeinfodoc on May 17, 2020, 12:16:57 pmThe output capacitance is not as important as the impedance and ripple current rating. The capacitor impedance at high frequencies is primarily composed of the ESR and the ESR combined with the ripple current determines the power dissipation.Aluminum and tantalum electrolytic capacitors are suitable at lower ripple currents however at high ripple currents, capacitors with a much higher ripple current rating, which is associated with a lower ESR, like polymer aluminum or tantalum capacitors are required.Ceramic output capacitors are sometimes used but have special considerations because their ESR is so low that it impacts feedback loop stability. The same would apply to a film capacitor.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 19, 2020, 04:56:02 pm
content: You get the ripple current fromdI/dt = V/Ldt = ontime (or offtime)V = voltage over inductor during the ontime (or offtime)Then look at the ripple current rating of the capacitor.You are pretty much done at that point, with electrolytic capacitors; if you get large enough part to handle the ripple current, it has quite some capacitance to deal with ripple voltage, as well.Buck output is about an order of magnitude easier on the caps than the buck input. Electrolytics are often OK, even as-is.Buck input requires lower ESR, and the input loop area should be small, easier done with small ceramics with less lead stray inductance.Never use tantalum unless you know what you are doing.