Current rating of film capacitors? - Page 1
Forum section: electronics / beginners

author: sean0118
author_rank: Frequent Contributor
timestamp: September 06, 2014, 01:03:08 pm
content: Hi everyone,This is probably pretty basic but still has me stumped.I'm searching for a capacitor to use in an application (LLC Resonant Converter) where it will be subjected to 2.6A(peak) at 100kHz.The two models of cap I've been considering are:http://www.farnell.com/datasheets/1740902.pdfhttp://www.farnell.com/datasheets/415222.pdfHowever the problem is I can't find anything on the datasheet about their current rating. Is it derived from the power or voltage ratings? Or am I looking at this the wrong way? This is something I really, really need to get right.

author: jmoreland79
author_rank: Regular Contributor
timestamp: September 06, 2014, 01:54:44 pm
content: It can be derived from the voltage rating and the ESR of the specific unit, but it's a rather involved multi-step calculation. It's usually only necessary for very high power applications with high ripple currents.It's a good concept to understand, but for a circuit reaching only 2.6 A, just stay within the voltage rating and you'll be fine.

author: Whales
author_rank: Super Contributor
timestamp: September 06, 2014, 02:02:34 pm
content: This is complicated -- you can find the impedance of the cap form the impedance vs frequency plot on the datasheet, but current will only flow through this impedance due to voltage ripple.Your circuit will /probably/ be fine, provided your vsupply is relatively stable.EDIT: This is a resonator. Why did I assume it was a voltage smoothing cap?

author: Rufus
author_rank: Super Contributor
timestamp: September 06, 2014, 02:42:48 pm
content: Quote from: sean0118 on September 06, 2014, 01:03:08 pmDon't know why they choose to specify it this way but the current handling capability is defined as an rms voltage rating at specified frequency. The Vishay datasheet has plots. You can pick some points on the plot and work back to an rms current rating which I suspect will be fairly constant or have some other curve against frequency which would be helpful to have in the datasheet to start with.

author: sean0118
author_rank: Frequent Contributor
timestamp: September 06, 2014, 02:52:39 pm
content: Ah, very interesting, thank you. It sounds like it's more complicated than I thought.Just to elaborate a bit, in the LLC the resonant capacitor is in series with the main current path of the primary. It's going to have AC voltage applied across it, with current swing from -2.6A to 2.6 at max load. This should only be at 50kHz, but worst case could be up to 100kHz.Do you still think either cap I linked in would survive? I was originally going to go forthesebut everyone has sold out of the capacitance size I need.

author: sean0118
author_rank: Frequent Contributor
timestamp: September 06, 2014, 02:55:29 pm
content: Quote from: Rufus on September 06, 2014, 02:42:48 pmThanks, I might try that.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 06, 2014, 03:02:31 pm
content: The current rating can also be derived from their loss factor at your target operating frequency.The Panasonic caps (the first ones you linked) have a ~2% loss at 100kHz while the Vishay ones are under 0.1% if I read their graph correctly so the Vishay should be able to handle resonant tank currents with much lower losses.Your original Panasonic ones that you cannot get anywhere do look even better than Vishay's.

author: sean0118
author_rank: Frequent Contributor
timestamp: September 06, 2014, 03:24:22 pm
content: Quote from: DanielS on September 06, 2014, 03:02:31 pmThanks, that sounds like a good approach as well.QuoteWell, Digikeyhas them, but I'm not paying $34 postage for a $3 capacitor.

author: PChi
author_rank: Frequent Contributor
timestamp: September 06, 2014, 03:38:11 pm
content: The Panasonic general purpose metallized film capacitors are probably unsuitable because the current carrying capability of the metallization is unlikely to be sufficient.The Vishay parts are designed to carry current. I have seen the AC voltage derating with frequency being used before so a bit of maths is needed.EPCOS make it easy and their power capacitors do specify the current capability,http://www.epcos.com/inf/20/20/db/fc_2009/MKP_B32774_778.pdf.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 06, 2014, 07:15:04 pm
content: Polyester is too lossy for resonant applications. At best, you might get away with using those Panasonic caps for supply bypass, limited AC line use (filtering, PFC, voltage drop, etc.) (see notes at the bottom of page 5), or RC timing sorts of things. But not for real ripple current, let alone lots of voltage.Polypropylene is what you're looking for. The V(F) graphs in the Vishay datasheet give you what you need to know. For example, the 630V 0.1uF curve intersects 50V at 1MHz, or a whopping 31A! That's a lot of current for those poor 0.8mm leads.Only other caps I've seen like those are Illinois Capacitor's PPB series. Good to know Vishay has a competing line. Hmm, rather pricey from Mouser though. Reasonable at Newark/Farnell. Digikey doesn't carry them.Anyway, they'll certainly work for your purpose -- they'll be rather large and expensive though! You can save a buck or two with a milder one, maybe, EPCOS B3265x series?Tim