Why are X2-caps so much larger than non-rated caps? - Page 1
Forum section: electronics / projects

author: andersjm
author_rank: Contributor
timestamp: November 05, 2016, 08:34:17 pm
content: I'm designing a low power SMPS power supply, and have been using this cap as an input cap:Kemet R474N31005001K - "Capacitor, Film, Metallized Polypropylene, 0.1 uF, +/-10% Tol, -40/+110C, Safety: X2, 440 VAC, Lead Spacing=22.5 mm"(datasheet url :http://capacitoredge.kemet.com/capedge2/DataSheet/Datasheet-R474N31005001K.pdf?pn=R474N31005001K)It does take up a lot of space on the board, and I don't really need the X2-rating (I'm not using it across directly across mains), so I started looking for smaller alternatives. Then I found this:Kemet C1808C104KBRACTU - "Capacitor, Ceramic, SMD, MLCC, High Voltage, Temperature Stable, 0.1 uF, +/-10% Tol, 630 V, X7R, 1808 (4520 metric)"(datasheet url :http://capacitoredge.kemet.com/capedge2/DataSheet/Datasheet-C0402C100F5GACTU.pdf?pn=C1808C104KBRAC7800)At a cursory glance, it seems to have comparable performance (voltage rating and capacitance). Granted, the datasheet lists 2.5% dissipation factor for the latter (non-rated cap) vs 0.1% for the former (X2-rated). However, the latter is just under 20 cubic millimeters, and the X2-cap is almost 3000 cubic millimeters. A huge difference in size!Is it the X2-reliability requirements that make the X2-cap so much larger? Or is it something else?I realize X2-rating a cap has implications on design. So I'm not surprised it's larger, I'm just surprised by the magnitude of the difference in size and wanted to share it with someone :-)

author: rob77
author_rank: Super Contributor
timestamp: November 05, 2016, 08:39:17 pm
content: the main difference is the polypropylene in the X2 rated cap... basically it's 2 sheets of metal with a polypropylene film in between... it takes up lot of space but on other hand gives the cap the low dissipation and high reliability.

author: rob77
author_rank: Super Contributor
timestamp: November 05, 2016, 08:51:13 pm
content: and don't forget.... X7R might(and will) have even 80% capacitance loss at rated voltage while the polypropylene one will have the same value across all voltages from 0V to rated voltage.

author: tggzzz
author_rank: Super Contributor
timestamp: November 05, 2016, 08:51:49 pm
content: When using ceramic caps with dielectrics other than C0G, beverycareful to assess the affect of capacitance as a function of DC applied voltage.Decent caps have such specifications, and often the actual capacitance is only20%of the headline capacitance! Who knows what the other caps will/won't be.

author: Niklas
author_rank: Frequent Contributor
timestamp: November 05, 2016, 09:00:52 pm
content: Also be careful about the location of such a large ceramic capacitor to avoid cracks. Avoid anywhere near the edges of the PCB or areas with possible mechanical stress, for instance near screw holes or connectors.

author: wraper
author_rank: Unknown
timestamp: November 05, 2016, 09:23:36 pm
content: There are smaller polyester X2 capacitors too. Polypropylene capacitors are large by their nature, not only X2.

author: Gyro
author_rank: Super Contributor
timestamp: November 05, 2016, 09:26:22 pm
content: MKP capacitors are also inherently self-healing whereas multilyer ceramic would be anything but (in fact self detonating in an across the line transient situation). So in reliability terms, yes, there is a big penalty for the volume reduction.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 05, 2016, 10:14:48 pm
content: Plenty of things!1. The 440VAC cap is rated to withstand surges of 4kV or thereabouts. It's unlikely to weather that without cumulative damage, but it's also rated to self-heal rather than fail shorted, or catch fire (or if it does catch fire, that it's quickly self-extinguishing..). You're paying for a lot of other things in there, besides pure capacitance!2. If you need smallest film caps, there are stacked SMT films, general-purpose polypropylene, and polyester parts (use only the voltage rating you need). Some of the smallest I've ever seen are CDE's DME series. They're cheap, small, and somewhat lossy (being polyester).Even regular X2 caps can be damn small. Illinois Capacitor and Vishay/BC MKPs come in 0.1uF, 250VAC, X2, polypropylene, and 10 and 15mm lead spacing. It's uh, a fair bet these things will self-heal a little more aggressively, leading to shorter lifespan in across-the-line use.3. Ceramic caps with class 2 dielectrics, are much the same as ferrite beads: they're fine to use within ratings. But they rarely tell you theelectricalratings. Only the physical ratings (what voltage or currentis safe to apply, not that you'll actually get the rated valueunder that condition).Go on Kemet's K-SIM and look up the next closest part that theydohave data for: C1808C473KBRACIt's -30% of rated value at only 150V. You're talking about twice the value in the same footprint, so it's likely the 0.1uF drops off at an even lower voltage (though it will probably still have more capacitance than the 0.047uF, even under bias).When using class 2 dielectrics, you must always keep in mind that the value under bias isnotthe rated value. You can design with this in mind, but the excess capacitance under startup conditions, and the change with voltage, can be trouble for control loops. (Not to mention aging: the value decreases as log(time) since soldering temperature was last applied.)You can use class 1 dielectrics (C0G/NP0, SL, etc.), but they are much more expensive. They do store quite a lot of energy, though: more than electrolytics!Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 05, 2016, 10:16:45 pm
content: Quote from: Gyro on November 05, 2016, 09:26:22 pmI forget if there are any advertized, and approved, for X1/X2 (across-the-line) duty, in usefully large values. You're right about meltdown: power is fuse-limited only, in that position!Surprisingly, Y1 rated SMT ceramics are available. Probably, they're designed to arc over, externally, first (which is also usually the case for conventional THT parts).The value under bias is still bad, but when they're designed to withstand over 2.5kV peak, and you're talking values of a few nF tops, it's actually quite tolerable. For either kind (TH discs exhibit it, too -- they're usually Y5P, not C0G!).Tim

author: Seekonk
author_rank: Super Contributor
timestamp: November 06, 2016, 06:21:44 am
content: Isn't the simple answer that they are basically two caps in series to lower corona effects between the windings. Therefore take up more space.