How can a 1206 MLCC used in a design meet 2kV rating? - Page 1
Forum section: electronics / projects

author: SgtSiff
author_rank: Contributor
timestamp: February 01, 2021, 11:31:39 am
content: Im using the XP Power IES0105S24 isolated 5-24V DCDC converter in a design, and the datasheet suggests a filter for class B emissions:For C4, they recommend to use a 2kV rated 1nF MLCC. I didn't think this would be possible - I used the Saturn PCB calculator and it says that for 2KV, the minimum conductor spacing for uncoated traces is 10mm. Even for a PCB with a conformal coating, it says the spacing should be 5.38mm.To my surprise, there's many available such as the C1206C102KGRACTU, however this is in a 1206 package so the entire cap is only 3.2mm long.Am I misunderstanding things, or perhaps getting my terminology (rating, isolation) wrong?Another option would be to drop the filter entirely but that's an entirely different discussion. It's only low volume right now but I imagine at some point we would need to get EMC testing done to get a CE stamp.

author: strawberry
author_rank: Super Contributor
timestamp: February 01, 2021, 12:22:17 pm
content: X1 X2 Y1 Y2 Safety Class capacitors used in line mains applicationsthere should be isolation class for DC-DC converter it self to be safe for HV2kV is DC MLCC offset saturation voltageEEVblog #626 - Ceramic Capacitor Voltage Dependency

author: penfold
author_rank: Frequent Contributor
timestamp: February 01, 2021, 12:40:54 pm
content: First thing to note is that it would be generally improper to use an MLCC right up to its data sheet spec voltage. It would be generally designed around some voltage derating factor to prevent permanent damage to the cap by small deviations to normal operating conditions and to work around the whole voltage coefficient of capacitance issue.. so just because they've made a 2kV 1206 cap, the chances are its not actually intended to be used at such a voltage especially within the scope of every possible target safety standard.The XP app example is only there to highlight their recommended filtering arrangement. It is up to you to design your circuit so that it meets your requirements for 'safety'.I would recommend that you work out exactly what your requirements for safety criteria and EMC are before committing to any design choices, Saturn PCB calculator is useful, but its not going to tell you what regulatory requirements you need to apply.

author: Gyro
author_rank: Super Contributor
timestamp: February 01, 2021, 12:41:43 pm
content: I'm surprised that they would put such dangerous information in an App note. As strawberry says, C4 needs to be a Y rated capacitor with appropriate agency approvals!A Y cap is going to be much longer than 1206. You'll probably find that a through hole disk ceramic part is cheaper, and easier on the PCB layout.P.S. If this is a Class II double insulated product, Primary-Secondary creepage and clearance needs to be at least 8mm, regardless of solder resist / conformal coating.

author: Yansi
author_rank: Super Contributor
timestamp: February 01, 2021, 12:49:41 pm
content: Whats dangerous on using the 2kV rated ceramic?Jeez, it is just some pisspoor basic insulation DCDC converter rated with just 1.5kVDC. This is definitely not the DCDC you would use to isolate yourself from the mains, so depending on the application, the cap soes not even have to by Y rated!

author: wraper
author_rank: Unknown
timestamp: February 01, 2021, 12:49:59 pm
content: Quote from: strawberry on February 01, 2021, 12:22:17 pmQuestion has nothing to do with safety capacitors.QuoteWhat this should even mean?Quote from: SgtSiff on February 01, 2021, 11:31:39 amDistance is enough for functional insulation.

author: Yansi
author_rank: Super Contributor
timestamp: February 01, 2021, 12:50:35 pm
content: Quote from: Gyro on February 01, 2021, 12:41:43 pmIt is not a reinforced insulation converter!Use of a non-Y rated 2kV 1206 ceramic is just fine.

author: wraper
author_rank: Unknown
timestamp: February 01, 2021, 12:50:43 pm
content: Quote from: Gyro on February 01, 2021, 12:41:43 pmIt doesn't. Look what is the device in question.

author: SgtSiff
author_rank: Contributor
timestamp: February 01, 2021, 01:02:18 pm
content: Quote from: strawberry on February 01, 2021, 12:22:17 pmThanks for the quick replyYes, the DCDC converter is rated at 1.5kV as standard.I figured as much RE the sat. voltage, so I just wonder why the datasheet specifies a 2kV MLCC as it seems impossible to maintain the correct separation that way.The DCDC converter is not being used in conjunction with mains, it is being used to power the field side of an ISO1212 so that I can have isolated digital inputs. Instead of supplying 24VDC externally, I will supply it from the device itself.

author: Gyro
author_rank: Super Contributor
timestamp: February 01, 2021, 01:04:18 pm
content: Quote from: wraper on February 01, 2021, 12:50:43 pmDamn, I saw the diagram and interpreted it as offline!Sorry OP.

author: wraper
author_rank: Unknown
timestamp: February 01, 2021, 01:07:16 pm
content: Quote from: SgtSiff on February 01, 2021, 01:02:18 pmFor example to ensure that capacitor does not fail due to static discharge. Functional distance is enough for 2kV, at least on component side of things and it's ceramic body.

author: SgtSiff
author_rank: Contributor
timestamp: February 01, 2021, 01:08:20 pm
content: Quote from: wraper on February 01, 2021, 01:07:16 pmI think this answers my question, thank you. Sorry for the confusion everyone I should have mentioned the application.

author: Gyro
author_rank: Super Contributor
timestamp: February 01, 2021, 01:09:47 pm
content: Nope, Entirely my fault for not reading it properly!

author: wraper
author_rank: Unknown
timestamp: February 01, 2021, 01:19:47 pm
content: QuoteBTW datasheet does not state it should be SMD MLCC. There are plenty of TH MLCC with much larger distance between terminals. I was under impression they recommended SMD MLCC after reading what you wrote, but nope. Though it still would be valid, just not in every case possible.

author: SgtSiff
author_rank: Contributor
timestamp: February 01, 2021, 01:21:44 pm
content: Quote from: wraper on February 01, 2021, 01:19:47 pmGreat point, yes, I don't know where I read that it must be SMD.

author: strawberry
author_rank: Super Contributor
timestamp: February 01, 2021, 01:23:36 pm
content: Quote from: wraper on February 01, 2021, 12:49:59 pmQuoteWhat this should even mean?Control logic at 0V reference level and High side gate driver is offset 300V for example 2kV MLCC will see ~300Vif capacitance drops EMI attenuation might as well

author: wraper
author_rank: Unknown
timestamp: February 01, 2021, 01:29:17 pm
content: Quote from: strawberry on February 01, 2021, 01:23:36 pmYou're still confusing things and what you wrote here makes little sense. How much capacitance drops due to applied DC voltage, and how much of it remains at max rated voltage is different for different capacitors. Rated voltage is not a function of capacitance drop as such. Capacitor rated for 2kV must withstand 2kV, period.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 01, 2021, 02:01:11 pm
content: Several things:- Spacing assumes nominal plus transient voltages. Mains for example might be 250V nominal but 2.5kV peak. The wiring shouldn't arc over at the higher of the two, but also does not need to sustain it continuously.- Spacing assumes some combination of creepage or clearance (which one, isn't stated above). Clearance is the straight-line distance through air and around insulators. Creepage is the distance along insulating surfaces. Creepage can be avoided by cutting slots in the PCB, using internal layer connections*, or adding flanges to insulating standoffs.- If creepage, the comparative tracking index (CTI) may be relevant. In particular, PCB material is about as bad as it can be (class I), meaning, if a sustained arc occurs across the material, it will quickly char to carbon in a runaway fashion. Ceramics tend to have very high CTI figures, for example even if dust and soot happen to collect on the surface, it may simply burn away, leaving it actually cleaner than before. I'm not sure offhand what CTI specifically applies to the ceramic used in capacitors (barium titanate something or other); I've certainly seen some melted into ugly greenish blobs before (the ceramic isn't very strong, and the fine layers of metal within can fracture, short out, and, the rest is history... a briefly incandescent history). But it's likely much better than PCB material.- Even if the creepage and clearance aren't enough for the voltage under any combination (i.e., functional only), it could be potted*. I've seen resistors as small as 0603, rated in this way -- it's rather stark to see a "10kV" rating in a datasheet, but once you finally locate the fine print, ah yes, that's a rather important gotcha. (Such resistors might be useful for ESD protection -- few chip resistors can handle such voltage or power, and whether or not they survive, they would just arc over.)*Standards usually refer to this as acemented joint, and the rating is typically not much below the dielectric withstand voltage of the enclosing materials. Inspection or test clauses may apply. (Soldermask and conformal coating may count as well.)As for the real deal -- Y2 compliant capacitorsareavailable in SMT types. They are 1808 or larger size, to the best of my knowledge. You might want to use one over a PCB slot, for precious creepage; and be very careful of board flex (especially with the slot cut out under it), position it away from strain and flex, or add additional slots to isolate it from such. Can also use multiple in series for redundancy.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 01, 2021, 02:04:04 pm
content: Quote from: wraper on February 01, 2021, 01:29:17 pmAs for this -- most types in this service (Y5P) are dropping off above a few hundred volts, regardless. That's fine for EMI: line voltage is the expected case. If a 20us surge comes by and lets through a couple cycles of switching noise, well so what, no one is going to hear it over the arcing, or lightning strike, that caused the damn surge.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: February 02, 2021, 12:20:01 am
content: Quote from: T3sl4co1l on February 01, 2021, 02:01:11 pmFor what it's worth, even though there are Y-rated SMD caps available, I would be cautious to actually use them. In particular, I would probably not use the types in the standard brown/earth colored ceramic package, if I could avoid it. If you must use SMD Y-caps, use a molded plastic type with gull-wing lead. In order of preference:1. Plastic molded package with gull-wing leads (e.g. Murata DK1)2. Normal ceramic package with soft/flexible-termination (e.g. Vishay VJ with "W" termination)3. Normal ceramic package with normal termination (e.g. Vishay VJ with "X" termination)If you have space for a through-hole Y-cap(s), I'd just use those. They're going to be cheaper (I think) and they have good built in strain-relief by virtue of through-hole leads.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 02, 2021, 08:22:59 am
content: The key points in a nutshell:Why 5 - 10 mm is required for 230V mains is not because it's 230V, it's becauseit can be way more, mains isn't clean. If it always was just 230V guaranteed, 1mm would do fine. But because it'ssafety critical, it can't be allowed to arc during peaks, possibly leaving carbonized residue.Functional isolationmeans non-safety critical use case. Safety margin is not needed. If the cap fails, product just fails, no one is hurt. As a result, the 2kV rating is the actual rating, it has no excess margin for any peaks exceeding that. You can easily make it arc by maybe going to just 3kV. The designer is responsible for understanding the PCB surface creepage, that's not the part of the capacitor.