Common mode chokes - why so much insulation? - Page 1
Forum section: electronics / projects

author: ivonenand
author_rank: Contributor
timestamp: July 09, 2015, 01:40:52 pm
content: Hi guys,I'm looking into power line common mode cokes (like the ones on the inputs of AC power supplies). Most of them have a plastic spacer between the windings to increase the creepage distance to > 3mm, offering a withstand voltage of about 3kV. My question is, why is so much insulation needed between the two windings? One is connected to L and the other to N, so the maximum withstand voltage should be 265V * sqrt(2) = 375V. Yet, they are designed for 3kV. Why?Regards,Ivo

author: Alphatronique
author_rank: Regular Contributor
timestamp: July 09, 2015, 02:06:06 pm
content: Hifuse or circuit breaker that you put after that filter have 10KA rating even if it 10A fuse ,all for the same reason safetynot Design for working voltage but for the FAULT one ,at least if you that your product pass UL certification

author: amyk
author_rank: Super Contributor
timestamp: July 09, 2015, 02:11:26 pm
content: So that surges don't arc over.

author: mikerj
author_rank: Super Contributor
timestamp: July 09, 2015, 02:25:21 pm
content: Would you be happy with a product that had only just enough insulation to support the working voltage?

author: splin
author_rank: Frequent Contributor
timestamp: July 10, 2015, 01:04:36 am
content: Quote from: amyk on July 09, 2015, 02:11:26 pmDon't lots of designs have spark gaps so that they do exactly that?Still you probably don't want an uncontrolled arc over at some random point in a component not designed for the purpose which may catch fire or explode as a result.

author: coppice
author_rank: Super Contributor
timestamp: July 10, 2015, 01:12:25 am
content: That's not a common mode choke. That's the blue flashing light which indicates that fast pulse transient testing is in progress.

author: calexanian
author_rank: Super Contributor
timestamp: July 10, 2015, 03:06:28 am
content: Insulation becomes a negative resistance device once it hits its zener breakdown voltage

author: Siwastaja
author_rank: Super Contributor
timestamp: July 10, 2015, 07:32:35 am
content: Quote from: ivonenand on July 09, 2015, 01:40:52 pmYou have it fundamentally wrong here; although it's common knowledge that 1 kV creeps 1 mm, it's kind of a rule that itreallydoes happen. I.e., if you have an unknown high voltage and it jumps 10 mm, you can estimate it's around 10 kV.So 1mm/1kV is a borderline case for breakdown. If you want the breakdown to happen, or not to happen, you need to add margin either way.Now, as you might see, when designing for the case where you want it NOT to jump, you need to factor in a lot of safety margin. You also want to think about moisture and contaminants in the air, or any dirt that forms on the PCB / plastic / core / whatever the wires are on. Then there are manufacturing tolerances - what if that 3mm sometimes becomes 2mm? If it was 2mm to begin with, it would be 1mm, and with moist, dirty air, and some slight surge on the mains, it's suddenly almost guaranteed to breakdown.

author: HKJ
author_rank: Super Contributor
timestamp: July 10, 2015, 08:19:45 am
content: Quote from: ivonenand on July 09, 2015, 01:40:52 pmThe mains voltage may be below 265VAC most of the time, but there are transients at much higher voltage and you do not want the first transient to damage the coil.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 11, 2015, 11:32:37 am
content: Another reason there is a large separation between each winding in a common mode choke is because this results in a lot of uncoupled (aka "leakage") inductance which provides some differential mode filtering. It's a rare case of where leakage inductance does something useful, rather than blow up switches and diodes and such.That said, yes, the main reason for the tall insulator between windings is to withstand specified transient overvoltage in worst-case environmental conditions (ie - from surface contamination, condensation, etc).