What is isolation level of isolated DCDC module? - Page 1
Forum section: electronics / beginners

author: ocset
author_rank: Super Contributor
timestamp: February 25, 2018, 11:05:37 am
content: Hello,We need an isolated DCDC module to give 15V output from a 13V input. Its to provide isolated power to a DALI comms bus from a non mains isolated source.We need full mains isolation. This is usually described as being able to withstand a hi-pot test at “3000VAC at 50Hz for one second”.However, the Traco TEN 3-1213 DCDC module states isolation of “1500VDC for one second”.Do you think that this is therefore safe to use for our DALI comms bus? (ie, does it equate to full mains isolation)?Traco TEN 3-1213 DCDC module datasheethttp://www.farnell.com/datasheets/2200535.pdf?_ga=2.114611086.845751712.1519485949-287702491.1506211291

author: madires
author_rank: Super Contributor
timestamp: February 25, 2018, 12:21:33 pm
content: If you need an isolation withstanding at least 3kV AC the Traco is the wrong DC-DC converter. It could be just a ferrite toroid with some magnet wire only relying on the enamel coating. For mains isolation there should be also insulating tape between the windings.

author: ogden
author_rank: Super Contributor
timestamp: February 25, 2018, 12:22:22 pm
content: Quote from: treez on February 25, 2018, 11:05:37 amStandards does not say "full mains isolation", they are much more specific. You shall not rely your safety-related design decisions on public forum knowledge where anyone can say anything.If it is required to withstand a hi-pot test at “3000VAC at 50Hz for one second”, then isolation of “1500VDC for one second” does not meet the specs no matter how you are trying to twist it or look at it.You shall either change specs of your product and state it's isolation as “1500VDC for one second” or just meet what's required by DALI or whatever document you are referring to regarding 3000VAC.

author: Ian.M
author_rank: Super Contributor
timestamp: February 25, 2018, 12:38:54 pm
content: 3000V RMS is 4243V peak. You therefore need a module with a 5KV DC isolation rating *BUT* as your design requirements call for 3KV AC RMS rating, you should check with the module manufacturer that its suitable unless it also has an AC rating >=3KV.

author: ocset
author_rank: Super Contributor
timestamp: February 25, 2018, 01:16:48 pm
content: Thanks, we need the type of isolation that you get from primary to secondary in an offline isolated power supply for eg a domestic product....where human hands might touch the output. "basic" insulation level.The alternative NXJ1S1215 has 4.2kV of isolation. 3000VAC * sqrt(2) is 4240VpkMaybe that just scrapes inside the regs?So i reckon the 4.2kvDC of isolation should give us "basic" insulation level?NXJ1S1215http://www.farnell.com/datasheets/2282933.pdf?_ga=2.13815998.845751712.1519485949-287702491.1506211291

author: daveshah
author_rank: Unknown
timestamp: February 25, 2018, 01:24:28 pm
content: According to the datasheet for that part, it says "The NXJ1 series has been recognised by Underwriters Laboratory (UL) to UL 60950 for reinforced insulation to a working voltage of 200Vrms and for basic insulation to a working voltage of 250Vrms." - so it would be OK for basic insulation for UK mains (unless you're in a part of the UK with 290V mains)

author: max_torque
author_rank: Super Contributor
timestamp: February 25, 2018, 02:03:35 pm
content: But you are driving that DCDC with a 13v supply? so presumably already have some HV isolation upstream?If you have say a fixed transformer, then the HV winding is wound onto the same physical location as the secondary, and so high levels of isolation voltage are sensible, but driving an isolated LV module, hmm im not so sure? (because it would take multiple failures to cause a breakdown)But, it's your product, you need to evaluate the risk / hazard and set appropriate levels, and provide suitable matching engineering solutiions

author: ocset
author_rank: Super Contributor
timestamp: February 25, 2018, 02:31:59 pm
content: QuoteThanks, the 13V supply is non isolated from the mains.It comes from a High voltage, non isolated buck converter called LNK304. (from power.com)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2018, 02:37:26 pm
content: Basic insulation means the secondary side must be grounded.Tim

author: ocset
author_rank: Super Contributor
timestamp: February 25, 2018, 04:20:31 pm
content: Thanks, in fact, ive dug out a DALI spec which saysQuoteBut we dont have IEC61347-1 and it costs £120.

author: max_torque
author_rank: Super Contributor
timestamp: February 25, 2018, 05:36:39 pm
content: Quote from: treez on February 25, 2018, 02:31:59 pmUnder what failure modes can that 13v output exceed 50vdc? As it isn't "Normally" at HV, then you don't need downstream isolation (unless you have other reasons to do so, for example, for galvanic isolation to AC supply E / N etc)A large clamping diode on the output of the 13v supply (at say 22v), and a fuseable link that blows, would result in the system failing "safe" in the case of a single failure. In that case, the HV applied to the DC:DC is only "short term" rather than continuous, which changes the safety case.How many "cascade" failures are you going to design to prevent?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2018, 12:24:11 am
content: Quote from: max_torque on February 25, 2018, 05:36:39 pmNot mains isolated --> mains referenced --> probably has 240VAC, -160VDC on it, plus infrequent 2.5kV+ common mode surges.Basic insulation is required if the secondary side is grounded, otherwise Reinforced insulation is required.Tim

author: ocset
author_rank: Super Contributor
timestamp: February 26, 2018, 06:09:14 am
content: Thanks, do you think that the 4.2kVDC of insulation in the NXJ1S1215 DCDC module conforms to "reinforced insulation"?NXJ1S1215http://www.farnell.com/datasheets/2282933.pdf?_ga=2.13815998.845751712.1519485949-287702491.1506211291

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 26, 2018, 07:33:39 am
content: Quote from: treez on February 26, 2018, 06:09:14 amDude, you alreadythankedthe post that has your answer:Quote from: daveshah on February 25, 2018, 01:24:28 pmAlso, 290V, nice catch daveshahTim