Isolated converter common-mode voltage (AC) tolerance? - Page 1
Forum section: electronics / projects

author: Dago
author_rank: Frequent Contributor
timestamp: July 08, 2016, 07:09:09 am
content: Does anyone have any experience with the common-mode AC voltage tolerance of small isolated power converter modules?I am thinking of using them to power the the gate drive for a mains powered H-bridge, so for the top switches the converter would be floating and see a common-mode voltage of 330 V as a ~100 kHz square wave in relation to the other isolated side. Most of them are rated for some 1500 VDC isolation voltage but do not specify anything about common-mode voltages. Most do not even specify the coupling capacitance. The common-mode signal will at least push a current through the parasitic capacitance of the converter but other than that I have no idea will they work in an application like this. Does anyone have experience using them like this or a guess will it work or not?An example of a converter type I am talking abouthttp://www.cui.com/product/resource/pds1-m.pdf

author: Siwastaja
author_rank: Super Contributor
timestamp: July 08, 2016, 08:10:48 am
content: None of those typical converters provide enough isolation to meet any safety standards anyway, so they are out of question for mains-voltage bridges.There are some that have safety qualifications listed, but they are expensive as hell. If this is an one-off project, those 50EUR modules might do the job.In practice, you are on your own on designing it ground up. AVAGO has a optocoupler IGBT gate driver chip which integrates flyback controller, only a transformer is needed. And small mains-rated flyback transformers with proper safety should be relatively easy to get; something similar to what exists in every mobile phone charger.

author: Phoenix
author_rank: Frequent Contributor
timestamp: July 08, 2016, 09:00:15 am
content: Quote from: Dago on July 08, 2016, 07:09:09 amMost products designed for this application have a dv/dt rating as well as the isolation rating.- The isolation rating gives the allowable excursion of voltage- The dv/dt rating gives the allowable rate of change of the rising/falling edge of the voltageThat CUI block doesn't list any dv/dt, and is probably not suitable.Quote from: Siwastaja on July 08, 2016, 08:10:48 amThis application (gate driver supply) will be functional isolation only, so it doesn't need safety standard approval.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 08, 2016, 09:16:49 am
content: Yeah, if the LV source is mains referenced too, it doesn't matter.I wouldn't recommend anything that's not reinforced, for two reasons: reliability and capacitance.Tim

author: Dago
author_rank: Frequent Contributor
timestamp: July 08, 2016, 11:42:38 am
content: Quote from: T3sl4co1l on July 08, 2016, 09:16:49 amYep I am only concerned about functional isolation not safety isolation. I am just concerned the large common mode voltage swing will not play nice with the converters if they are not designed for it.My backup plan is to make a simple open loop flyback converter with four secondaries to provide the gate drive voltages but I'd rather avoid it if I can.

author: Phoenix
author_rank: Frequent Contributor
timestamp: July 09, 2016, 01:43:15 am
content: Have you considered if you need a negative gate supply voltage too?