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Non-Isolated Electronic tranformers w/UL approval?

  1. Mar 14, 2012 #1
    I’m looking at a project to make a electronic tranformer for use within a product, and driving its teathered external devices for a control system. I want to make the output AC voltage configurable between 24VAC and 120VAC. This can easily be done with PWM of the AC signal and a feedback path. In its simplest form it is just back-to-back MOSFETs in a AC switch with a little filtering, and the PWM controls.

    I’d like to get UL approval on the device. Electronic transformers typically empoly a small transformer for isolation (smaller than typical transformer due to high freq operation). However in this application where 120VAC is an output option, the transformer would be 1:1… and thus be seemingly of dubious value for isolation.

    It is not clear when UL requires isolation. There are lots of non-isolated AC-DC converters that are UL approved. I also noticed that there are new rules that allow non-isolated PV inverters that are high-voltage DC-AC connected the mains (high stakes consqeunces).

    So the basic questions are:
    1. Under what circumstances is non-isolation acceptable to UL?
    2. What safty measure would be required for this class of control device?
     
  2. jcsd
  3. Mar 14, 2012 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    The main issue is exposing users to voltages above SELV (Safety Extra Low Voltage):

    http://en.wikipedia.org/wiki/SELV#Separated_or_safety_extra-low_voltage_.28SELV.29

    The voltage level varies with country, but is generally around 60V. I'm not aware of any devices like you mention here:

    "There are lots of non-isolated AC-DC converters that are UL approved."

    Can you give specific examples?

    You will generally need to have isolation from the AC Mains (which is above SELV) in order to get UL approval. Some systems have exceptions and there are some grandfathered systems, but those exceptions would not seem to apply to your application as described. You will need to do some very good insulating in order to have your secondary 120V running around...
     
  4. Mar 14, 2012 #3
    You are apparently too young to remember transformerless ac-dc AM radios. The filaments of the 5 vacuum tubes were in series and connected directly to the 120 volts. The 120 volts was rectified directly by the 35Z5. The power plug (I think) was unpolarized. There was an output transformer, however. See

    http://books.google.com/books?id=Lb...QDQ&sqi=2&ved=0CDsQ6AEwAw#v=onepage&q&f=false

    Bob S
     
  5. Mar 14, 2012 #4
    Yes,

    Its very common to have non-isolated AC-DC converter ICs such as this circuit from TI:
    http://www.ti.com/lit/an/slyt391/slyt391.pdf

    Also there are very simple direct regulation schemes such as this:
    http://ww1.microchip.com/downloads/en/appnotes/00954A.pdf

    Also there non-isolated PV inverters that have 600 VDC on one side and 230VAC on the other side:
    http://www.tsrec.net/sitebuildercontent/sitebuilderfiles/faqstranformerlessinverters.pdf [Broken]

    However I do not know when non-isolation is acceptable and when it is not, and if there are different application classes, and what safety mean are required.
     
    Last edited by a moderator: May 5, 2017
  6. Mar 14, 2012 #5

    berkeman

    User Avatar

    Staff: Mentor

    And SELV applies to your links how?
     
    Last edited by a moderator: May 5, 2017
  7. Mar 15, 2012 #6
    And SELV applies to your links how?

    I don't think I ever refered to SELV.

    THe question ask only those above: i.e. when is non-isolation is acceptable and when it is not, and if there are different application classes, and what safety mean are required.

    For example when is it important to have a isolate AC-DC converter and when is it Ok to be non-isolated.
     
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