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purpose of isolation

gannamraju kumar
Intellectual 900 points

Hi,I have a doubt.I am working on a battery charger.My circuit is ,a cascaded combination of diode bridge rectifier,Power factor correction and dc/dc converter.Everywhere i see an isolated converter for the dc/dc converter but can i use a relay at both sides of charger.Is there any other purpose of isolation than safety?.Can someone explain me what happens if I use hi speed solid state relay?

Thanks in advance.

  • 0
    TI__Expert 7395 points
    Gannamraju,
    Thanks for your interest in TI here.

    I'm not sure how you are proposing to use a relay as a form of isolation but in general isolated supplies are used for safety from an AC or high voltage input, as a way to easily step down the voltage to have better control and efficiency, and as a way to provide a clean power from a noisy supply voltage.

    If you need any one of those it is recommended to use an isolated supply.

    You mentioned that you seem to have an AC input from the comment on the rectifier so if people have access to your battery charger( not locked away and protected for use by qualified professionals only), it seems it needs to be isolated for safety.

    Regards
    John
  • 0 in reply to John Stevens
    Intellectual 900 points
    Hi,thanks for the reply.If noise is an issue,I have an EMI FILTER in between ac supply and diode bridge rectifier.If efficiency is an issue,i go for ZVS,ZCS techniques.For safety I have Solid state relay or High speed relay which acts in some milli seconds.I hope my battery pack will not create any problem for milli seconds.Correct me If I am wrong.Is there any regulation that mandates the use of isolation.Please provide me any document related to it.Once again thanks a lot, John.
  • 0 in reply to gannamraju kumar
    TI__Expert 7395 points
    There are numerous applications that require isolation. TI though comment on regulatory requirements for your given application end product.
  • 0 in reply to John Stevens
    Intellectual 900 points
    John,can you please provide those regulatory requirements when you are free.Because I read in one of the paper,that there is no regulatory requirement for Isolation.I will attach the paper once i get it.
    Thank you.
  • 0 in reply to gannamraju kumar
    TI__Guru* 85616 points

    Hello Gannamraju,

    In most cases if the human can touch the output voltage isolation is required.

    I don’t have a copy of the safety standard that I can share with you. However, I found this EDN article that describes the safty standards for isolation.  It goes into detail with regards to creapage and clearance.

    isolator_safety-1.pdf

    In offline (85V to 265V RMS) adapters and charger design to meet isolation saftey requirements tripple insulated wire is used primary to secondary.  This generally gives roughly 4.5 kV of isolation if done correctly.  To give enough margin for creepage I generally have an 8 mm barrier between primary and secondory.  If an air gap is used you can cut this distance in half to 4 mm.

    Table 2 of the attached article gives in greater detail what the IEC 60950-1 creepedge and isolation requirements.  I hope this information helps you with your design.

    Regards,

    Mike