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UL requirements for 600Volts

Discussion in 'Electronic Design' started by Jamie Morken, Feb 14, 2008.

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  1. Jamie Morken

    Jamie Morken Guest

    Hi,

    I am trying to pick out gate drive transformers for a 600Volt SMPS,
    and am wondering what the required clearances are between PCB pads
    for the transformer primary to secondary for this to meet UL
    requirements for 600Volts operation. I remember reading 0.1" for
    500Volts was enough, but I've also read 8mm (0.31") somewhere.

    Anyone have experience on this?

    cheers,
    Jamie
     
  2. Paul Mathews

    Paul Mathews Guest

    The requirements are not based on the nominal voltage difference in
    potentials across the insulation barrier. Instead, they are based on
    peak voltage measurements, which can be very much influenced by how
    much ringing exists in your circuitry. This is because the peak
    voltage determines the distance that corona discharge can form
    'tracks' through contamination on the circuit surfaces. In my
    experience, different certification labs interpret the standards
    (e.g., UL908 and the newer 'harmonized' standards) differently. In any
    case, the relevant standard generally includes a graph, equation, or
    table that you can use to determined required creepage and clearance
    dimensions. In general, isolating transformers, whether linear or
    switchmode, must meet 'reinforced' or 'double' insulation standards.
    Very likely, this would put you somewhere upwards of 3.1 mm.
    Paul Mathews
     
  3. Jamie Morken

    Jamie Morken Guest

    Thanks,

    Does this 3.1mm spec apply to spacing between pins/pads of the actual
    transformers themselves?

    I am looking at a couple possible gate drive transformers:

    this one would seem to meet all the UL requirements but is huge
    and expensive:
    http://www.coilcraft.com/pdfs/pwrbase.pdf



    this one is nice and tiny and has 4mm+ clearance between primary
    and secondary PCB pads. I'm not sure about the transformer winding
    insulation itself though if it can meet the UL specs, but it specifies
    a 2250VDC primary to secondary isolation.

    http://www.coilcraft.com/pdfs/fa2659.pdf


    cheers,
    Jamie
     
  4. Paul Mathews

    Paul Mathews Guest

    What kind of circuit do you have where the gate drive transformers are
    mains isolating? In most topologies, the primary switches and gate
    transformers are entirely on the mains side of the isolation barrier.
    Their construction is therefore not based on safety, per se, but is
    based on good engineering practices. I'm aware that some types of
    synchronous rectifiers and some types of feedback circuits, for
    example, use transformers across the insulation barrier. In
    particular, many types of high rel standards prohibit the use of
    optocouplers, so you end up using transformers. In those cases, the
    same standards apply to all transformers that bridge the isolation
    barrier. To make them smaller, you end up using insulating barriers to
    increase creepage distances, encapsulation, special pinouts, and so
    on.
    Paul Mathews
    Paul Mathews
     
  5. legg

    legg Guest

    How the hell have you ended up with 600V as your supply rail
    ..........or is this still on paper?

    This is a class of service that isn't normally adopted for domestic or
    consumer applications, Again, you have to determine what is a safety
    isolation issue and what is a functional isolation issue. For safety
    isolation, consult the standard - this is not something you want to
    rely on anecdotes to achieve.

    RL
     
  6. Jamie Morken

    Jamie Morken Guest

    600VDC is the maximum voltage going into the SMPS, its from a brushless
    DC generator.

    cheers,
    Jamie
     
  7. legg

    legg Guest

    I think that's still UL1012 (if at one output is not marked as
    class2). Class 2 circuits are UL1310.

    Internal transformer spacings are table 19.2 on page 41 - 6.5mm
    between safety-isolated conductors or conductive structures.

    External and printed wiring spacings are listed in table 28A.1 on page
    50 - 9.5mm cl / 12.7mm cr for safety isolation at 600V or less.
    The latter refers to UL840 - which is a standard that only covers
    creepage and clearance - as guiding functional distances.

    If your output is hazardous (ie greater than 42V), isolation between
    the two hazardous sections could be considered as requiring functional
    isolation.

    The standards I'm referencing are not coordinated. I'm not sure if
    there has been an effort to do this, for industrial devices, since
    Y2K. You should inquire, if your intended use is outside the US.

    RL
     
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