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Mains powered SMPS.....Earthing?

Discussion in 'General Electronics Discussion' started by eem2am, Jan 20, 2013.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    Hello,

    For any given transformer isolated 230VAC mains powered SMPS, how do you know how to manage the Earth grounding configuration?, ……


    …..and how do you know whether or not Earth should be wired to the SMPS at all?……………..
    …and how do you know whether the chassis should be connected to earth ground or not?
    [“chassis” being the chassis of the product in which the smps resides (ie the thing the smps powers)…….and/or the chassis of the smps if its metal]

    And should the output return be………..
    1….directly connected to earth ground?
    2….Capacitively connected to the chassis and earth ground?

    If earth ground is brought into an smps, should it always be connected to the chassis of the product in which the smps resisdes (the product which the smps powers)?

    The powerint.com 20007 Datebook and design guide……………
    http://www.powerint.com/products/product-documents/product-selector-guide

    …….is one of the worlds best references on smps, …….inside this databook, there are many different earth configurations, but its not clear when you should use which one.

    Eg inside the databook…….

    Page 2-130 has an AC split y cap connected to earth at the centre……and the output is connected to earth ground.
    Page 2-50 has only a y cap from primary to secondary, and no earth ground at all.
    Page 2-185 has no earth ground brought to the smps.
    Page 3-18 has input neutral and output common connected.
    Page 3-25 has Earth connected to the centre of the two y caps connecting primary and secondary…..and this point is also chassis connected.
    Page 3-222 has earth connected to output common.
    Page 3-292 has output return capacitively connected to chassis, but there is no earth connection to the chassis


    Which earthing configuration should you use, when should you use it, and why?
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Any mains-powered appliance with exposed metal should either be double-insulated (does not apply to computer-type switching supplies) or should have the metalwork earthed to the mains supply.

    Every computer power supply you will see has its metal case connected to mains earth and this is the only sensible and safe approach with a unit like that.

    The 0V output of switching supplies is normally also connected to mains earth. The exception is small adapters and laptop supplies which are double insulated and do not have a mains earth connection.

    It is always safer to connect the 0V output of a mains-powered power supply (switching or linear) to mains earth if this is available.

    The AC input circuitry on a switching supply includes capacitors for interference suppression. In a power supply that has a mains earth connection, these are normally connected from phase to earth and from neutral to earth.

    It's common to have a low-value, high-voltage capacitor connected across the isolation barrier in switching supplies, to reduce extremely fast transients across the isolation barrier that could disrupt the regulator. (There may be other reasons too.)

    In supplies where the 0V output is earthed to mains earth, the leakage current caused by this capacitor flows back to mains earth; in double-insulated supplies with no mains earth, this is one reason why you will feel a "tingle" or "rubbing sensation" if you stand in bare feet and touch the supply output with a sensitive part of your body such as the back of your middle finger. The leakage current is not enough to be dangerous but it is detectable.

    To summarise. Unless you can guarantee compliance with the isolation requirements for double insulation, the only way to be SAFE is:
    1. Use a mains cable with an earth connection.
    2. Connect the mains earth to all exposed metalwork.
    3. Connect the mains earth to the 0V connection of the DC output.
     
    Last edited: Jan 21, 2013
  3. eem2am

    eem2am

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    Aug 3, 2009
    Thanks, thing is, i thought part of the idea of transformer isolation is that it isolates the secondary from the mains...making it safe. -so i am not sure i understand why the secondary common should be earthed from a safety view point?

    If the secondary common is earthed, and then one touches the secondary v(out), then there exists a potential current path for electrocution through the body, and down to earth via the feet, and finishing the loop off through the circuit's connection to earth.

    If secondary comnmon is not earthed, then one can happily touch the secondary v(out) and suffer no electrocution?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    As Kris mentioned, use double insulation or at least ground the secondary.
    In case of a single fault (e.g. breakdown of the transformer's insulation), you have either a second sinsulation barrrier (when double insulation was used) or a grounded secondary side. A grounded secondary will cause the fault current to flow via ground safely away from you and ideally the fuse will blow thus disconnecting the power supply from mains.

    Sorry, but that's nonsense. Ground is considered safe - you can touch ground without getting an electrical shock. So a grounded circuit is (in principle) safe, too. That is why metal chassis of mains operated devices very often have the three-prong plug with the separate ground connnection for safety.
    Only if the mains circuit within such a device is doubly insulated one can (under certain rules) forego the ground wire.
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    In theory, a transformer provides perfect isolation and there is no need to earth the output side. In practice, there are many leakage paths between the primary (live) side and the secondary (output) side and leakage and sometimes breakdown can occur. One example is the capacitor that is sometimes deliberately put across the isolation barrier. It is critical to the safety of the system, if the secondary side is not earthed. There is also the transformer, the optocoupler (that provides feedback from the output side to the regulator on the input side so that regulation is possible) and the PCB itself, which has creepage and clearance requirements and may have slots cut into it in areas where the barrier is narrower.

    As I mentioned, there is an exception to this rule, and that is double-insulated power supplies. These have (or at least are required by law to have) extra safety precautions to ensure that isolation cannot be compromised, and they have no mains earth connection. It is still safe (and in my opinion, wise) to connect the 0V connection of a double-insulated power supply to mains earth; this connection is sometimes made inadvertently, for example if a laptop's headphone socket is connected to an amplifier which is earthed to mains earth.

    In this discussion I am talking about power supplies with output voltages low enough that they don't present an electrocution hazard. The output voltage of PC and laptop power supplies is under 20V DC. Obviously, power supplies that produce dangerous output voltages are dangerous whether or not the 0V rail of the output is earthed, and proper safety precautions must always be used in either case. In this thread I am only talking about low voltage power supplies used for consumer products.

    You can certainly argue that a power supply with a dangerous output voltage is safer if the secondary side is not earthed, assuming the power supply is not earthed through the device it is powering, because in that case you can touch either the positive or the negative side separately and not get a shock to ground. But when you're dealing with dangerous voltages, you should be taking safety precautions that will prevent you from touching either side!
     
    Last edited: Jan 22, 2013
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    BUT you in your first sentance of your first post said it was a mains powered SMPS
    there is NO isolating transformer as such ... not in the AC mains anyway
    Do a read up on SMPS units and you will find that the 115 or 240VAC ( whatever in your country) is directly rectified and smoothed to a high DC voltage that is then switched at hi freq through a small transformer to produced your low voltage DC lines after further rectification and filtering

    Sooooo ... are you really talking about SMPS supplies or linear supplies ?

    Dave
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Dave, I think he is talking about switching supplies. The isolating transformer he's referring to is the high-frequency transformer. That's what provides the isolation in an SMPS.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    That capacitor is used to reduce conducted HF and is standard in SMPS. Therefore you will find it in most SMPS. ANd yes, it can fail, as can the other components KRIS mentioned.
    Also you should take into consideration the coupling capacitance between primary and secondary winding of the transformer. Specially at the comparatively high frequencies of SMPS the impedance of tis capacitance can be a factor worth regarding.
     
  9. eem2am

    eem2am

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    Aug 3, 2009
    OK thanks, but i wasnt sure why the Y cap that goes from pri to sec of smps transformer is for safety?
    -i though this was entriely for EMC sake only?
     
  10. davenn

    davenn Moderator

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    I pretty much knew that ... but I wanted him to confirm

    because of the way he was speaking/wording it was pretty obvious that he was expecting there to be mains AC isolation. Where in reality the mains voltage goes "far into" the SMPS with lots of "potential" for electrocution unlike a linear supply where the mains goes pretty much directly to the transformer and the transformer does directly provide isolation and substantially reduces the risk of anyone/anything coming into contact with the mains AC volts.

    just wanted to clear the misconception that appeared to exist :)

    Dave
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    The cap across the isolation barrier is not there "for safety". It's there to improve the operation of the circuit, and that might include improving EMC.

    But it is critical to safety, because it connects straight across the isolation barrier, which is a safety feature. If it goes leaky or fails short, the barrier is compromised.

    If the secondary side is earthed, and the capacitor (or any other part of the isolation barrier) fails, the mains fuse will blow.

    If the secondary side is NOT earthed, and this happens, the output will become live.

    Hence my strong recommendation to earth the 0V rail of the secondary side if possible.
     
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