Running AC wall power through PCB trace

Discussion in 'General Electronics Chat' started by UWGRAD11, Nov 20, 2010.

  1. UWGRAD11

    Thread Starter New Member

    Nov 20, 2010
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    Hi,

    I've been all over the internet and cannot find an answer to my question. In my design I have 5 transformers, each convert U.S. wall power (120 Vrms) to 12.6 Vrms. I want to connect the wall power (120 Vrms) to the transformers through PCB traces. I will be using a 4 layer board made from FR4. I know FR4 (in general) has a breakdown voltage of 750 V/mil. I was thinking of running the the two AC power lines on layers 2 and 3 and putting a copper pour on layers 1 and 4 which will connect to Earth ground.

    What I know: 115 Vrms = 340 Vpp = 170 Vmax. Since the breakdown is 750 V/mil I should be OK in placing these on layers 2 and 3 with Earth on layers 1 and 4. I've made the PCB traces wide enough to handle 10x the total current that will be used.

    Please let me know if there is anything I am missing or got wrong. I don't want my board to blow up. Prefer people with experience with this problem or design.

    Thank you.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Are these PC mount transformers? If not that's a lot of trouble to go to when things like terminal strips are available.
     
  3. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    typically, you would keep your AC section short and direct, and physically away from the isolated circuits. I wouldn't think that inner layer would be a good approach as failure downstream would likely rupture the board over heated traces.
     
  4. UWGRAD11

    Thread Starter New Member

    Nov 20, 2010
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    Yes, they are all PCB mount (through hole) transformers. Each transformer will draw a max of 50 mA, so 250 mA total on the power trace. I've made the traces wide enough to handle 2.5 A of current so I can avoid all heating problems. The traces are at least 1.5" from any other circuits in the system. I am very concerned about breakdown/arcing etc. But it seems FR4 has a very high break down voltage. Again, I am not sure if I'm missing something.
     
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    But the problem will lie at the via or layer jumping mechanism, because of it's very small contact surface.
    If you consider the implications, surface layer soldering creates a bridge that would be a hundred times that of your innerlayer contact.
     
    Last edited: Nov 21, 2010
  6. UWGRAD11

    Thread Starter New Member

    Nov 20, 2010
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    I've made the vias large enough to handle the current; again, each via can handle 10x the current that will pass through it, on top of the vias being completely filled with solder (large volume). The vias also have a large clearance around them that will be filled with solder mask and silk screen to reduce creepage.
    The only issue I am concerned about is the inter layer voltage breakdown. Please, I have layed out the traces so there will be no current issues. I am only concerned with voltage issues.

    Thank you.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Keep the AC on one edge of the board. e.g. All transformers along the left side.

    Then ALL layers should have an open spot over 1/4" before going to the low voltage side this gap is bridged with the transformers. The ground plane should be low voltage and not connected to the AC hot side.

    In other words, all 4 layers should have a 1/4" gap of etched area at the same spot on the board where high voltage goes to low voltage. You can see this design inside computer power supplies as well as notebook power supply boards.

    You want to have noise isolation in addition to simple safety isolation. creating the break does this. Some designs I've seen go so far as having the transformer and rectifier on one board, and use a multi-connector cable to connect to the low voltage power supply board's input.
     
  8. UWGRAD11

    Thread Starter New Member

    Nov 20, 2010
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    thatoneguy:

    If I understand correctly, I should put a 1/4" gap between any power traces and Earth ground (all layers) on the primary side and any traces/planes on the secondary side?

    In my design, space is very limited, I have the transformers, rectifiers, etc, all on one PCB. I want to add the Earth ground to layer 1 and 4 as to produce noise isolation from the power line traces. All of my transformers are in one row/line and the AC power lines hot and neutral are on layers 2 and 3, respectively.

    Added Info:
    Looking at the IPC-2221 design specs, I can apply up to 300 V (either DC or AC peak) on a buried trace as long as it is at least 0.2 mm (7.9 mils) from a trace in another layer, or at least 0.2 mm layer spacing. Look at page 39 (page 51 of the PDF). This info is added for others that are interested in this topic.

    Thank you.
     
    Last edited: Nov 21, 2010
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    You only want the outer layers as hot/ground in the High Voltage section of the circuit board. After the transformer's low side output, there shouldn't be any high voltage in any layer that can be accidentally connected to/get noise from.

    Essentially, isolate the high and low voltages, both physically and electrically through layout.
     
  10. UWGRAD11

    Thread Starter New Member

    Nov 20, 2010
    5
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    Thank you.

    I am still wondering if I can get away with the following layer sequence with FR4 since FR4 has a break down voltage of 750 V/mill:

    Layer 1: Earth Ground
    Layer 2: AC Hot - 170 V peak (60 Hz wall power USA). 11 mils below layer 1
    Layer 3: AC Neutral (60 Hz wall power). 40 mils below layer 2
    Layer 4: Earth Ground. 11 mils below layer 3

    This will be the last time I ask, I am just curious and I'm sure others are getting frustrated.
     
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