Can I place 220V/10A max Live on top and bottom of PCB and let them cross over on 2 different layers. Is it safe? Is there a minimum distance of some rule that we need to follow. Attached image with PCB image. Please advice.

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Typical FR-4 PCB will have a Dielectric Strength (or Dielectric Breakdown voltage) in the order of 10s of kilovolts per millimeter. Ask your PCB manufacturer to provide the datasheet of the material they use, so you can check the value yourself.

Therefore, running Line traces on the top layer and Neutral traces on the bottom is fine.

However, you need to be careful when the Line voltage appears on the bottom layer, such as the areas circled in red. In such cases, you need to ensure that the proper clearance and creepage distances are maintained.

The safe distances will depend greatly on the environment your PCB will be used in, but the typical rule of thumb is to leave a 2mm gap between mains and any low-voltage trace. The strictest requirement for voltages with peaks up to 420V is 6mm.

You can refer to UL 60950-1 Section 2.10.3 for full details.

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Why would the manufacturer leave such a narrow space between pins?

There are several considerations:

  • TO-220 is probably cheaper than a larger package, such as TO-247.
  • The manufacturer thinks that TO-220 is sufficient for the power dissipation of the device.
  • Not every customer is going to use the device at high voltage.

By making these tradeoffs, they can cater for a larger market, than would otherwise be possible.

And for those customers who do use high voltage, there are several ways to go about using such a device:

  • Interleave the pads and form the legs of the device manually.
  • Use conformal coating or potting.

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See this app note from infineon for some ideas: https://www.infineon.com/dgdl/Infineon-ApplicationNote_MOSFET_CoolMOS_Electrical_safety_and_Isolation-AN-v01_00-EN.pdf?fileId=db3a30433d1d0bbe013d20e0cbf017fe

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    \$\begingroup\$ +1. That's why transistors for high-voltage applications usually come in wider packages (like TO-3P) with wider spacing between legs (5mm), and/or with longer leads that allow to form them in such a way that increases the distance between center pin and side pins. \$\endgroup\$ – Ale..chenski Nov 24 '17 at 3:14
  • \$\begingroup\$ @Armandas Just wondering, the Triac I am using here is a BT136 wich is rated for 600V/4A. Why would the manufacturer leave such a narrow space between pins. I guess they all comply to some standards. \$\endgroup\$ – Suresh M Nov 24 '17 at 22:29
  • \$\begingroup\$ @SureshM I have updated the answer. \$\endgroup\$ – Armandas Nov 26 '17 at 15:04

in the U.S. for most mains-powered or battery-powered information technology equipment, the minimum allowed PCB spacing should be determined from UL 60950-1 2nd Edition Tables 2K, 2L, 2M or 2N. For voltages under 300ac a good rule of thumb is 4mm of clearance. You should be able to run on top and bottom of the pcb with no problem however you must maintain at least 4mm clearance between the high voltage and any other trace or pad on the same side. More details can be found by googling the above document numbers. Please be careful and double check everything. People's lives including your own depend on it


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