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Ground/Power plane on PCB, need advice — Parallax Forums

Ground/Power plane on PCB, need advice

Timothy D. SwieterTimothy D. Swieter Posts: 1,613
edited 2010-01-27 01:30 in General Discussion
OK Gurus, I need some straight feedback.

Usually when I design a board I have a ground plane (copper pour, etc) on the top and bottom of the PCB. Right now though I am designing a PCB that needs wide power tracks (12V or 24V DC). The PCB is two layers, so I was thinking of making the top layer a power plane and the bottom layer a ground plane. Is this a good idea?

My thoughts on it:

-I need to be sure to label the plane so that if someone trys to experiment with the board they know it is a power plane on top and ground on the bottom.
-The PCB is for internal company use, but in deployed systems. In other words, not packaged in consumer products, but in a limited run of testing.
-I am sure there may be opportunities for the power plane to short to the ground plane, so I need to watch my via clearances and such.

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Timothy D. Swieter, E.I.
www.brilldea.com - Prop Blade, LED Painter, RGB LEDs, 3.0" LCD Composite video display, eProto for SunSPOT
www.tdswieter.com

Comments

  • Lee HarkerLee Harker Posts: 104
    edited 2010-01-26 15:10
    I regularly make one side of a 2 layer PCB a "ground plane". I usually selectively create a copper pour not the whole PCB to route the main power bus on the other side. That way you still leave plenty of space to run signal traces. Sometimes the signal traces will carve up the planes severely so I have to be creative about vias and trace spacing to allow plenty of "flooding".
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-01-26 17:36
    Assuming this is an SMT board, I would put the ground plane on the bottom. You can add power traces to the bottom, too, so long as they can skirt the periphery and not break up the ground plane. That leaves most of the top free for signal traces. If you need to run signal traces on the bottom, keep them short and bunched together to minimize their effect on the ground plane. And when you're done, examine a view that includes the ground plane and it's connections only. What you're looking for are the routes that the ground currents will take back to the negative supply. You want these to be as short and direct as possible. This step may entail rerouting some of the bottom signal traces to improve the overall feng shui, so that the currents (chi) can flow more smoothly.

    -Phil
  • mctriviamctrivia Posts: 3,772
    edited 2010-01-26 18:28
    The only down side to power plane is you can't easily allow return current to jump traces cutting your ground plane.

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  • TubularTubular Posts: 4,718
    edited 2010-01-26 20:42
    Tim,

    Its fine to do whatever you like with the copper pours. Everyone else does. Labeling the planes would be a very thoughtful gesture.

    Since its for commercial application you might like to follow the guidelines Parallax published for the planes around the prop chip (to minimize potential PLL failure)

    cheers
    tubular
  • Timothy D. SwieterTimothy D. Swieter Posts: 1,613
    edited 2010-01-26 23:07
    Thank you for the feedback guys.

    This particular PCB design doesn't include the Propeller. This design is such that a smarter board plugs into this board and that smarter board would have a Propeller on it.

    One of the reason I am looking at a power plane is that there several power points in my design and each of those power points could take 2 or 3 amps of current. I thought the overall routing would go easy by making the top plane thh power and the bottom plane the ground. I can do selective copper pours to make a wide bus to the power points, I have done this before, but I thought it may be easier/faster to put a plane on top.

    The board is mostly SMT. The connectors and plug-in board mounts are TH. The mosfets and ADCs and passives are all SMT, single sided assembly.

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    Timothy D. Swieter, E.I.
    www.brilldea.com - Prop Blade, LED Painter, RGB LEDs, 3.0" LCD Composite video display, eProto for SunSPOT
    www.tdswieter.com
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-01-27 00:19
    Timothy D. Swieter said...
    I thought the overall routing would go easy by making the top plane thh power and the bottom plane the ground.
    It may or may not depending on how crowded the top layer is with components and signal traces. At least when you run a power trace, you know what you're getting. With a copper pour, you've really got to be careful not to deceive yourself that you're getting good current paths, especially considering all the choke points the current might encounter. You've also got more control with power traces over single-point-of-origin matters, although those can also be addressed by the strategic placement of breaks in a power plane.

    -Phil
  • BradCBradC Posts: 2,601
    edited 2010-01-27 01:29
    Phil Pilgrim (PhiPi) said...
    It may or may not depending on how crowded the top layer is with components and signal traces. At least when you run a power trace, you know what you're getting. With a copper pour, you've really got to be careful not to deceive yourself that you're getting good current paths,

    I usually draw the board up with sufficiently thick power traces, and then just drop a copper pour over that net as the last step.

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  • Timothy D. SwieterTimothy D. Swieter Posts: 1,613
    edited 2010-01-27 01:30
    True, true - all very true. My board is looking like less than 10 IC and a handful of passive, but the board is large for mechanical/mounting/connector reasons. I will investigate a power plane with the advice in mind and report back.
    Phil Pilgrim (PhiPi) said...
    Timothy D. Swieter said...
    I thought the overall routing would go easy by making the top plane thh power and the bottom plane the ground.
    It may or may not depending on how crowded the top layer is with components and signal traces. At least when you run a power trace, you know what you're getting. With a copper pour, you've really got to be careful not to deceive yourself that you're getting good current paths, especially considering all the choke points the current might encounter. You've also got more control with power traces over single-point-of-origin matters, although those can also be addressed by the strategic placement of breaks in a power plane.

    -Phil
    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Timothy D. Swieter, E.I.
    www.brilldea.com - Prop Blade, LED Painter, RGB LEDs, 3.0" LCD Composite video display, eProto for SunSPOT
    www.tdswieter.com
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