Question about drawing POWER PLANE in 4-layer PCB.

[Anonymous]

Hi, everyone.

(This is not about a Eagle's function. so maybe you think this is not proper for this forum.)

I have a question about drawing Power Plane.

I use 4 layer PCB, I use one of the inner layers for Power plane.

but this point, I'm wondering that this is common way for designing.

I have two converter and one LDO in my circuit,

so there are several types of VCC.

I draw all of my VCC as a polygon.

Is this too many polygon? I'm wondering that I rather use wide track than polygon.

because of polygon I think I couldn't use this layer for signals.

Please let me the common method of drawing POWER PLANE.

I'm full of components in my both side of pcb. and 3rd layer will be used as GND PLANE.

I'm looking forward to your answer!.

Big thanks.

[rachaelATWH4 · Accepted solution]

Hello,

There isn't a problem with dedicating layers like this and drawing segregated power polygons. Having not seen your circuit I can't comment on the validity of having so many power domains in your design but assuming they are all necessary then it's not an issue.

I would also question whether they are all power rails or just power paths which are part of a drive circuit though, for example your +12V_FAN_OUT rail. This would seem like it's just the power connection to the fan and while using a polygon is fine I would probably consider whether it's a cutout from your power plane or just a surface polygon. If you put it on an inner layer unnecessarily then you've got a bunch of additional vias everywhere to get it all connected up. Again, I don't know what your actual circuit is here.

Also, are they really all totally segregated or is there a main power rail from which all others are derived? If so I would flood the power layer with that, set the rank of its polygon to 6 and then draw your other polygons (for actual power rails) within that using rank 5 or above so they are automatically isolated from the main plane. If there isn't a main rail from which all is derived then if what you have connects everything up correctly with enough copper for the current then thats fine although I'd probably flood the remainder with GND in that situation. Remember though that when signals cross discontinuities in adjacent power planes they have to find a different route for their return currents so make sure you consider this in your routing, for example if you have any critical / high speed signals.

Best Regards,

Rachael

[Anonymous]

Hi, @rachaelATWH4 

I really appreciate about your continuous supporting.

My board have a main power source named "+5V_IN" or "+VBAT " (+VBAT is Battery input)

these are compatible.

and It is sourcing to two converter for making+5V and +12V

and also sourcing to 3.3V LDO for making input power of MCU

and as you said in your answer, it's just segregated by P channel mosfet for Enable signal which is controlled by MCU.

(this circuit, you can see a below picture)

so what your saying is that in my case I don't need to use polygon for +12V_FAN_OUT ?

and just using big wide track for it?  

so.. do I understand your answer correctly or not? Please let me know 🙂

and in fact, I don't know the real difference of power rails and power path.

"I would also question whether they are all power rails or just power paths which are part of a drive circuit though, for example your +12V_FAN_OUT rail. " (I want to use only this for quote, but I when I click quote symbol, it just makes all of your answers to be quote..)

I know my words maybe make a little confusing to you.

I'm not a native, so It's somewhat hard to explain in English what I think. 😓

Best Regards, 

[one-of-the-robs · Accepted solution]

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@Anonymous wrote:

and in fact, I don't know the real difference of power rails and power path.

"I would also question whether they are all power rails or just power paths which are part of a drive circuit though, for example your +12V_FAN_OUT rail. "

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I'm sure @rachaelATWH4 could explain this clearer than me but...

The distinction I believe she is making is that a "power rail" supplies power to a number of components on the board, to their power pins, whereas a "power path" is merely a route that carries quite a lot of current. If your "+12V_FAN_OUT" is a high-side switched output to a cooling fan, allowing the CPU to turn it on and off as required, then this is a "power path" - conceptually the same as any other output signal, just beefier.

I would agree with Rachael that you probably want just a fat track for these, although I have sometimes found it convenient to use a polygon to get best use of available board space and convenient thermals on the pads. However, you don't want to "flood out" such tracks because there's no benefit - they run point-to-point and are constrained at their narrowest point, and you don't want the (minimal) extra capacitance to any ground planes on nearby layers.

Does that make sense?

[rachaelATWH4]

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@one-of-the-robs wrote:

I'm sure @rachaelATWH4 could explain this clearer than me but...

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Nope, that's absolutely fine and pretty much how I would have put it 🙂

[Anonymous]

Big thanks for detailed explanation.

It's really helpful to me.

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and you don't want the (minimal) extra capacitance to any ground planes on nearby layers.

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I agree with you, I don't want unwanted capacitance.