Re: Amp Design project - PCBs

[Scott Dorsey]

David Grant > wrote:
>I'm a 4th year E.E student designing/building a class d audio amp for my
>design project course. Our curriculum hasn't included anything on (or even
>mentioned) PCB design, so I'm hoping for a little help here. How important
>is part placement and trace routing in the quality of the finished product?

Pretty important.

>Where could I go to get some beginner info on this sort of thing (my online
>searches bring up nothing but explanations of PCB design rules, etc)?

Your department doesn't have a PCB layout class? Some good discussion of
layout for conventional amplifier design can be found in Doug Self's series
of articles on amp design in Wireless World.

Your issues are a little different since your goal is to keep trash from
leaking into the output and keep the output from coupling into the modulator.
There is an Intel application note on RFI issues which you might want to look
at, which discusses digital layout and design.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Pooh Bear]

David Grant wrote:

> I'm a 4th year E.E student designing/building a class d audio amp for my
> design project course. Our curriculum hasn't included anything on (or even
> mentioned) PCB design, so I'm hoping for a little help here. How important
> is part placement and trace routing in the quality of the finished product?

It's very important to really good designs.

I've been known to reduce the thd of power amplifiers by simply re-routing
traces for example.


> Where could I go to get some beginner info on this sort of thing (my online
> searches bring up nothing but explanations of PCB design rules, etc)?

Various manufacturers have written applications notes on this kind of thing.
You're most likely to find it relating to the likes of switchmode power
supplies ( which actually isn't too different to a class D amplifier ).

A few things to bear in mind. Remember that pcb traces have resistance - so
voltages will be developed across them when current flows in them For that
reason be especially careful with the routing of ground traces. High speed
switching signals should have short traces with minimum 'loop area' - *area*
since there has to be a return current as well as 'send'.


Graham

and check out the design rules listed here

http://www.smps.us/pcb.html

[Pooh Bear]

David Grant wrote:

> I'm a 4th year E.E student designing/building a class d audio amp for my
> design project course. Our curriculum hasn't included anything on (or even
> mentioned) PCB design, so I'm hoping for a little help here. How important
> is part placement and trace routing in the quality of the finished product?

It's very important to really good designs.

I've been known to reduce the thd of power amplifiers by simply re-routing
traces for example.


> Where could I go to get some beginner info on this sort of thing (my online
> searches bring up nothing but explanations of PCB design rules, etc)?

Various manufacturers have written applications notes on this kind of thing.
You're most likely to find it relating to the likes of switchmode power
supplies ( which actually isn't too different to a class D amplifier ).

A few things to bear in mind. Remember that pcb traces have resistance - so
voltages will be developed across them when current flows in them For that
reason be especially careful with the routing of ground traces. High speed
switching signals should have short traces with minimum 'loop area' - *area*
since there has to be a return current as well as 'send'.


Graham

and check out the design rules listed here

http://www.smps.us/pcb.html

[U-CDK_CHARLES\\Charles]

On Fri, 12 Nov 2004 13:33:44 -0500, David Grant
> wrote:
> I'm a 4th year E.E student designing/building a class d audio amp for my
> design project course. Our curriculum hasn't included anything on (or even
> mentioned) PCB design, so I'm hoping for a little help here. How important
> is part placement and trace routing in the quality of the finished product?
> Where could I go to get some beginner info on this sort of thing (my online
> searches bring up nothing but explanations of PCB design rules, etc)?
>

Somewhere between "Critical" and "Essential."

You need to isolate line currents from everything else.

You need to minimize the lengths of high frequency transmission lines.

You need to minimize the distance between bypass caps and the thing
they're bypassing.

The inductance of a high-impedence (iirc) trace is 6nH/in (again iirc,
it's been a while). Does inductance matter at all in a Class D circuit?
(Probably)

There is always a mutual capacitance between adjacant traces.

If your circuit is fairly simple, "Minimize trace length" will get you
far.

Oh . . make sure your mounting holes are isolated from the rest of the
board. Don't ask how I know that.

OTOH, I recall the PCB teams running layout software on high-end
workstations for 30 hours or so once they had the mechanical minimumsm
placed by hand (components that needed to be screwed down, heat sink
mounting points, and other things) for a B-sized VXI card of moderate
complexity.

[U-CDK_CHARLES\\Charles]

On Fri, 12 Nov 2004 13:33:44 -0500, David Grant
> wrote:
> I'm a 4th year E.E student designing/building a class d audio amp for my
> design project course. Our curriculum hasn't included anything on (or even
> mentioned) PCB design, so I'm hoping for a little help here. How important
> is part placement and trace routing in the quality of the finished product?
> Where could I go to get some beginner info on this sort of thing (my online
> searches bring up nothing but explanations of PCB design rules, etc)?
>

Somewhere between "Critical" and "Essential."

You need to isolate line currents from everything else.

You need to minimize the lengths of high frequency transmission lines.

You need to minimize the distance between bypass caps and the thing
they're bypassing.

The inductance of a high-impedence (iirc) trace is 6nH/in (again iirc,
it's been a while). Does inductance matter at all in a Class D circuit?
(Probably)

There is always a mutual capacitance between adjacant traces.

If your circuit is fairly simple, "Minimize trace length" will get you
far.

Oh . . make sure your mounting holes are isolated from the rest of the
board. Don't ask how I know that.

OTOH, I recall the PCB teams running layout software on high-end
workstations for 30 hours or so once they had the mechanical minimumsm
placed by hand (components that needed to be screwed down, heat sink
mounting points, and other things) for a B-sized VXI card of moderate
complexity.

[Ben Bradley]

On Fri, 12 Nov 2004 13:33:44 -0500, "David Grant"
> wrote:

>I'm a 4th year E.E student designing/building a class d audio amp for my
>design project course. Our curriculum hasn't included anything on (or even
>mentioned) PCB design,

It makes me wonder if the curriculum or lab classes covers
oscilloscope operation...

>so I'm hoping for a little help here. How important
>is part placement and trace routing in the quality of the finished product?
>Where could I go to get some beginner info on this sort of thing (my online
>searches bring up nothing but explanations of PCB design rules, etc)?

Yes, the book "High Speed Digital Design," and also the authors'
follow-on book. The books go into PCB design for high-speed logic
families, so it's a little bit overkill for what you're doing, but
it's good stuff to learn anyway.
Perhaps more targeted to what you want would be SMPS data sheets
and app notes. I especially found the ones from Linear Technology to
be useful.
To quote Jim Williams in "Switching Regulators for Poets" AN-25,
page 18, point 4, "Layout is vital." Go directly to
http://www.lintech.com and get that pdf, and read the rest of the
paragraph. Then read the whole appnote, and read some other stuff on
switching regulators. Put the scope ground at the PCB ground that's
electrically "closest to" the component you're looking at (see the
scope connections in "High Speed Digital Design") so you'll see
something that resembles the actual waveform, rather than the ringing
of the ground clip wire. And a few dozen other little tidbits of
knowledge ...
Every trace is an inductor as well as a resistor. You can lower the
inductance and resistance by making a trace wider and shorter, but you
can't make every trace one inch wide and 1/4 inch long, so you have to
look at currents and switching speeds to determine which traces can be
skinny and which ones need to be phat.

>Thanks,
>
>Dave

-----
http://mindspring.com/~benbradley

[Ben Bradley]

On Fri, 12 Nov 2004 13:33:44 -0500, "David Grant"
> wrote:

>I'm a 4th year E.E student designing/building a class d audio amp for my
>design project course. Our curriculum hasn't included anything on (or even
>mentioned) PCB design,

It makes me wonder if the curriculum or lab classes covers
oscilloscope operation...

>so I'm hoping for a little help here. How important
>is part placement and trace routing in the quality of the finished product?
>Where could I go to get some beginner info on this sort of thing (my online
>searches bring up nothing but explanations of PCB design rules, etc)?

Yes, the book "High Speed Digital Design," and also the authors'
follow-on book. The books go into PCB design for high-speed logic
families, so it's a little bit overkill for what you're doing, but
it's good stuff to learn anyway.
Perhaps more targeted to what you want would be SMPS data sheets
and app notes. I especially found the ones from Linear Technology to
be useful.
To quote Jim Williams in "Switching Regulators for Poets" AN-25,
page 18, point 4, "Layout is vital." Go directly to
http://www.lintech.com and get that pdf, and read the rest of the
paragraph. Then read the whole appnote, and read some other stuff on
switching regulators. Put the scope ground at the PCB ground that's
electrically "closest to" the component you're looking at (see the
scope connections in "High Speed Digital Design") so you'll see
something that resembles the actual waveform, rather than the ringing
of the ground clip wire. And a few dozen other little tidbits of
knowledge ...
Every trace is an inductor as well as a resistor. You can lower the
inductance and resistance by making a trace wider and shorter, but you
can't make every trace one inch wide and 1/4 inch long, so you have to
look at currents and switching speeds to determine which traces can be
skinny and which ones need to be phat.

>Thanks,
>
>Dave

-----
http://mindspring.com/~benbradley