Motherboard inductors

[John Kimble]

Hi all,

Just a short question on something that's been bugging me for a
while...

On motherboards, it is common to see various inductors placed around
the board. What, in general, is the purpose of such components? It was
my understanding that inductance is something that is generally
undesirable in digital systems that are switching at high clock
frequencies (hence the need to a heirarchy of decoupling capacitors).
Are they used for clock generation?

Stephen

 

[Guest]

Hi all,

Just a short question on something that's been bugging me for a
while...

On motherboards, it is common to see various inductors placed around
the board. What, in general, is the purpose of such components? It was
my understanding that inductance is something that is generally
undesirable in digital systems that are switching at high clock
frequencies (hence the need to a heirarchy of decoupling capacitors).
Are they used for clock generation?

Stephen

You're right. Those inductors are slowing your motherboard WAY down.
The manufacturers put them in there for their low-end boards and if you
pay a lot more, they simply take them out and presto, instant speed demon.
That way they make a profit because they don't have to build two versions
with a lot of different parts.

Jim

 

[John Devereux]

Hi all,

Just a short question on something that's been bugging me for a
while...

On motherboards, it is common to see various inductors placed around
the board. What, in general, is the purpose of such components? It was
my understanding that inductance is something that is generally
undesirable in digital systems that are switching at high clock
frequencies (hence the need to a heirarchy of decoupling capacitors).
Are they used for clock generation?

They are part of the switch mode power supply circuitry, used to
convert from one supply voltage (e.g. 5V) to another (e.g. 1V).

 

[Larry Brasfield]

Hi all,

Hi, one.

Just a short question on something that's been bugging me for a
while...

Can't have that.

On motherboards, it is common to see various inductors placed around
the board. What, in general, is the purpose of such components?

In general, to store some energy and give it up later. A
great many more detailed purposes fall within that rubric.

It was
my understanding that inductance is something that is generally
undesirable in digital systems that are switching at high clock
frequencies (hence the need to a heirarchy of decoupling capacitors).

Au contraire. Without inductance, signals would never
get anywhere. Wave propagation would be halted.

Are they used for clock generation?

Yes, indirectly. Take them out and any number of
clocks will stop being generated, as the power forms
they help condition or generate collapse.

 

[Pooh Bear]

Hi all,

Just a short question on something that's been bugging me for a
while...

On motherboards, it is common to see various inductors placed around
the board. What, in general, is the purpose of such components?

Almost invariably as part of voltage step down on board power supplies
needed to generate the low voltages needed for modern CPUs and GPUs.

It was
my understanding that inductance is something that is generally
undesirable in digital systems that are switching at high clock
frequencies (hence the need to a heirarchy of decoupling capacitors).
Are they used for clock generation?

No.


Graham

 

[John Popelish]

Hi all,

Just a short question on something that's been bugging me for a
while...

On motherboards, it is common to see various inductors placed around
the board. What, in general, is the purpose of such components? It was
my understanding that inductance is something that is generally
undesirable in digital systems that are switching at high clock
frequencies (hence the need to a heirarchy of decoupling capacitors).
Are they used for clock generation?

Are you talking about soldered on components, or serpentine traces?

 

[Paul Burke]

inductance is something that is generally
undesirable

Resistance is futile! Capacitance is immoral!

 

[Fred Bloggs]

Hi, one.

Can't have that.


In general, to store some energy and give it up later. A
great many more detailed purposes fall within that rubric.

Au contraire. Without inductance, signals would never
get anywhere. Wave propagation would be halted.




Yes, indirectly. Take them out and any number of
clocks will stop being generated, as the power forms
they help condition or generate collapse.

Unbelievable garbage response...

 

[Winfield Hill]

John Devereux wrote...

They are part of the switch mode power supply circuitry, used to
convert from one supply voltage (e.g. 5V) to another (e.g. 1V).

Just now my 3.3V is running at 3.15V, and my Vcore = 1.52V, for an
AMD Athlon64 3200+ processor. This beast's current draw varies
dramatically as a function of whatever program is executing, a buck
switching regulator located on the PCB must be able to handle this.
This switcher is sourced from the +12V supply. The peaks currents
are 70A, 120A or whatever, so the switcher is broken into multiple
sections in parallel, four is common. There may be another supply
for the ram or other chips. That's why you see so many inductors.

 

[Tim Shoppa]

The peaks currents are 70A, 120A or whatever,
so the switcher is broken into multiple
sections in parallel, four is common.

Out of curisosity, is this breaking-into-sections for form-factor
reasons (bigger inductors would stick up to high and block plug-in
boards or otherwise stick outside of the ATX form factor), for PCB
reasons (for high currents you'd like to have thicker traces but
instead you compromise to thinner copper because most traces are logic
not power), component price reasons (sweet spot for inductors or
semis), or something else?

I'm completely flabbergasted that umpteen-layer ATX motherboards are
availble for $30 with all that stuff already on it.

Tim.

 

[Graham Orme]

Out of curisosity, is this breaking-into-sections for form-factor
reasons (bigger inductors would stick up to high and block plug-in
boards or otherwise stick outside of the ATX form factor), for PCB
reasons (for high currents you'd like to have thicker traces but
instead you compromise to thinner copper because most traces are logic
not power), component price reasons (sweet spot for inductors or
semis), or something else?

There are two reasons for dividing the load among multiple stages. It is
much easier to design a 30 Amp switcher than a 120 Amp one and the
efficiency is likely to be much higher. The main reason is that in a
buck converter current is only drawn for a small proportion of the time.
When converting from 12 V to 3 V the switching device conducts for 25%
of the time. This means you need capacitors with enormous ripple current
ratings. By running four stages in synchronism but each phased 90
degrees after the previous one, the input and output capacitors see
almost constant current.

You're probably right about the thickness of the traces. 120 Amps is a
lot for a pcb.

 

[Mike Monett]

Graham Orme wrote:

[...]

You're probably right about the thickness of the traces. 120 Amps is a
lot for a pcb.

Graham

Especially for writing ascii text messages to send to a newsgroup

Mike Monett

 

[Ted Edwards]

efficiency is likely to be much higher. The main reason is that in a
buck converter current is only drawn for a small proportion of the time.
When converting from 12 V to 3 V the switching device conducts for 25%
of the time.

While I have had a fair bit of experience with sitch mode, I haven't
seen on of these mother boards. Two questions:
Is what you are calling a a "buck converter" what I have always called
a switching regulator? It sounds like it - i.e in your example, switch
on 25% connecting 12V to LC filter?

This means you need capacitors with enormous ripple current

ratings. By running four stages in synchronism but each phased 90
degrees after the previous one, the input and output capacitors see
almost constant current.

I assume, then, that four switches feed four inductors from one input
capacitor and one output capacitor. Neat idea! The caps should see
only minor switching transients.

Ted

 

[Joel Kolstad]

While I have had a fair bit of experience with sitch mode, I haven't seen
on of these mother boards. Two questions:
Is what you are calling a a "buck converter" what I have always called a
switching regulator?

A buck converter is a "switching regulator" that specifically has a lower
output voltage than input voltage. There's also boost converters,
inverting, and buck-boost, which have self-evident meanings.

I assume, then, that four switches feed four inductors from one input
capacitor and one output capacitor. Neat idea! The caps should see only
minor switching transients.

Well, when you're talking about e.g., 100A, even with multi-phase converters
you still have to pay attention to the capacitors' ripple current specs...

 

[Tim Shoppa]

When converting from 12 V to 3 V the switching device
conducts for 25% of the time. This means you need
capacitors with enormous ripple current ratings. By
running four stages in synchronism but each
phased 90 degrees after the previous one, the input and
output capacitors see almost constant current.

Makes perfect sense. My experience with buck regulators is at the
much-lower-current end (a few amps, not hundreds!)Thanks, Graham!

Tim.

 

[Graham Orme]

A buck converter is a "switching regulator" that specifically has a lower
output voltage than input voltage. There's also boost converters,
inverting, and buck-boost, which have self-evident meanings.


Well, when you're talking about e.g., 100A, even with multi-phase converters
you still have to pay attention to the capacitors' ripple current specs...

Yes you do, but the capacitors have a much easier life. With four stages
each running at 30 Amps the current through the first inductor might go
from 25 to 35 Amps during the time it is being energised. Then the
second inductor does the same and so on. The current drawn from the
supply is a sawtooth waveform of 30 Amps d.c. plus or minus 5 Amps. The
input cap' only sees the +/- 5 Amp bit.

A single stage with the same percentage ripple would draw zero current
before the inductor is energised. This would jump to 100 Amps when the
switching device turns on and then rise to 140 Amps before returning
rapidly to zero for the remaining 75% of the cycle. Now THAT is what I
call ripple current.

The output cap' also has a much easier time of things.

 

[Winfield Hill]

Winfield Hill wrote...

ASUS P4C800-E, socket 478, 3.2GHz Intel P4 cpu. I posted a photo
on a.b.s.e., asus_p4c800.jpg Note the three big inductor coils.

A followup zoomed photo is on a.b.s.e., "Re: ASUS P4 mobo photo,
for s.e.d. "Re: Motherboard inductors" - asus_p4c800.jpg"

 

[Winfield Hill]

Winfield Hill wrote...

To add a bit of concrete information to this discussion, here are
some details about the switching supply in my last computer's mobo,
an ASUS P4C800-E, with socket 478 for a 3.2GHz Intel P4 cpu.

[ snip ]

Let's do a simple analysis of the two FETs as they do their job.
First, the '60T03 high-side 12V part, with its 12-milliohm Rds(on),
will drop 300mV at 25A. That may seem a lot, but the 12V switch
output is still 11.7V, so it's not so bad. Second, consider the
'90nh02 on the ground side: its 5.2 milliohms drops only 130mV,
but compared to the 1.52V output it's significant (8.6%). Now we
begin to see why the ASUS designers carefully selected this part.
Oh, one more thing, during the low part of the cycle the FET drain
voltage is negative 130mV, not +130mV. Think about why this is so.
And remember, these are N-channel FETs (just a little subtlety).

The voltage across the inductor is 11.7 - 1.5 = 10.2V during the
short high part of the cycle, and -1.52 -0.13 = -1.65 during the
longer low part, ignoring the resistive drop in the inductor.

The inductors are wound with three paralleled #14 wires, for a
calculated Rdc = 0.8 milliohms (and measured, below 1 milli-ohm).
We know the inductor's ac resistance may be higher due to skin and
proximity effects, so this issue bears more detailed examination.

 

[legg]

Hi all,

Just a short question on something that's been bugging me for a
while...

On motherboards, it is common to see various inductors placed around
the board. What, in general, is the purpose of such components? It was
my understanding that inductance is something that is generally
undesirable in digital systems that are switching at high clock
frequencies (hence the need to a heirarchy of decoupling capacitors).
Are they used for clock generation?

Ignoring the big toroids used in the buck regulators, there are other
inductors on your board - smd beads gathered generally around the I/O
ports to assist in obtaining electromagnetic compatability.

RL

 

[Alan Turner]

Hi Win,

[fascinating switching regulator analysis snipped]

Thanks very much for the description of this power supply Win! I found it
very interesting. Posts like this one make s.e.d really worth the reading
time

Regards,
Alan