# RE: [SI-LIST] : Charge moving from decoupling capacitors

From: Jian Zheng ([email protected])
Date: Fri May 12 2000 - 09:03:11 PDT

Hi, Barry:

I think your questions and observations are very interesting. From what I
know, the electric system is similar to the water system.

(1) When you apply a voltage to some circuit, the charge will move and carry
the power. However, it does not mean the charge from the source will arrive
the destination immediately. A charge will collide to other charges ahead of
it and pass the power to the other charges. It is like the water in the
tower will apply pressue to the water below it and pass the power to the
water in your faucet. Thank about it. The charges from the source of AC
current will never reach your computer. They are going back and forth in a
cycle to pass the power to other charges in order to deliver the power to

(2) I think the problem is that if the distance from the source to the
destination is too long, there will be a damping effect there. It will take
a while for the power carried by the source charges to pass the power to the
charges in your computer. I think that is the reason why we need to shorten
the electric distance between the source and the distination.

(3) Another interesting topic is that: what is carrying the power? Some
people claim it is from the voltage and current (or charges) applied on the
pair of conductors, because power can be represented as P = V * I. However,
some people claim it is from the E and H fields surrounding the conductors,
because they can represent the power as: P = Cross_Section_Surface_Integral
of ( E X H ), where X means cross. Inside a perfect conductor, the E and H
are 0 and there is no power delivered inside a perfect conductor. From what
I can see, they are in the same system. You can not say it is V and I on the
conductors carrying the power or it is E and H in the space surrounding the
conductors carrying the power. It is the whole system. When you have E and I
on the conductors, you will have E and H surrounding the conductors or
vice-versa.

Thank you for your attention.

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Jian-X. Zheng, Ph.D
Zeland Software, Inc., 39676 Mission Blvd., Fremont, CA 94539, U.S.A.
Tel: 510-797-8109, Fax: 510-797-8241, Web: http://www.zeland.com
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> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Barry Ma
> Sent: Thursday, May 11, 2000 3:50 PM
> To: EMC-PSTC; [email protected]
> Subject: [SI-LIST] : Charge moving from decoupling capacitors
>
>
> Hi,
>
> As the speed of digital signals gets faster and faster, people
> begin being concerned with the distance for electric charge to
> move on power and ground planes of multilayer PCB during the
> signal rise time from a decoupling capacitor (cap) to a chip it
> serves. I would like to raise two questions.
>
> (1) The charge is moving in a metalic plane, not inside the
> dielectric between pwr and gnd planes. Please let me know why you
> have to use the propagation velocity in the dielectric, instead
> of that in the metal.
>
> (2) The second question is regarding distance between the cap and
> the chip. Do we really have to limit the distance letting the
> charge have enough time to move from the cap to the chip during
> the rise time interval? I doubt it.
>
> Take the running water system for example. When we open, then
> close the water faucet within one second, does the water we've
> got in basin come from water tower (or water station, or
> reservoir)? No, it is the water that resides in the pipe. As a
> matter of fact, we have a very large pipe - pwr/gnd planes. Well,
> of cause you know, I did not mean we don't need water tower - the
> cap. ......
>
> Regards,
> Barry Ma
> [email protected]
> Morgan Hill, CA 95037
> Tel. 408-778-2000
>
>
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