Some simple calculations will give you a good indication of where you
are on your power planes. Given 1000 Watts and 5 Volts, you are
consuming 200 amps. If you want to keep your power supply regulation
within 5% x 5V = 250 mV, you will have to meet a target impedance of
1.25 mOhm. The first thing to look at is your power plane resistance.
One-half ounce copper planes are .7 mils thick and have about .97 mOhms
per square. Assuming a power and ground plane, your DC distribution
resistance is about 2 mOHms per square. One square is more than you
can tolerate, even at DC. The problem becomes more interesting at high
frequencies. Your board will dissipate about 200amps x .250V = 50
watts. If it is large enough, it will just heat up, not burn. The
biggest challenge will be to regulate the voltage at the several loads
with DC drop being a big issue.
The direction that we are headed is multiple pairs of 1 oz planes in
parallel. The best bet is to keep the DC resistance under 20% of the
target impedance. That way you have a good chance of staying within
the power supply tolerance spec and won't dissipate too much power in
> From: "Pat Zabinski" <email@example.com>
> Date: Mon, 5 Apr 1999 09:28:48 -0500
> To: firstname.lastname@example.org
> Subject: [SI-LIST] : high-power board
> Mime-Version: 1.0
> We're looking into a board that has much higher power requirements
> than we're familiar with, and I'm looking for some advice. Here's
> a summary of the board:
> * roughly 1000 Watts (1 KW) off a single +5 V DC supply
> * full-swing CMOS
> * ~50 ASICs
> * 200 MHz clock and data paths
> * up to 300 simultaneous switching outputs per chip
> * between 12x12 and 24x24 inch multi-layer PCB
> * ASIC packages have yet to be determined/designed
> To some extent, we have a good handle on the standard SI issues, but
> we're looking for input in two areas: decoupling and power.
> With 300 full-swing switching outputs per chip, what's the best
> (or a good) way to decouple the supply? First, at the chip-level.
> Second, at the board-level.
> At this high of power level, what are the primary concerns? How
> do we best address them? Is there a limit on how much current
> a half-ounce copper sheet in FR4 can tolerate? How much current
> can I push through a standard via before it melts? How can
> we effectively remove the heat?
> Any suggestions, ideas, and paper references will be appreciated.
> Pat Zabinski
> Pat Zabinski ph: 507-284-5936
> Mayo Foundation fx: 507-284-9171
> 200 First Street SW email@example.com
> Rochester, MN 55905 www.mayo.edu/sppdg/sppdg_home_page.html
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