Re: [SI-LIST] : coming up with average power estimates for

S. Weir (weirsp@a.crl.com)
Mon, 02 Aug 1999 15:47:29 -0700

Pat,

If we deal in energy and time, then you can get an accurate answer for any
circumstance: This is exactly the method which is required when designing
systems such as switch-mode power supplies.

Integrate V*I over time at each point of interest to obtain the energy
drawn at said point, and perform the integration over your repeating
interval. RMS is simply a way to obtain that value when the load is
resistive, and only V or I is known. Since you now know the energy for
some period of time at each point, you simply scale to seconds to obtain
power, aka average. This works, because from a state-space averaging
viewpoint, all the variations in voltage and current have been encapsulated
in your integration interval. If you had a case where one pulse occurred
every 100 cycles, then you would need to integrate 100 cycles. Since, your
model is only a couple of cycles long at 50/50, that is all you need to
integrate over.

When evaluating heat loss in the ASIC's, remember to subtract out the power
delivered to the loads. You will still need to account for the load power
in your cabinet cooling budget.

It sounds like your power cycle is very fast compared to any thermal time
constants you are likely to have, and the duty cycle is high as well.
Therefore, we can leave those effects to another time.

I hope this helps.

Regards,

Steve.

2.
At 10:32 AM 8/2/1999 -0700, you wrote:
>RMS is the heating effect, so unless your thermal time constants can follow
>the instantaneous values (doubtful), RMS should work.
>
>50 kW? Get rid of those vacuum tubes and use solid state! (couldn't resist)
>
>Larry Miller
>
>At 10:57 AM 8/2/99 -0500, you wrote:
>>I'm working on a rather large system (over 50,000 Watts!), and we
>>are trying to come up with some detailed power estimates for
>>each component.
>>
>>The system is essentially comprised of LOTS(!) of identical
>>parallel processing ASICs, each ASIC having roughly 600 I/O.
>>The basic I/O is full-swing CMOS with a 2.5 VDD supply running
>>at roughly 100 Mbps. A small change in one ASIC will have
>>a three-orders of magnitude higher impact on the system, so we need
>>to pay very close attention to details. The I/O have been
>>designed and tweaked to account for impedance, edge rates,
>>packaging effects, etc., so we have a high level of confidence
>>they are going to work; at this point, we simply need to
>>obtain a power estimate for them.
>>
>>In order to come up with good system-level power estimates (which
>>will determine cooling requirements and power supply requirements),
>>we need to have an accurate ASIC power estimate. We've got pretty
>>good numbers for the core circuitry, but we're trying to develop
>>an estimate for the custom I/O buffers.
>>
>>To get the power for one buffer, we simulate the buffer with
>>a 1010101... pattern, toggling every possible bit period.
>>The buffer is loaded with an average-length transmission
>>line, and we use spice to plot the power vs time for at
>>least two bit-transitions. Overall, we get a power
>>vs time plot that is relatively flat except during the logic
>>transitions (no surprise here), and the peaks vary in amplitude
>>depending upon a rising or falling edge.
>>
>>In the past, we have used the "simple average" power, meaning
>>taking the integral of the power over two bit periods (to ensure
>>we've captured one falling and one rising edge)and dividing
>>it by the time. We have used this figure as
>>our average power for the worst-case-bit-pattern.
>>
>>However, a colleague recently suggested using the "RMS average"
>>of the power, which is computed slightly differently. For our
>>case, the RMS average resulted in a power estimate that was
>>50% higher than the average value.
>>
>>>From my experience, taking the integral of the power curve will
>>result in the effective energy consumed by the buffer, and dividing
>>this by the time will provide the average power. However,
>>RMS is used so frequently in power estimates, I could not provide
>>a good answer why it shouldn't be used.
>>
>>Can anyone tell me how to best determine the average power
>>for a buffer? Am I anywhere on the right track? Which is better,
>>simple average or RMS average?
>>
>>One other point to note: as we increase the transmission line
>>length, the RMS power goes up as well (as expected). However,
>>this trend continues to a certain point, then the power actually
>>reduces with increased line length. Can someone explain why
>>the RMS power would be reduced with increased length? We're only
>>seeing a small percentage change (~10-20%), but it's got
>>me curious.
>>
>>
>>Thanks,
>>Pat Zabinski
>>
>>
>>
>>
>>--
>> Pat Zabinski ph: 507-284-5936
>> Mayo Foundation fx: 507-284-9171
>> 200 First Street SW zabinski.patrick@mayo.edu
>> Rochester, MN 55905 www.mayo.edu/sppdg/sppdg_home_page.html
>>
>>
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