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Old 25th July 2009, 01:45 PM   #491
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Quote:
Originally posted by scott wurcer
It is an important distinction when talking about "time constants" that heat obeys the diffusion equations and can never be modeled exactly in SPICE by R's and C's.

BTW there is a beautiful paper by (I think) Alberto Billoti (sp?) that derives the behavior of temperature vs. arbitrary device geometry from first principles, rectangular coordinates only IIRC. Such maths would make a Russian jealous .
Very good point, Scott. Using lumped time constants and RC representations is just a starting point, but it does lend valuable insight.

We use FlowTherm a lot at work for these kinds of things. The thing cranks for quite a long time when modeling the temperature of all of the components on a board in a shelf.

Cheers,
Bob
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Old 25th July 2009, 01:54 PM   #492
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Bob,
I'll try again.
Using your 20W as a medium term output from a 200W amplifier, we can expect many seconds even minutes operation at the 2W (-20dB) level.
This output will fall to tens of milliseconds at the 20W (-10dB) output level and for a 200W amplifier that has a 2pair output stage is just 5W per device. I can't tell you the device dissipation since that will depend on the load reactance, but I could estimate it will be of the same order as the output power. The devices will dissipate on alternate halves of the overall cycle and thus have cool down periods between half transient.

The short term transients should last just a millisecond or so and hopefully they will be short of clipping.

It's on this basis that I suggested that PMA's posting show a power dissipation that is likely to indicate temperature variations that are worse than what happens in an amplifier reproducing music.

I am not specifically referring to change in bias conditions nor how the bias compensation should work. Just variations in output device junction temperature.

I do however support PMA's contention that the higher the bias, whether optimum or higher, helps swamp out deltaTj with variations in output power that follow normal music waveforms.
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Old 25th July 2009, 02:07 PM   #493
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Quote:
Originally posted by KSTR
[snip]@Jan: I think there is no way to get constant power with real world complex loads and EF's/SF's unless one uses cascodes which generate an (approximated) optimum load line for the instanteous current, regardless of voltage (as they aren't corelated).

Other way would be not to use plain EF's/SF's, then...

- Klaus
Klaus,

I was wondering for resistive loads, because intuitively you can image that there is some condition where a doubling of output current leads to exactly halving of Vce which would mean no change in dissipation. Worth looking at I thought.

The math however seems to indicate that there isn't such a condition except for one specific Vout (which depends on Ibias, Vcc and Rl), because the Vout is in both Ic (Vout/RL) as well as in Vce (VCC-Vout) which leads to a quadratic term in Pc which cannot be cancelled in the general case.

Ohh well.

jd
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Old 25th July 2009, 03:57 PM   #494
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Quote:
Originally posted by scott wurcer
It is an important distinction when talking about "time constants" that heat obeys the diffusion equations and can never be modeled exactly in SPICE by R's and C's.

BTW there is a beautiful paper by (I think) Alberto Billoti (sp?) that derives the behavior of temperature vs. arbitrary device geometry from first principles, rectangular coordinates only IIRC. Such maths would make a Russian jealous .
We Russians when learned how to deal with dissipated heat studied as well works of Grashof Prandtl. Were they Italians as well?
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Old 25th July 2009, 05:18 PM   #495
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Thanks, PMA.
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Old 25th July 2009, 05:19 PM   #496
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Quote:
Originally posted by janneman
I was wondering for resistive loads, because intuitively you can image that there is some condition where a doubling of output current leads to exactly halving of Vce which would mean no change in dissipation.
You can approximate that with cascoding, an example being the ZV9
with slightly different resistor values. It has the advantage of being
current dependent, so that the value and phase of the load are
not of primary importance.

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Old 25th July 2009, 08:16 PM   #497
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Yes, I learned this trick from you, Nelson
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Old 25th July 2009, 10:44 PM   #498
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Now, we are cooking.
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Old 25th July 2009, 11:13 PM   #499
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Quote:
Originally posted by AndrewT
Bob,
I'll try again.
Using your 20W as a medium term output from a 200W amplifier, we can expect many seconds even minutes operation at the 2W (-20dB) level.
This output will fall to tens of milliseconds at the 20W (-10dB) output level and for a 200W amplifier that has a 2pair output stage is just 5W per device. I can't tell you the device dissipation since that will depend on the load reactance, but I could estimate it will be of the same order as the output power. The devices will dissipate on alternate halves of the overall cycle and thus have cool down periods between half transient.

The short term transients should last just a millisecond or so and hopefully they will be short of clipping.

It's on this basis that I suggested that PMA's posting show a power dissipation that is likely to indicate temperature variations that are worse than what happens in an amplifier reproducing music.

I am not specifically referring to change in bias conditions nor how the bias compensation should work. Just variations in output device junction temperature.

I do however support PMA's contention that the higher the bias, whether optimum or higher, helps swamp out deltaTj with variations in output power that follow normal music waveforms.
Hi Andrew,

I'm not sure we're really disagreeing here. My main point was that we should not underestimate the amount of variation that can occur in junction temperature as a result of program power variations. It is certainly true that the heat sink has a long time constant, but such program-dependent variations in junction temperature will occur even if you bolt the output transistors to the Brooklyn Bridge.

It is certainly true that the extent of such variations depends strongly on the nature of the program material and how loud it is being played. Hopefully, our amplifiers will sound good with all kinds of program material. Consider the 1812 Overture, with extended periods of high program levels followed by very soft levels and vice versa. Consider also other material that might have bass notes that drive an amplifier to 1/3 power for over 100 ms, where output stage dissipation may be near maximum.

A simple 100W/8-ohm amplifier with 45V rails might have output stage dissipation of 10 watts quiescent and nearly 50W at 1/3 power. That's quite a swing. Such an amplifier might typically employ two output pairs.

Junction temperature variations do have consequences, and those include possible intervals of over-bias or under-bias.

Cheers,

Bob
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Old 26th July 2009, 12:59 AM   #500
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Try an 800W amp into 4 ohms.
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