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Old 3rd February 2004, 01:02 PM   #11
Jennice is offline Jennice  Denmark
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As much as I respect Rod for his designs, I can't really see where he wants to lead with that article.

On the bottom line, the limit is set by the allowed junction temperature (inside the transistor).

He makes his calculations, and determines that there's only room for 23W of power "left". Well, the max rms power is already running, so where's he heading?

In my oppinion the only thing shown, is that a 1K/W heat sink for a powerful stereo amp is not enough.
Then again, the typical home amplifier is often driven to deliver less than 1/10 of full power to have headroom for dynamic transients (and btw., 7W sound power is quite a bit of sound, unless youy have very inefficient speakers!)
This would leave his heat sink sufficient, but in any case I would have one heat sink for each amp channel.
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Old 3rd February 2004, 07:28 PM   #12
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It seems Rod has done a good job calculating the instantaneous power dissipation of the semi's..


A semi is derated with this rule: at a particular case temperature, what is the power that will cause the die surface to rise to a specified number.

Some semi's use 100, 125, 150, or 175 C.

At a heatsink temp, an instantaneous dissipation can cause the die temp to climb over that prescribed limit, where failure of the device can occur.

The failure can be as easy as current crowding..or as complex as lateral early effect gradients, causing a hot spot to latch permanently on, destroying the device.

A speaker can easily force this, if it's reactive "kickback" causes the I * V of the devices to cross the derating line.

Embedded within the derating curve are also the bonding wire limits and the secondary breakdown limits...in addition to the simple thermal limit.

Exceeding the derate is an invitation to disaster..manybe now, maybe later...but it will happen.

Many semi's are tested bvceo sus at high current, say 200 mA and 1 or two hundred volts..That checks the die attach integrity as well as the die's ability to handle the dissipation without lateral instabilities.

Cheers, John
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Old 4th February 2004, 02:16 AM   #13
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Quote:
Would improved heatsinking result in greater power output and more powerful sound?
Every amplifier has a weak link which sets its limits. It's usually one of these:

1. Rail voltage / PS
2. Thermal limit / Overheating
3. Device dissipation rating

If your amp is hungry for more voltage and clips, a larger heatsink won't do a thing for you. If your amp can dissipate enough heat to get close to its maximum Pd rating, chances are it already has a good heatsink. For example, I use TIP series devices in my amplifiers, with a small heatsink, I may be able to play upto 50-75w before it gets too hot to touch. Now, if I take the same devices and mount them on a huge heatsink, I can play over 200w while the heatsink barely warms up. If cooling is the weak link in your system and your amplifier overheats, a larger heatsink will help keep your output devices within safe temps during higher power amplification. So in a way, yes a large heatsink will help you "unlock" more power from your amp.
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Old 4th February 2004, 04:44 AM   #14
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I feel the point has been missed in these responses.

Quote:
Can a bigger heat sink result in greater power output
I think the qualifier here is time; a 150-watt rated device with no heat sink will output 150 watts but will not last for more then a few ms's.

If you were able to keep your output devices at room temp no matter what conditions your amplifier was subjected to, then your output device's heat derating curve wouldn't matter which is always considered in commercial amps.

Can someone explain why this wouldn't be true?

So an infinite heat sink would only suffer from transistor to sink junction losses but aside from that, the normal heat generated in your typical output devices driven near max output would be able to maintain the same gain curve as they would at room temp.

But this is an absolute question and absolute's are impossible, so the answer on a less then absolute margin would be yes, you would be able to maintain a larger output figure with a larger heat sink that is less then absolute yet larger then commercially accepted. .
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Old 4th February 2004, 07:21 AM   #15
Jennice is offline Jennice  Denmark
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To John/jnewton,

I suppose EasyAmp is right... we were getting a little off-topic here.

However, as my last response to Rod's article, my claim is that he cannot extrapolate the results and calculation for his "head-room" (margin) to see how near he is to the edge of opertation limits.

True, short power peaks in the semiconductor will cause die temperture to rise, but not with the same calculation formula as the average power calculation. For millisecond transients, the heat sink temperature will remain unchanged, and transistor case temperature will not rise a lot either (depending on the thermal resiatance ratio betreen Rth (die-case) and Rth(case-sink), and keeping in mind the thermal capacity of the case of a power transistor.
Therefore the main temperature rise for power transients will be limited by Rth(die-case), which is not the rule used for Rod's margin calculation.

Just my 2 cents...
Jennice
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Old 4th February 2004, 08:14 AM   #16
dumdum is offline dumdum  Wales
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Thank you everyone!

You have given me some good stuff to think about...
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Old 4th February 2004, 03:02 PM   #17
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Quote:
Originally posted by Jennice
To John/jnewton,

I suppose EasyAmp is right... we were getting a little off-topic here.

However, as my last response to Rod's article, my claim is that he cannot extrapolate the results and calculation for his "head-room" (margin) to see how near he is to the edge of opertation limits.

True, short power peaks in the semiconductor will cause die temperture to rise, but not with the same calculation formula as the average power calculation. For millisecond transients, the heat sink temperature will remain unchanged, and transistor case temperature will not rise a lot either (depending on the thermal resiatance ratio betreen Rth (die-case) and Rth(case-sink), and keeping in mind the thermal capacity of the case of a power transistor.
Therefore the main temperature rise for power transients will be limited by Rth(die-case), which is not the rule used for Rod's margin calculation.

Just my 2 cents...
Jennice
You are horribly wrong...it is jneutron...not jnewton...

I am a cartoon character, not a fig..

You are quite close to correct...his simulation indeed covers only the sink to junction number. Without going into a really huge ansys simu, that will be the best that can be done..

I have seen no simulation packages with the strength to simulate the transient thermal response for a die in the vertical and lateral planes..

If you look in the IR catalog, you will see plenty of transient graphs. But they help set operational numbers allowing for the heat capacities and vertical resistances. Very good stuff, but hugely difficult to include the information in a reasonable simulation..

So, yes, you're correct..Rod's in not totally complete, but it is as close as most will be able to get....and it will tend to err on the side of caution..

Cheers, John
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