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Old 13th April 2010, 12:16 PM   #11
guyst is offline guyst  Israel
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sorry andrewT, i asked you..(and i meant to write why not way..)
anyway, why divide by 4? safety factor? (i'm playful with words today)
maybe i didn't understand you: did you mean devices rated for total 1200W, not actual dissipation?
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Old 13th April 2010, 10:58 PM   #12
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The thing that bothers me about this board is that the photograph of the board doesn't match the downloaded diagram.

The diagram shows 3 pairs of 2SC5200 and 2SA1943 transistors whereas the photo shows 5 pairs.

The transistors are rated at 15A max and 150W dissipation, so about 75A and 1500W total, collectively.

Given the power factor divisor of 4, then the board should be capable of dissipating 375W.

The problems arise when you want to drive the board into low impedance loads.

If you wanted to run this power into 2R, then your maximum rail voltage would be 38.7v and peak current of 19.35A to top out the power at 375W.

The output transistors would supply that current easily but the limited rail voltage would give a maximum power into 8R of only 93.6W

It's all swings and roundabouts, I'm afraid.

Sandy
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Old 14th April 2010, 07:53 AM   #13
guyst is offline guyst  Israel
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i dont understand why the giant safety factor when power dissipation can be calculated w/o a lot of hustle. all the data is in the dsheet..
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Old 14th April 2010, 12:27 PM   #14
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by guyst View Post
i don't understand
at least you can admit it. Most refuse to believe there are ways to find the answer and plod on regardless.

Download Bensen's spreadsheet or look at the many other SOAR calculators in this Forum to learn why that factor of ~4 becomes necessary, rather than simply saying "it can't be".
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Old 14th April 2010, 05:36 PM   #15
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Quote:
Originally Posted by guyst View Post
i dont understand why the giant safety factor when power dissipation can be calculated w/o a lot of hustle. all the data is in the dsheet..
It is indeed but the question is, "how are YOU interpreting the data"?

If you're assuming that the output transistors can dissipate 150W each and that's an end of it, then you're going to have some interesting smoke effects from your amplifiers.

Have a look here;

http://akizukidenshi.com/download/2SC5200.pdf

and go down to the Safe Operating Area (SOA) plot.

The main things to understand is that highest power is obtained with very short pulses and ONLY if the Collector junction is kept at 25 deg C.

The first has virtually no place in the real world of audio signals and unless you have a refrigerated coolant system, the 25C junction temperature is not feasible with any decent power going through the transistors.

Unfortunately, in the real world, you might very well wish to go down to DC and this brings serious consequences.

The board from Ebay only has a + or - 65v rail, and maximum transistor dissipation happens when the conducting transistor is at half rail voltage, i.e. 32.5v and fron the SOA plot, this will give a MAXIMUM current per device of about 4.5A and thus a maximum dissipation of 150W

Looks good, so far, doesn't it but if your junction temperature goes up to 100C (not outside the realms of possibility), the dissipation drops immediately to 62.5W per device. Obviously, this will get worse, the hotter the junction gets.

In fact, you just have to go up another 18C, to 118C and you're down to 37.5W, i.e. 1/4 dissipation.

The simple fact is that unless you have MASSIVE heat sinking capability, standard bipolar transistors MUST be run very conservatively or they will blow up with monotonous regularity.

Sandy
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Old 14th April 2010, 06:15 PM   #16
AndrewT is offline AndrewT  Scotland
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SOA and temperature de-rating applies equally to BJT and FET output stages, not just BJT.

Bensen's spreadsheet was designed originally for FETs only.
I added the BJTs to make the modified version.
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Old 14th April 2010, 06:59 PM   #17
russo is offline russo  Portugal
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And always remember that a speaker is an reactive load, not a pure resistive load.
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Old 14th April 2010, 07:19 PM   #18
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Quote:
Originally Posted by AndrewT View Post
SOA and temperature de-rating applies equally to BJT and FET output stages, not just BJT.
Quite correct but as the thread concerns the LM49810 on Ebay and THAT board uses BJT's, I disregarded FETs. I was simply attempting to explain the reasoning for the "apparently" high over specification of the output stage.

Quote:
Originally Posted by russo View Post
And always remember that a speaker is an reactive load, not a pure resistive load.
Irrelevant in the spirit of the thread, as amplifiers should always be rated into a purely resistive load to maintain consistency in measurements.

Sandy
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Old 14th April 2010, 07:35 PM   #19
guyst is offline guyst  Israel
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andrewT,
you wrote:
"If the load is reactive then the current can be out of phase with the voltage and now Vds can be high and output current can be medium, but the dissipated power can be very high. This is the condition that really stresses the FET's SOAR"
is this the root of all evil here?
is my mistake looking at average dissipation figures derived from minimum efficiancy?
isnt this figure (~4) dependant on heatsink&contact thermal resistance?

i appologize for asking rather than searching, but this forum holds masive amounts of data. i get lost here more often then not.
thanks
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Old 15th April 2010, 06:44 AM   #20
guyst is offline guyst  Israel
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mnemneth, is it possible you are confusing case temp with junction temp?
please, some credit here...when i said the data is in the dsheet i didnt mean just take max Pd. there is a lot more dissipation data there: thermal resistances and max Tj is what i used, apparently naively so, as i use the steady state numbers derived from 50% eff (dissipate the same as the speakers see).
if i understand you and andrewt correctly, you use max instantaneous Pd?
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