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Old 14th April 2013, 07:02 PM   #1
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Default Transistor Dissipation and Turbo Projects

In another thread, the question of cranking up the bias of an amplifier
to the absolute maximum has come up. As this is a tutorial opportunity,
I have started a new thread:

Attached is the portion of the IRF244 datasheet which lists the maximum
ratings. We see that the dissipation figure is 125 watts at 25 deg C, and
you must derate that by 1 watt per degree above that temperature.

The maximum dissipation calculation for this is predicated on the notion
that the maximum junction temperature is 150 deg C. This is pretty
universal for Silicon transistors.

The formula is:

Tj = Watts X (Rj + Rc + Rh) + Ta , where:

Tj = the temperature of the junction (deg C)
Ta = ambient temperature
Rj = thermal resistance of junction to transistor case (deg/watt)
Rc = thermal resistance of transistor case to heat sink
Rh = thermal resistance of heat sink to ambient (usually 25 deg)

Keep in mind that Rh is the amount of heat sink allocated to one transistor,
so if there are 2 transistors on the sink, the Rh figure is doubled.

From the 125 watt rating, we can infer that Rj = 1 deg/watt, and from
experience we know that the Rc value is also about 1 deg/watt.

If we simply know the temperature of the heat sink or the case, the
calculation is more simple:

Tj = Watts X (Rj + Rc ) + Th (Th is temperature of heat sink)

or

Tj = Watts X (Rj) + Tc (Tc is temperature of transistor case)


Let's take the case of a 65 deg heat sink. If the part is dissipating 50 watts,
then

Tj = 50 X (Rj + Rc) + 65

Tj = 50 X (1 + 1) + 65

Tj = 165

This a lot more than the manufacturer thinks is reliable. The rule of thumb
is that the lifespan doubles for every decrease by 10 deg C of the junction.

And contrariwise.

Lets say an Aleph 2 has 12 devices dissipating about 200 watts, or about
16 watts each. The temperature on the heat sink at the devices is about
60 deg C. This means that the Rh (experienced by each transistor) is
(60 - 25) / 16 or about 2 deg/watt

The maximum dissipation per device here can be estimated by

150 = W * (1 + 1 + 2) + 25

125 = W * 4

w = 125 / 4

w = 31 watts.

In our Aleph 2 example

Tj = 16 * (1 + 1 + 2) -25

Tj = 116 deg C.

And this is why Aleph 2's don't break.

Attached Images
File Type: gif IRF244 B.gif (6.3 KB, 320 views)
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Old 14th April 2013, 07:05 PM   #2
Gyuri is offline Gyuri  Hungary
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For me, the absolute maximum, where the smoke is remaining inside the semiconductor.
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Old 14th April 2013, 07:08 PM   #3
Gyuri is offline Gyuri  Hungary
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Do you remember, we are fearless BA builders?
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Old 14th April 2013, 07:36 PM   #4
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yes, but for us a bit more fearfull...... good info.
thanks
i am not fearless yet.
maybe somday
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Old 14th April 2013, 07:49 PM   #5
Gyuri is offline Gyuri  Hungary
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Actually, what's crazier?
You crank up your amp bias a little bit, or
you are traveling with your car, you know it though, the brake is wrong?
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Old 14th April 2013, 07:53 PM   #6
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Default Killer quote!!!!

Quote:
Originally Posted by Gyuri View Post
For me, the absolute maximum, where the smoke is remaining inside the semiconductor.
let's not talk numbers, but actual failures.. Like I said im dumb, well sorta, for some, we just need to push it and find out! Crank it up till it blows up and back it off was great!!
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Old 14th April 2013, 08:13 PM   #7
n3szd is offline n3szd  United States
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Default Burn it?

Quote:
Originally Posted by n3szd View Post
let's not talk numbers, but actual failures.. Like I said im dumb, well sorta, for some, we just need to push it and find out! Crank it up till it blows up and back it off was great!!
maybe im a crazy diyer, the articile was appricated, thank you np. Let's go where no man has gone before. Sometimes the only way to find out is try.. I like the smoke thing, rock ON!
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Old 14th April 2013, 08:15 PM   #8
Gyuri is offline Gyuri  Hungary
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Quote:
Originally Posted by Gyuri View Post
Actually, what's crazier?
You crank up your amp bias a little bit, or
you are traveling with your car, you know it though, the brake is wrong?
No, the latter is not insane, but rather irresponsible.
But this crazy world forced to do so.
I think I have finally will learn to say no.
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Old 14th April 2013, 08:23 PM   #9
n3szd is offline n3szd  United States
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"This a lot more than the manufacturer thinks is reliable. The rule of thumb
is that the lifespan doubles for every decrease by 10 deg C of the junction."

sounds like what a tranny cooler would do for a th 350 gm. If the amp will benifit from more bias why not try, not much to loose... Exept the power supply...
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Old 14th April 2013, 08:56 PM   #10
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In that case, have a fire extinguisher handy.

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