Understanding Class-B Tr Dissipation graph - diyAudio
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Old 4th January 2013, 05:03 AM   #1
kroto is offline kroto  Indonesia
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Question Understanding Class-B Tr Dissipation graph

Vs= 70V, Load=4Ω
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circuit
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Q: Why the power curve was parabolic?
i.e. low and high output voltage has low dissipation.
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Last edited by kroto; 4th January 2013 at 06:11 AM. Reason: copyright issue
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Old 4th January 2013, 05:08 AM   #2
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P=Volts X Current

all current no volts, no power

all volts no current, no power

When the transistor is supplying lots of current with lots of volts still across it, dissipation is high.
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Old 4th January 2013, 05:09 AM   #3
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You know it's copyright material so why post the graph here?
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Old 4th January 2013, 06:27 AM   #4
kroto is offline kroto  Indonesia
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@Ian : I've change the image now. thanks.

@Andrew :
the LTspice formula to express Transistor Q1 dissipatioan is as follow:
V(Vcc,out)*Ic(Q1) + V(Vin,out)*Ib(Q1)

what is V(Vcc,out)? is that mean V(Vsupply-Vout)?
if I want to calculate power dissipation at output=10V,
then it would be :
V(70-10)*2.5A = 150W + V(Vin,out)*Ib(Q1) [which is low enough]

that should be correct, right?
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Old 4th January 2013, 06:38 AM   #5
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That's right. That's also why an amp with 70v rails designed for a 4 ohm load will have many parallel output devices.

Last edited by Andrew Eckhardt; 4th January 2013 at 06:43 AM.
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Old 4th January 2013, 09:25 AM   #6
DF96 is offline DF96  England
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Quote:
Originally Posted by kroto
if I want to calculate power dissipation at output=10V,
then it would be :
V(70-10)*2.5A = 150W + V(Vin,out)*Ib(Q1) [which is low enough]
This appears to be a DC calculation. Fine for a servo mechanism, but for an audio amplifier you need to do an AC calculation and average over a complete cycle. To do this properly involves some simple calculus.
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Old 4th January 2013, 09:29 AM   #7
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Now throw in driving a reactive load like a speaker where the voltage is out of phase with the current. There will be situations where most of the voltage is across the transistor and most of the current flows at the same time. Once the instantaneous die temperature gets to critical it's sayonara! And for BJT's there is secondary breakdown.......
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