Understanding Class-B Tr Dissipation graph
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 4th January 2013, 06:03 AM #1 kroto   diyAudio Member     Join Date: Jun 2009 Location: Bandung Understanding Class-B Tr Dissipation graph Vs= ±70V, Load=4Ω circuit Q: Why the power curve was parabolic? i.e. low and high output voltage has low dissipation. __________________ Last edited by kroto; 4th January 2013 at 07:11 AM. Reason: copyright issue
 4th January 2013, 06:08 AM #2 Andrew Eckhardt   diyAudio Member   Join Date: Nov 2005 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.
 4th January 2013, 06:09 AM #3 Ian Finch   diyAudio Member   Join Date: Apr 2010 Location: Coffs Harbour You know it's copyright material so why post the graph here? __________________ Ian
 4th January 2013, 07:27 AM #4 kroto   diyAudio Member     Join Date: Jun 2009 Location: Bandung @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? __________________
 4th January 2013, 07:38 AM #5 Andrew Eckhardt   diyAudio Member   Join Date: Nov 2005 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 07:43 AM.
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Join Date: May 2007
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.

 4th January 2013, 10:29 AM #7 CBS240   diyAudio Member     Join Date: Aug 2005 Location: K-town 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....... __________________ All the trouble I've ever been in started out as fun......

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