Understanding Class-B Tr Dissipation graph

[kroto]

Vs= ±70V, Load=4Ω

An externally hosted image should be here but it was not working when we last tested it.

circuit

An externally hosted image should be here but it was not working when we last tested it.

Q: Why the power curve was parabolic?
i.e. low and high output voltage has low dissipation.

 

[Andrew Eckhardt]

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.

 

[Ian Finch]

You know it's copyright material so why post the graph here?

 

[kroto]

@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?

 

[Andrew Eckhardt]

That's right. That's also why an amp with 70v rails designed for a 4 ohm load will have many parallel output devices.

 

[DF96]

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.

 

[CBS240]

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.......