Question is about maximum chip heat dissipation (and not about efficiency or correctness of this design).
Circuit is quite simple: put in parallel 30 Dual Opamps in 8 pin DIL package (i.e 60 opamps in parallel in unity gain configuration) to pilot 4ohm load. For each opamp there is a 5 ohm resistor (Rout) at their output to avoid they fight each other.
Vin is a sine wave of 1KHz (about 12 VACmax) In theory 12VAC on 4ohm would be 36W @ 3A This split by 60 opamp, means roughly 600mW @ 50mA load each opamp.
RShared= 4ohm * 60 opamp = 240ohm
Rout single = 5ohm
RL = RShared + Rout = 240 + 5 = 245ohm
So I suppose each opamp will be current-like loaded by 245 ohm. (Correct?)
Then I need to calculate package dissipation for a DIL 8 pin plastic package:
Tj Max = 150°C
OJA = 130°C/W (8-LEAD PDIP PACKAGE) from datasheet
Current max (from datasheet) for each opamp is
IS Max = 8.7mA
Vs+ = 15V
Vs- = -15V
PSupply Max each opamp= (15 + 15 ) *8.7 = 261mW
Datasheet says (pg 10): "worst case power dissipation occurs at the maximum supply current and when the output voltage is at 1/2 of either supply voltage (or the maximum swing if less than 1/2 supply voltage)".
For each amplifier PDMax is:
= PSupply Max + (Vs+/2)^2/RL
= 261mW + 230mW = 491mW
For total chip (dual op-amp):
PDMax chip= PDMax each * 2 = 982mW
TJ Chip: Tj= TA + (PDMax * 130 °C/W) = TA + (0,982 * 130) = TA + 128°
TA Max = Tj Max - 128° = 150° - 128°= 22° (<< 70°C !!!)
As my ambient temperature is 25° or even above, any heatsink on the DIP 8 chips would not work, and will my amplifier smoke in any case??
I really need to change the Op-amp or increase the number of op-amp or give up to this project?
Is this correct? Are all above assumptions correct? Many thanks.