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.
R=4ohm
No Chip=30
No OpAmp=60
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:
PDMax each=
= 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.
Best Answer
The issue with this math is it works in theory, but in practice it may be widely off.
The math only works if all 60 op-amps work exactly in unison. That is a big ask. Any propagation delay in the input signal from the first to last op-amp will create a standing wave of higher current through the devices. Moreover, variances in gain and slew rate from device to device will cause hot spots in your array. These effects will also add distortion into your resultant output signal.
These are the reasons we do not typically double up (or more) devices to increase current drive.