[sdiy] IC Temperature

Mattias Rickardsson mr at analogue.org
Mon Aug 3 14:11:50 CEST 2020

Hi Mike,

Den sön 2 aug. 2020 19:53Mike Beauchamp <list at mikebeauchamp.com> skrev:

> On 8/2/20 5:26 AM, Mattias Rickardsson wrote:
> > A heat camera can prove useful for design as well.
> Hi Mattais, I borrowed the FLIR for this exact reason - to check a
> prototype PCB design.
> The first thing I noticed was that all of my IC's get very hot. TL074's
> are all going up to around 50C in 20 minutes, NE5532 a bit hotter and
> the LM13700's stay pretty chilly. After running the board for about 20
> minutes, the whole thing is warm to the touch - something I'm not really
> used to in previous designs. But this is my first surface-mount design
> and it is probably twice as dense as previous stuff. I'm using a +/-15V
> power supply.

Around 50 degrees is what I'm experiencing as well on TL074s on densely
mounted double-sided SMT designs at +/-12 V supplies. At first quite
unexpected since we're used to old hole-mount synth circuits not going very
warm at all, but when measuring the current draw and summing up the
expected supply currents of all circuits it seems to be correct. I guess
the density is the main reason here, both having the heat sources much
closer to each other AND having much smaller area & less material for
dissipating the heat.

Btw, TL064s are hardly visible in the heat camera - they pull so little
current. Applying TL064 op-amps where noise & slewrate aren't critical
factors is an important trick in designs where power & heat & cost are.

> I checked the datasheet for the TL074 I'm using and it's "Operating
> Temperature" is 70C Maximum. I can't help being worried that my measured
> 50C is pretty close to this, although I'm 95% certain I'm not doing
> anything electronically "wrong" with all of these chips to put them in
> danger.
> Maybe "Operating Temperature" refers to the ambient temperature that the
> chips can operate in, not the temperature they reach?

It would be very hard to actually measure the temperature on the hottest
spot on (or even in) an IC, so from a design perspective I just assume that
"Operating Temperature" refers to the surrounding air and PCB - and then
the internal IC temperature lands on some reasonably well-known temperature
above this, since the delta temperature is given by the expected power draw
of the IC. Please do chime in, if someone can correct or clarify this! :-)

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