Voltage regulator, how hot is too hot?

[Demon]

LR1084 at 5V:

Doing some testing to see which are the best heatsinks that I have on hand.

These thin black aluminium set up back to back get real hot at 300mA (2 per IC); 115C on VR front, 110 on HS back (bare, only held by screw/nut).
https://www.aliexpress.com/item/32882672963.html

(didn't take pics)


These mega aluminium get to 91C / 35C (bare, only held by screw).
https://www.amazon.ca/gp/product/B07H5GQVTD/?th=1
I would have expected much more "coloration" on the back, it's nearly like they entomb the heat in there.

(didn't take pics, gotta retest and take pics)


So I tried these on the front; 9x9 bare ALUM with installed tape:
https://www.aliexpress.com/item/1005004079109702.html

These on the back:
https://www.amazon.ca/gp/product/B0B68HPC65/

Along with Arctic MX-4 for improvement thermal exchange:
https://www.amazon.ca/gp/product/B07L9BDY3T


Setup looked like this (unit on the left was running at only 150mA or so):




Front temps at 300mA:



The front HS is at 50C, but the perimeter of the IC is at 128C. Is that adhesive tape holding back heat?


Rear temps at 300mA:



Despite a relatively good thermal paste (I use it on CPUs), the center of the HS is only at 48C, but the screw is over 108C.


Is it common practice to use a fan to assist the heatsinks?

The LR1084 is rated at 5A; I'm not even running 1/2 amp at 5V...?

5.3 Recommended Operating conditions

Control section 125C max
Output section 150C max

I prefer not adding a fan if at all possible. I'm not seeing a lot of EXCELLENT rated tiny units.

I somehow expected the heatsinks to do "more", especially that last "tandem" setup (unless that chinesium adhesive tape is garbage).

[Ioannis]

Everything has resistance. Junction (of semiconductor) to Case, Case to Ambient, Heat sink to ambient, thermal paste... All add up to a big resistance of transfering heat from junction to the nearby air.

Adding a thermal paste does the opposite of what you think it does! The proper use of thermal paste is not to make a thick layer between your part and the heatsink but to just fill the tiny holes of the metals surfaces, so you do have more contact area.

The technique is to spread some paste and then with some tool (e.g. a spatula) to remove the rest, leaving only a very little amount in the areas the may not be very smooth (either TO-220 part or the heatsink).

As I have noted on other thread, you need much larger heatsink (around 2-3°K/W) for the losses you have. Or you can add a decent fan (mag lev or with ball bearing) to last longer. In a similar case I opted for a lower cost fan but with 3 wires. The third wire is the internal sensor of the fan and I check it with the PIC. If the fan slows down or stops completely, then an alarm is triggered and the device stops operation to avoid any damage from excessive heat.

You have to either add a bigger heatsink, add a fan or lower the power losses (by lowering the input voltage to the regulator. Or select a SMPS and never worry about heat, heatsink, fans etc. I think it may cost less, even though is a bigger circuit.

Ioannis