A pair of LM2940CT-s getting hot - is this normal?

16V-22V in, 12V out = 4V-10V drop.

(the other two are negligible).

0.18A * 4V-10V = 0.72W-1.8W.

Thermal Resistance 3-Lead TO-220 4 °C/W Junction-to-Case 3-Lead TO-263 4 °C/W

Thermal Resistance 3-Lead TO-220 60 °C/W Junction-to-Ambient 3-Lead TO-263 80 °C/W

For the TO-220 (it's worse for the TO-263):

1.8W * 60°C/W = 108°C junction-to-ambient. 1.8W*4°C/W = 7.2°C junction-to-case, which implies 100.8°C case-to-ambient. IOW, if ambient is 25°C, the case will be at 125.8°C, which probably fits the definition of "hot".

Also, the junction will be at 133°C, which exceeds the maximum operating temperature.

Add a heatsink. One barely larger than the package will provide ~25°C/W, which is enough to stop it cooking itself.

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Thanks. That was very useful and interesting too. I've never paid much attention to thermal requirements until now - I just took the power supply for granted.

Regards,

- Alex

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Thanks. That was very helpful and interesting too. So far I've never paid much attention to the thermal requirements - I just took the power supply for granted.

The main problem was that I had in my head that the LM2940-s work in switching mode, so the power dissipation surprised me.

- Alex

Did you read the part about using low ESR caps on the output ?

They must be close to the output leg and with a good run of copper to reduce board R.

some circuits become a little unstable and may cause a small internal oscillation due to the output not having a low ESR storage.

in any case, something to look at, I know what you're saying about using the 78xx with out heat sinks, I've done that also in small circuits like that and they operate cool how ever, they are not designed the same way as the 2940.

"

Can you get away with a lower supply voltage and use a low drop out regulator for the 12 V?

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Use only Genuine Interocitor Parts" Tom Servo  ;-P

That's an idea, if the things get too hot. For now however, with added heatsink, I have no problem. I was just not sure that the heating was normal.

Thanks,

- Alex

I read that and gave it a try (temporarily attaching a small cap on the other side of the board) but it didn't help. So, I guess this is not the problem.

Thanks,

- Alex