Electronics > Beginners

LM317AHVT and Heatsink

(1/17) > >>

mike_mike:
Dear Forum,

I built the power supply from the attachment (with a few modifications: protection diodes and a capacitor in parallel with the potentiometer) and I mounted on one heatsink the power transistor and on another heatsink the rectifier and the LM317AHVT (I will note LM317 for LM317AHVT from now)
The heatsink with LM317 is having the following dimensions: 165x80x35mm.
I mounted the LM317 using a very thin piece of mica and I used thermal grease and a M3 screw and a mounting clip.
The problem is that I noticed that LM317 is heating up to about 70 C while on the power supply output is connected a overload.
The current through the LM317 is 0.7A and the voltage drop on the LM317 is 33V, while on the output is an overload.
It is a problem if LM317 is heating up to about 70C after a few minutes of overload ?
It is normal to heat up to 70 C or this temperature is too high ?

james_s:
That sounds pretty reasonable, with 33V on the input you are dissipating quite a lot of power. Is the heatsink getting hot too? If the IC is running much hotter than the heatsink I would look at the mounting to make sure it is properly mated. If the heatsink is getting hotter than you like then use a larger heat. There is no getting around a linear regulator dropping a large voltage at significant current getting hot, you can't fight the math.

Zero999:
70oC isn't too hot.

Another thing to note is the safe operating area protection is kicking in and reducing the current limit. When the voltage differential between the input and output exceeds 15V, the output current limit is reduced, to protect the pass transistor from secondary breakdown. With a voltage differential of 60V, the current is typically limited to 300mA.
https://docs-emea.rs-online.com/webdocs/12cf/0900766b812cfcaa.pdf

The solution is to either route more current through the pass transistor, or add another transistor in series with the LM317HV, to reduce the voltage across it.

mike_mike:
Thanks for the answers.
James, the heatsink is getting hot after about 30 mins of overload. The LM317 is properly mated (I think). I used a very thin piece of mica insulator  (about 0.05mm thickness).

The temperature was measured using a thermocouple and was the case temperature, not the junction temperature.
In this case, your answers are still the same ?

james_s:
What is the heatsink temperature?

70C case temperature doesn't sound unreasonable, it would worry me if it was above about 85-90C, but 70 is within the range that is acceptable.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod