What are Heat Sinks??

[Ramji1]

Hi all,
Im a starter in Electronics as well as starter in this forum. I have learnt somthings about heat sinks and how they limit the heating up of transistors. But im not able to grasp it fully. Can anyone provide me with any explanation or notes on heat sinks preferably with some diagrams and equations? And do they work only for transistors or for other devices as wel?

 

[indulis]

Heat sinks is better lumped into Physics than electronics. Basicaly there are 3 ways to transfer heat, conduction, convection and radiation. The most effective of these is conduction. Metal/s, conducts heat a lot better than air. The magic is in in the amount of surface area exposed to the air (that's why you have all those fin's...

 

[Ramji1]

Yes i get what u mean in theory but i want to know its application in electronics. To be frank i dont know how a heat sink wil actualy look. Is it somthing external that u connect to a transistor or does it come built-in to the transistor? Can u help me with some diagrams or pictures? I myself am searching in the net too. And thanks very much fr ur rply.....

 

[indulis]

A "google search" for heat sinks will show you all the different sizes and shapes.

Heat sinks are rated in watts/degrees... in other words, a heat sink can dissipate a certain amount of watts and only increase in temperature so may degrees.

Say you had a MOSFET with 10 mOhms on resistance. You push 100A thru it so the MOSFET dissipates (I^2*R) 100 watts. Now, say the MOSFET manufacturer says the the thermal resistance is 2 degrees/watt junction to tab (we'll say it a TO-220 case), so that means the the tab will rise 200 degrees (that's above ambient, and this hot is a bad thing). But they also tell you if you have the device on a heat sink rated xx degrees/watt the thermal resistance is now .5 degrees so the temperature now will rise only 50 degrees above ambient (this is better and a good thing). In general you don't want junction temperatures to go above 150

 

[audioguru2]

150 degrees C is pretty darn hot!
My computer's Pentium4 2.93GHz processor has a heatsink with a variable-speed fan on it. Intel's program shows me the internal temperature of the processor is usually only 52 degrees C and the fan speeds up to keep that temp when the processor is thinking very hard.

 

[indulis]

Yeah.......... OK 150 is a bit high. Our work de-rating guide lines call for max junction temp of 130

 

[Ramji1]

Thanks fr ur responses. I understand the general outline of a heat-sink and how it works. Now i am keen to see the real device. I'l just browse through some sites and get back to u guys if i have any more doubts.

 

[audioguru2]

Here is a pic of a power transistor (shiny silver color) bolted to a cast aluminum heatsink (black with many fins for high surface area) with a mica insulator between (white color).
This is from the power supply in our Projects Section.

View attachment 39448

 

[GreekPIC]

Here is a pic of a power transistor (shiny silver color) bolted to a cast aluminum heatsink (black with many fins for high surface area) with a mica insulator between (white color).
This is from the power supply in our Projects Section.

Audioguru, heatsinks are actually extrusions, not casts. I've seen a cast heatsink from an old Russian (or should I say Soviet) appliance and it was UGLY!

Nikolas

 

[ante1]

Hi Zeppelin,

I try to use only compound and no mica if not in conflict with polarity and/or electrical insulation, this gives better thermal conduction. If I need the electrical insulation I use the rubber type insulators which do not need compound (less mess)! ;D

 

[Ramji1]

Hi ante,
            What exactly do u mean by "compound"? I understand that the transistor must be mounted on the heat sink through an insulator(should it necessarily be an insulator??) having high thermal conductivity. And what r the benefits u get from using mica or "compound"?

 

[indulis]

A correction is needed to my explanation above (thanks to Zeppelin who caught it), heat sinks are rated in degrees C per watt, not he other way around.

Heat sink and component surfaces are not perfectly flat or smooth. To maximize surface area contact, you want to fill all those imperfection with some thermally conductive material. It can be thermal grease, sil pads, gap fillers... etc. The point is to lower the thermal resistance as much as possible. In some cases (in the SMT world),  ceramic substrates are used, or even copper clad aluminum instead of the "old" G10, FR4... etc.