# Heatsink for 0-30V 20 A powersupply

Discussion in 'The Projects Forum' started by jm-a, Mar 4, 2013.

1. ### jm-a Thread Starter Active Member

Oct 20, 2010
105
2
Hi all,

I only need to be sure about right size of this heat sink:V4511....

Project: High power supply 0-30V DC and up to 20A. See schematic included.

I found some maths :

Choice: T0 220 package

Θjc :jonction > case 1°C/W

ΘjA :jonction > ambiant ( No Heat sink ) 35°C/W

With 4 LM338 and 400 Watts, 100W > LM338

100= ( Tj - Ta )* Rth Tj: junction temperature
Ta: ambiant temperature

100 = ( 125 - 55 )* Rth

Rth = 1,42 °C/W

In fact , I found calculations with Rth jc ( junction to case )

Rth ja ( junction to ambiant )

Rth ca ( case to ambiant )

Rth ch ( case to heatsink )

Where to find Rth ca and Rth ch,in LM338 datasheet?

Also V4511 has data in K/W, Kelvin degree or ? o° K -273°C so???

Is it a good or bad choice for this power supply, and why???

Thanks a lot.

jm

http://www.eleccircuit.com/high-power-supply-regulater-0-30v-20a-by-lm338/

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2. ### #12 Expert

Nov 30, 2010
16,283
6,795
Go to http://www.aavid.com/ and use their calculators.
You seem to have a good start at understanding heat flow. What you need to do now is figure about .2 for the theta ch and what you have left is theta ha. AAvid will help you find heat sinks to match that number.

edit: a Kelvin degree is the same as a centigrade or Celsius degree. Just ignore the -273 number.

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3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
I believe that the Avvid calculator's solution will be the Pacific Ocean.

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4. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
No offense to the OP, but if you think you can use TO-220 devices to construct a 0 - 30V/20A linear regulator....... you really need to start over.

The basic design is not feasible. You would need a heatsink that could safely handle about 700 Watts.

And a fork lift to carry it to the work bench.....

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5. ### takao21203 Distinguished Member

Apr 28, 2012
3,577
463
More than that. Or do you want a 95C cooler just at the margin?

You need forced cooling here anything else is nonsense.

All what I use in terms of linear is 78L05.

A variac + a large toroid are better if OP wants to go analogue.
Or a TRIAC circuit, which will make nasty noises at 20A transformer output. At least.

I have a welding transformer, starts buzzing from the Harmonics at just 1A.

The rectifier- needs a good cooling system as well. Capacitors will turn hot- OP needs these with thick terminals or better bolts.

It is a considerable project even with forced cooling. TO220- OP would need a bank of these. 20 to 30W is the max. you can crank out of these with good cooling.

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6. ### jm-a Thread Starter Active Member

Oct 20, 2010
105
2
Thanks to all,

I choose this project, in order to avoid building Switch Mode Power Supply, due to cooper foil cost ......

I know now that it's impossible with a '' small '' heatsink.

Jm

7. ### jm-a Thread Starter Active Member

Oct 20, 2010
105
2
Hi all,

Same result with TO3 package?

Thanks.

jm

8. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Yes.

The only way to get the currents and range desired is with a switching supply. Even then, there will be thermal considerations.

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9. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
+1

If a linear output is needed (for less ripple or faster transient response), you build a switcher to provide about +3V higher than output and have a linear stage after. With 20A, this is doable but still very high power dissipation. This is not a project for a beginner.

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10. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
I had a heatsink from a quality 5v 25A commercial linear supply from the 1970's. Worst case it dissipated about 100W continuous. The heatsink was close to the size of a football.

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11. ### takao21203 Distinguished Member

Apr 28, 2012
3,577
463
You can do a SMPS only with storage coils.

Normally this is not much done >150W.

A transformer bridge is used for larger SMPS.

If it is a single piece circuit, you can however bank smaller SMPS circuits.

I made a dual-coil SMPS, 80V to 0-80V with a large PNP or MOSFET.

Upto 10A continuous but then the rectifier turns very hot, as well the 4700uF input capacitor, wires, and the 500VA toroid.

The toroid needs forced cooling >200W.

You can for instance bank three of these TL494 circuits. Each is using 2 large storage coils.

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