TO3 & HEATSINK

good

the

The actual die is mounted to the base of the package. Your scheme increases the thermal resistance from the junction to the case.

Norm

the

it

It will be worse. You are increasing the thermal resistance between the die and heatsink quite a lot - the die is on the center of the base plate normally only 1 or 2 mm away from the heatsink. Try this.. build it or a test rig as you describe. Power it up and measure the temperature in the middle of the T03 base plate. Then calculate the die junction temperature (Tj) using....

Tj = (WxR)+Tc

where

R= Thermal res of the case = 1.25 C/W for a 2N5038 in a T03 W= power in watts = 50 Tc= Temp of case measured in the center of the base.

The calculated value of Tj must be below 200 C according to the data and

It's impossible to calculate how big a heatsink you will need because the case res R is specified from the die junction to center of the base not the mounting holes.

the

it

ok,

increases

After rereading your post, I see an even worse situation than poor thermal conduction (which can be alleviated with an appropriate H/S and that's the fact that the transistor case isn't at ground potential. With traditional mounting, you can get nice mica (or other material) insulators to isolate the transistor's electric potential from the H/S but not (as far as I know) with your scheme.

Exactly what are you trying to accomplish?

Norm

and

as

than

know)

Its not a good idea then, I was just trying to see if I could avoid routing the connection wires though the finn side of the heatsink. The device I have found is the 2N5038, its for a audio amplifier, its about the best gain and wattage I can find, The only other transistor is the TIP3055 which is no problem to mount, but lower gain and wattage, perhaps someone where knows of a good gain transistor ? 5amps 80V 100W or there about with a good gain, more there better I guess.

Chris

onto

screw

be

thermal

the

traditional

isolate

routing

gain

no

gain,

A little more expensive but good performance in the MJ150xx series of power transformers. The complementary pair MJ15003/15004 are higher voltage than you need but especially good for high power audio amplifiers. They're still TO-3 but there's generally no real problem in routing wires through the heat sink if you protect them against rubbing and overheating by installing a gromet in the hole. The 3055 is an old workhorse, best for low frequency designs like subwoofers.

For TO-220 packages, consider a MOSFETs instead of bipolar output stage -- there are some really good medium power units available in this package. Norm

power

than

still

heat

The MJ21194 power NPN looks nice, I have already tried some mosfets, They seem to sound the same, but when I look at set freqencys on the scope the wave is very much distorted, I've been trying this out for the past 2 days, ive tried everything I can think of, they just dont seem to work as good as transistors do. I'm all for using mosfets, though I think for my next project I'll design a class D amp and use some mosfets for that.

Chris

routing

Oh is that all... What you normally do is mount the transistor on the fin side so the legs stick through into the box.

Found this example (long URL)

id like to do that, though I cant find any "good" TO3 heatsinks, most heatsinks are expensive in general so I was just seeing if there is any other solutions. I can get heatinks where are fair in price, though id have to drill holes and route the wire between the finns, looks like this is the only solution.

Chris

frequency

stage --

days,

as

MOSFETs require different biasing and somewhat different drivers than BJTs -- you can't just substitute one for the other. There are a few "good" books on the subject -- Doug Self and Randy Slone are the latest ones I've read.

Norm

the

package.

They

the

"good"

yeah I spent a while alterting the bias, the audio output sounded ok, though somewhat lower volume, on sine wave test the sine on the falling edge seemed to slow down as it came to the crossover point, I imagine the capacitance of the fet isn't being discharged fast enough. I will look into those books, should make interesting reading. whats the titles of the books ?

chris

I've

though

seemed

of

I happen to have G. Randy Slone's "The Audiophile's Project Sourcebook" sitting on the desk next to me. He's also got "High Power Audio Amplifier Construction Manual" that I've seen. Doug Self's Audio Power Amplifier Design Handbook is somewhat more academic but contains a lot of good stuff. IIRC Amazon's prices aren't bad.

Norm

Whereabouts in UK are you?

I have 4 heatsinks together with professionally-mounted 2N3055 for disposal. You can have them for free if you're close to me, but I can't be arsed to post them!

In your suggestion, you need to be aware that the Collector is normally the case connection. Do you really want your heatsinks to be at something other than ground to external screwdrivers and the like?

good

the

I live in Uttoxeter, Staffordshire, yeah I know the collector is the case, didnt think about the electrical connection, the heatsink would be live, not good!. I'm looking into using CPU heatsinks at the moment, very cheap plus on most you get a fan, all for under a tenner! might give it a try.....

Chris

Sorry but you're too far away to make the gift worthwhile.

Don't forget if you've the heat sink vertical not to make the valleys vertical, otherwise only the bottom of the heat sink works. - The air there gets hot and starts to rise, but further up the valley there's no temperature difference and so the heat sink doesn't give off any heat. Mount the heatsink so that the valleys are horizontal - the turbulent flow that results gives cooling from the whole area. (Little known factoid)

something

case,

not

plus