Max power dissipation for 2N3055 without heatsink?

According to a reference the 2N3055 is good for 5.8 watts without a heatsin k at 25c ambient temp, and the case temp rise is 30 degrees per watt. So, i f ambient is 44C and power is 2 watts, case temp should be 104C, or boiling point of water. Any problem doing this, or should a heat sink be used?

Reply to
Bill Bowden
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I kinda wonder why. the whole idea of the TO-3 case style is to facilitate heat sinking. As far as space and cost go, a smaller transistor with a heat sink would be better in alot of ways.

Of course people have their reasons, maybe you just have some laying around ?

As the specs say, of course you CAN? get the transistor that hot. Just make sure you compensate properly if using it in the linear range. Of course as a switcher you will have to design the drive to accomodate the degraded sw itching characteristics at the temperature. With switching however, it prob ably doesn't need to track, just optimize it for 104C and it should be fine at 25C. Just watch that reverse B-E voltage.

With the surface area of the TO-3 case style, even putting it in contact wi th a circuit board will help, so it might not be totally zero heat sink, ot her than the surface area of the transistor itself. Plus, someting that hea vy, you might want to screw it down somehow, so that should help almost no matter what you screw it to. Well maybe certain types of rubber, or asbesto s or something. I odubt you would be looking specifically for a thermal ins ulator unless you have one wierd application there...

Reply to
jurb6006

keep in mind datasheets should not always be read literally.

Take a to-3 cased transistor. It's not even possible to use it with nothing attached to the case as that's an electrical connection. So while, yes you can probably dissipate 5.8w from one, it's not a real scenario that will even happen where the transistor is just floating in air with only the two leads connected to anything.

So as in the other followup post, even connecting the thing to a PCB will add some heatsinking and is probably fine.

Reply to
Cydrome Leader

Historically, the 2N3055 was the workhorse transistor for the linear power supplies of yesteryear. Almost every 10, 20, 100W multiple output, or even up to the 300W supplies used these as the highly heatsinked linear regulated output. Even HP's lab bench supplies used them as the final regulators.

I have NEVER found an equivalent transistor component. The Tjmax is 150C where most transistors have Tjmax in the 125C range. That extra 25C is where the allure of 2N3055 lies. The MOST important spec to never violate is the junction temperature. As you probably know, if you violate that maximum, you are 'baking' the NPN back into the ovens, the dopants continue to diffuse, and the beta goes to hell in a handbasket as the base junction just goes??? You can lose beta, or even end up connecting the collector region to the emitter region, which means, no more transistor.

So ALWAYS calculate Tj, keep below max, and assume NO margin and you will be alright. I know, I know, you have no control over the thermal conductivity between junction and case, so why not only 'think' case temp? Well, thinking junction temp keeps your mind concentrating on the important issue, not a 'sideways' observation. And, you'll know the true origin of the limit. And, if you have a spate of failures, can help you look for the failure mode, as an example, like when the IC house mounted the chip, they didn't do it right this run, and there were little voids all over underneath which destroyed the chip to case thermal conductivity, yet didn't touch the chip to case electrical conductivity. IMO, the way to think is ALWAYS to think in terms of the rudimentary, not in terms of the 'outsides' or 'secondary' terms.

PS: don't assume ambient air is the same everywhere. in a box ambient is not quite so ambient. at least without a fan.

Reply to
RobertMacy

Even with the linear power supplies of today, they still use the 2n3055. Even the knock off ones do. In fact, I can't think of the last time I saw a to-3 cased transistor that wasn't a 2n3055.

Reply to
Cydrome Leader

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This link claims the Tjmax is 200c and junction to case resistance is 1.52 per watt. So, at 2 watts the junction should only be about 3 degrees higher than the case. Seems low to me. I'm going to try a double sided 5 square i nch copper board which should increase the area about 3X. All the other com ponents will fit on the board. It's a shunt regulator for a small solar pan el and there isn't any headroom for a series regulator. I wonder how much e xtra thermal resistance will be added if I spray the finished board with Kr ylon?

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Reply to
Bill Bowden

"Bill Bowden"

This link claims the Tjmax is 200c

** That is the damage temp, never to be exceeded.

The max safe working case temp is more like 100C.

and junction to case resistance is 1.52 per watt. So, at 2 watts the junction should only be about 3 degrees higher than the case. Seems low to me.

** Many TO3s are better, down to 0.6C per watt for 200W rated BJTs.

Use one of these, takes hardly any PCB space.

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.... Phil

Reply to
Phil Allison

Actually there should be plenty of headroom for a MOSFET.

Shunt regulators are so inefficient. Actually you could have a switched mo de shunt regulator but if there is to be any filtering after it, there will have to be a diode to block the post filter's current from the shunt regul ator when it is switched on.

Another plane would be to use a ballast. you should know the maximum curren t the shunt needs to pul to regulate. Use a resistor that will shunt it eno ugh and shift the dissipatin to the resistor. At that point you can switch mode it if you feel so inclined, but you still aren't going to get very hig h efficiency.

Reply to
jurb6006

Right! It's 200C, and EVERYTHING else is 150C!! Some day will check facts and quit relying on memory. NOT!

To understand air cooling, someone once told me to think in terms of a 'stiction' layer of air. Air movement takes all the heat you can give it, but first the heat must travel through that 'stiction' layer. Easy to calculate the C/W based upon area and conductivity through that layer. A fan blowing across the component only makes the stiction layer thinner, that's why the curves don't go to zero [C/W], because there's ALWAYS a layer of air there. Don't know how accurate that mental model is, but certainly helps envision why it's so difficult to cool a component in air.

To find the effect of the Krylon coating, bet you can do a 'layer' calcuation. just like stacking dielectrics in a capacitor. Assume no 'hotspots', then have thermal conductivity thru Krylon to surface, then thermal conductivity into the air. Two terms in a row. Not much effect, but will be there.

Reply to
RobertMacy

Depending on application, thermal _capacity_ can matter more than thermal _resistance_.

For the TOW missile voltage regulator I used a steel TO-3 package with no heat sink. Its thermal capacity held temperature rise down long enough to reach the target without excessive temperature rise... after that it doesn't matter >:-} ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

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