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2n3055 or tip3055?

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Hi all

I'm building a power supply that uses the 2N3055. But now, after these transistors reached their breakdown temperature the 2nd time, I was wondering, would it be better to use the TO218 package TIP3055. Which one would dissipate heat better? If I was to use the same heatsink
 
2N3055 is a lot Better than the TIP alternative. With the. It has higher operating voltage and current - and power dissipating in form of heat is not a problem. Use the right heat sink.
However the TIP is smaller and easier to connect.

2N3055 is a lot Better than the TIP alternative. With the. It has higher operating voltage and current - and power dissipating in form of heat is not a problem. Use the right heat sink.
However the TIP is smaller and easier to connect.
 
Are you using thermal grease to mount the transistors to the heat sink? If so then you need a better heat sink. How much power are you dissipating and how large is the heat sink?
 
Yes, i am using thermal grease, 100Watts of heat is going to be generated, and the heat sink is that of an old CPU. One for each transistor.
The datasheet gives the max heat dissipation as 115W
 
115 watts with an infinite heat sink at 25C.

You don’t have an infinite heat sink at 25!

The TO-3 case is rated at 1.52C/watt. There for at 100Watts the junction is 152C above the case temp. If your heat sink was 25C then the junction temp would be 177C. The silicon breaks down at 200C. You heat sink is not at 25C. You are milting silicon.

The TO-247 case (ST Micro) is only 90 watts. It dies at 150C.
 
Yes, i am using thermal grease, 100Watts of heat is going to be generated, and the heat sink is that of an old CPU. One for each transistor.
The datasheet gives the max heat dissipation as 115W

As Ron has already pointed out, you're severely overloading the transistors and melting them - you can't dissipate 100W from a 2N3055 - as a minimum double up your transistors and heatsinks, I'm presuming the CPU heatsinks are fan cooled, and the transistors are somehow mounted to them efficiently (although I can't see how you would mount TO3 devices to them?).
 
115 watts with an infinite heat sink at 25C.

You don’t have an infinite heat sink at 25!

The TO-3 case is rated at 1.52C/watt. There for at 100Watts the junction is 152C above the case temp. If your heat sink was 25C then the junction temp would be 177C. The silicon breaks down at 200C. You heat sink is not at 25C. You are milting silicon.

The TO-247 case (ST Micro) is only 90 watts. It dies at 150C.
But the max operating temperature is 200'C. And that "100Watts I'm talking about, is only at full power operation of the power supply, namely 50V 2A.
 
WHOA, WHOA! HALT! I made a big mistake. My Vin is 54 Volts, my Vout is 45 Volts, and my Iout is 2 Amps.
Thus it gives me a total loss of heat of 18 Watts. I should do some more research before asking stupid questions.
Sorry everyone.
 
But I still don't quite understand what thermal resistance is.
Short course--

Thermal resistance is the resistance to the flow of heat in a system which causes a gradient in temperate, must as electrical resistance causes a gradient in voltage when current is flowing.

With semiconductors there are several parts to the thermal resistance:

1) semiconductor junction (chip) to case

2) case to air if mounted in air (this can be many 10's of degrees C/W depending upon the case type)

3) case to heat sink if mounted on a heat sink (typically a few tenths of a C/W if thermal grease is used)

3) heat sink to air (varies significantly depending upon the size and type of heat sink)

Look at this data sheet. On the second page there is a table labeled "THERMAL DATA". It states that the junction to case thermal resistance is a maximum of 1.5 C/W. This means that each watt the transistor is dissipating will raise the semiconductor chip 1.5 degrees C above the case temperature.

If you look at heat sink data it will also have a thermal resistance to air in degrees C/W.

To determine the temperature rise of the chip you add all the thermal resistances together.

You multiply that sum by the power being dissipated by the chip to determine the chips temperature rise above ambient.

You then add the maximum ambient temperature to that to get the maximum temperate the chip will reach.

This total can not exceed the maximum for the device as shown on the data sheet (in the case of the referenced transistor it is 200C). For good reliability you typically want to operate 25-50C below this maximum. If the total is too high then you need a better heat sink.

Make sense?
 
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Upon reading it the third time, it became clear. Just one thing, that "power being dissipated" , is basically the difference between input wattage and output wattage, right?
 
Upon reading it the third time, it became clear. Just one thing, that "power being dissipated" , is basically the difference between input wattage and output wattage, right?
Yes. It's also the average of the current through the device times the voltage across the device when it is "on".
 
But I still don't quite understand what thermal resistance is.



Hi,

If you know what voltage, current, and resistance is then you know what thermal resistance is. Resistance is V/I (voltage over current), while thermal resistance is T/P (temperature over power).
If you have a resistance of 10 ohms and 1 amp, then your voltage is 10 volts. If you have thermal resistance of 10degC/Watt and you have 1 watt, then you have a temperature rise of 10 degrees.
 
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Are these heatsinks large enough? I now know how to work out the heat generated, but how do I find the required heatsink?
You work backwards from the maximum ambient temperature and forward from the power dissipated. From that you can determine the maximum thermal resistance you can tolerate for the heat sink which will dictate the size of the heat sink.

To upload an image click on "Go Advanced" at the bottom of your new reply and then click on the paper-clip icon.
 
Are these heatsinks large enough?
 

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