Hi all,
I know the "Power dissipation" is a intentional waste of power in the transistor to achieve a task. But when I was searching for a 13009 transistor I got two types of cases: TO-220 and TO-3P. Almost all parameters are same. Same voltage, same collector current, same base current. But smaller case (TO-220) has just 2W power dissipation (2W waste) and the bigger ( TO-3P) has 80W power dissipation (80W waste). Really interesting difference! Does this mean I need to pick the smaller one to get same performance?

 

No you have it all wrong. Power dissipation is not necessarily "waste". What it tells you is that one package can dissipate more heat without the junction temperature rising to the point where the device is destroyed. Use the TO-220 package if your power dissipation requirements are modest. Use the larger package if you have a heavy duty application, like running a motor.

 

I would like to emphasize that the power dissipation specification has nothing at all to do with "waste". It only specifies how much power the device can pass without being destroyed. Whether it is "wasted" or not has no part in the number.

There are two limitations on the power handling ability of a transistor: average junction temperature and second breakdown. The power dissipation number is related to the ability for the device to heat up and still operate properly (or not fail catastrophically).

This figure from a power transistor datasheet shows a relationship between how hot the transistor gets and the power dissipation rating of the device. It would make no sense that it gets more efficient in some way as it heats up since ideally it wouldn't heat up at all but as you can see, the hotter it is, the less power you can safely pass through it.

 

Your datasheet does not say TA or TC. Here is the datasheet from Motorola (now called ON Semi) who invented the MJ series of transistors:

 

It only specifies how much power the device can pass without being destroyed.

A nit.
I'm sure you didn't mean that literally, since it's the power the device can dissipate, not how much it can pass ( which could be much higher if used as a switch).
I only mention this because that seems to be a common misconception among newbies.

 

A nit.
I'm sure you didn't mean that literally, since it's the power the device can dissipate, not how much it can pass ( which could be much higher if used as a switch).
I only mention this because that seems to be a common misconception among newbies.

To be more clear, it is proportional to how much power it can be used to switch.

 

To be more clear, it is proportional to how much power it can be used to switch.

Sorry, but that's not clear to me.
It what way is it proportional?
It may be proportional to the current, but the power switched is proportional to the current and the voltage.

 

(TO-220) has just 2W power dissipation (2W waste) and the bigger ( TO-3P) has 80W power dissipation (80W waste).

"apples & oranges"
The TO-220 with no heatsink is 2W. Depending on transistor it is also rated for 75W with the biggest heatsink in the world. So this is more about the heatsink than the transistor.

Next a transistor might be rated for 10A and 700V but it is not a 7000W transistor. The 700V is there at zero current and the 10A is there at 0.8V. (0W & 8W)

When building at 200W power supply you do not need a 200W transistor and heatsink.

 

Sorry, but that's not clear to me.
It what way is it proportional?
It may be proportional to the current, but the power switched is proportional to the current and the voltage.

Yes, to the current.The maximum voltage is also involved.