# Junction temperature and heatsink

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#### 18F4520

##### Junior Member level 1
My TRIAC datasheet says Tj max = 125C but should i really calculate my heatsink for that temperature, or use lower?

##### Member level 3
Hi

You should take some margin. This is definitely better in order not to have surprises.

I would recommend to calculate your heasink to a temperature between 100°C and 110°C

18F4520

### 18F4520

Points: 2

#### ravindragudi

##### Full Member level 3
Tj max will specify the max junction temp, the device can withstand ... you have to look into the graphs in DS to see what's the allowed rms current at this Tj. That means you may not be able run at max power what you need at this temp of junction. Also, with or without heat sink, you can calculate what can be the ambient temp for the device operation. This is more important from the product function point of view.

18F4520

### 18F4520

Points: 2

#### chuckey

Also allow for dust and fluff on the heat sink, that the life of a semiconductor is doubled for every 10 degrees C BELOW Tj max and you don't want people to burn them selves on the heat sink.
Frank

18F4520

### 18F4520

Points: 2

#### 18F4520

##### Junior Member level 1
I calculated heatsink for 500W(2.27A) load to be max 20 K/W (Tj max = 125C Ta=50)

Now this graph confuses me, because it shows maximum mounting base temperature for different loads.

##### Member level 3
hi

Are you sure you want to have a temperature coefficient of 20 Kelvin per watt, if you have a 500W load?

#### ravindragudi

##### Full Member level 3
I calculated heatsink for 500W(2.27A) load to be max 20 K/W (Tj max = 125C Ta=50)

As mentioned earlier, the power handling capacity for the triac at max junction temp will not be higher. It will be very less. Share the triac part DS so that we can look into the P vs jn temp graph. This can suggest what can be the max jn temp for handling 500W power.

Just for example, if for 500W power dissipation, the allowed max jn temp is 110C, then you have work backwards from here for heat sink Rth calculation. Take a safety margin for device jn operation and also for the heat sink Rth.

#### 18F4520

##### Junior Member level 1

An i used this as a guide **broken link removed**

#### KlausST

##### Super Moderator
Staff member
Hi,

500W is the power of the load. It has nothing to do with the heatsink nor with the power dissipation of the triac.

Power dissipation of the triac is max 3W (@ 2.27A RMS and 180°).

So a 20K/W (junction to ambient) heatsink causes a 60K temperature rise. If the max junction temp is 125°C then the max ambient temp is about 65°C.

******
The picture in post #5 is the safe operation area with tespect to load current and ambient temperature. Also knowns as derating curve.

Klaus

#### 18F4520

##### Junior Member level 1
Power dissipation of the triac is max 3W (@ 2.27A RMS and 180°).

So a 20K/W (junction to ambient) heatsink causes a 60K temperature rise. If the max junction temp is 125°C then the max ambient temp is about 65°C.

Actually, 20K/W is heatsink to ambient, add 3K/W for junction to case and 1.4K/W case to heatsink, and you will have 24.4K/W junction to ambient.
24.4C/W*3.07W = 75°C rise.
If maximum junction temperature is 125°C, then maximum ambient temperature is 125-75=50°C.

#### KlausST

##### Super Moderator
Staff member
Perfectly calculated.

Klaus

#### 18F4520

##### Junior Member level 1
According to datasheet, but i was looking for some practical advises.

#### KlausST

##### Super Moderator
Staff member
What do you mean?

Klaus

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