# When Is A Heatsink Required?

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#### MikeSD

##### Newbie level 3
I'm wondering if it's really this easy in deciding "IF" a heatsink is required.

I have a component (BTS555), that I'm using that has

Rjc = .4 K/W
Rja = 30 K/W

The device will be delivering a lot of current but the device itself will be disipating low power.
20A with 2.5 mOhm when conducting. Seems to be about 1W.

For a max junction temp of 150 deg. C and a maximum ambient of 85 deg C and using

Rja = (150-85)/1W = 65

If the part datasheet says that Rja = 30 K/W does that mean in this application that a heatsink is not required? The reason being that the required Rja is 65 but the component itself is already lower than the maximum resistance, according to the datasheet (i.e. 65 < 30).

Or conversly, does this mean that I could dissipate up to 2.1W before a heatsink was required?

Thanks,

As you are dissipating 1W and the device has a Rja of 30,it will have to run at 30 degs above ambient to dissipate 1W. If you max Tamb is 85 Degs, then the junction will run at 85 + 30 = 115 degs which is adequately lower then 150. If you bolted the device down to a heat sink of .5 deg/W then then total resistance is .4 +.5 = .9 degs/W so at your 1W then junction would be at .9 degs above ambient or conversely, with 150-85 = 65 Degs in hand , it could handle 65/.9 ~ 70W
Frank

You can not run that device at 150 deg C and expect it to live. For a silicon mosfet, you should stay under 110 deg C.

I'm wondering if it's really this easy in deciding "IF" a heatsink is required.

I have a component (BTS555), that I'm using that has

Rjc = .4 K/W
Rja = 30 K/W

The device will be delivering a lot of current but the device itself will be disipating low power.
20A with 2.5 mOhm when conducting. Seems to be about 1W.

For a max junction temp of 150 deg. C and a maximum ambient of 85 deg C and using

Rja = (150-85)/1W = 65

If the part datasheet says that Rja = 30 K/W does that mean in this application that a heatsink is not required? The reason being that the required Rja is 65 but the component itself is already lower than the maximum resistance, according to the datasheet (i.e. 65 < 30).

Or conversly, does this mean that I could dissipate up to 2.1W before a heatsink was required?

Thanks,

chuckey's answer is spot on, but here's a little more insight into the numbers.

Rja, in K/W or C/W, is the junction-to-ambient (ja) resistance (R) of the device. K = degrees Kelvin, C = deg Celsius, since they scale 1:1, they are both commonly interchanged. Thermal resistance calculations are very similar to electrical resistance calculations. Electrical resistance is the same as thermal resistance, voltage is analogous to temperature, and power flow (watts) is analogous to current.

So, just like determining the voltage at a node, if you know a resistor's value and the current flowing through it, you can calculate the junction temperature.
Given:
Rja = 30 deg C/W
Power = 1.0 Watts
Tambient = 85 C

So, Tjunction = Tambient + Rja*Power (deg C = deg C + C/Watt * Watt)
Tjunc = 85 + 30*1.0 = 115 C (just as chuckey noted), well below 150 deg C, max.

As biff noted, you want to stay away from Tj,max. That's the failure point of the device, so the further away you stay, the longer the part's expected lifetime will be (reliability goes down with temperature increase for transistor-based devices).

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