Transistors and heat sinks

Other Hardware General Electronics
1

Hi!

I'm looking for general advice on using heat sinks on transistors. Let's say I'm using a IRF520 MOSFET to drive a 1A load at 12 volts with gate voltage being 5V. The IRF520 is specced at 60W, has RDS(on) = 0.27ohms and can handle continuous drain current > 5A. 60W sounds so large that I'm guessing it must be with a heat sink, right?

In the example above I'm assuming I don't need a heat sink because I'm way below the 60W limit but how do I actually know this? Is there any rule of thumb how to know when a heat sink is needed? I've tried to interpret the datasheet but I cannot find anything about heat sinks in it.

2

if you are passing 1 amp and RDSon is .27 ohms this is .27Watts I2R COOL

3

LarryD:
if you are passing 1 amp and RDSon is .27 ohms this is .27Watts I2R COOL

Yes, I thought that too. My question is when do I actually need the sink? At 10W, 30W, 60W...?

4

My rule of thumb is when the case temperature is more than 120oF. i.e. IMHO

5

Question is slightly broader. Once temperature reached that point then your transistor requires a heatsink question will be which heatsink to use.

Open your datasheet. There will be a section on thermal resistance. For example for IRLB8743 there are two lines that are of interest:
Junction to ambient 62 C/W
Junction to case 1.11 C/W

Another piece of data you'll need is Operating junction temperature (thats in Absolute Maximum ratings section), 175C in case of IRLB8743.

Lets say ambient temperature in your room is 20 degrees. 175-20=155, thats 155Degrees headroom left.
155/62=2.5 Thats 2.5W for the gut of the transistor to reach max temperature without a heatsink.
With a heatsink you'll use the 1.11C/W value, and to estimate case temperature use dessipation value for your heatsink.
Best of luck on your project!

6

It is clear:
You need a heater when the junction temperature exceeds max temperature junction.
Max temperature junction can be found in datasheet, generally it is between 150°C and 170°C

It is easy to calculate:
You calculate the power in the mosfet.
In datasheet of your Mosfet transistor you have some thermal resistor (Rth).

Without heat sink you take Rthcase-ambient
The rise in temperature relative to the room is calculated by :
Delta T = P * Rthcase-ambient
Junction temperature = room temperature + Dela T

If calculated junction temperature exceeds device max junction temperature you need an heat sink.

To calculate the heat sink you have to take an other Rth -> Rthjunction-case.
The new Rthjunction-ambiant is now (Rthjunction-case + Rthheatsink)

You calculate the Rth max which give a temperature junction < device max junction temperature.
Now you can calculate the max Rthheatsink you need :
Rthheatsink = calculated Rth - Rthjunction-case

7

Ok, got it! Thank you very much!