Discussion in 'Math' started by 1213brett, Feb 9, 2015.

1. ### 1213brett Thread Starter New Member

Feb 1, 2015
13
0
I'm trying to find out if my mosfet needs a heat sink. my load draw 1.20 A and im triggering the mosfet with ardino pin 7 5V

P= current squared * RDS
1.20 A * .16 Ohms squared = .2302

TJmax - Tambient / ROJA
150 - 25 / 62 = 2.02

2.02 is larger then .2302 so I need a heat sink

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2. ### ScottWang Moderator

Aug 23, 2012
4,930
777
How is your load V/I, do you have a circuit?
You can choosing some other mosfets has lower Rds then it could reducing the power consumption.

3. ### Magician New Member

Aug 18, 2014
13
6
No, you don't. MOSFET is o'k up to 2W w/o heatsink, and it's needs to dissipate 0.23W. Are you using PWM? Losses may be more in case PWM

4. ### 1213brett Thread Starter New Member

Feb 1, 2015
13
0
No I'm not. And thank you!

5. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Note that
150 - 25 / 62 = 149.60

Remember order of operations!

If you mean (150 - 25) / 62, then say so.

Pmax = (TJmax - Tambient)/ROJA = (150 °C - 25 °C)/(62 °C/W) = 2.02 W

Pactual = I²·Rds = (1.2 A)²·(0.16 Ω) = 0.23 W

Since Pactual < Pmax, no heat sink needed.

6. ### uwed Member

Mar 16, 2015
64
17
Rds=0.16 ohm is usually normalized (room temperature). If you operate at Tj=150°C we would have Rds=2.2*0.16=0.35ohm (see Fig.4 in the datasheet).