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I am designing PCB a in which I am placing a BSP296 N-channel MOSFET, for switching solenoids:

  • MOSFET supply voltage is 24V
  • MOSFET max power dissipation is 1.8W
  • Max operating temperature is 150°C.

I have done calculations as follows:

  • My operating temperature = 100°C (not taking maximum, I need info about heat sinking)

    • \$ R_{th} = \dfrac{(100°C -25°C)}{1.8W}\$
    • \$ R_{th} = \dfrac{75°C}{1.8W}\$
    • \$ R_{th} =41.6 °C/W\$

I would like to know how to calculate how many ounces of copper I need, what area of copper and if I need a heat sink for this.

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Take a look at this publication from Texas Instruments (you can go directly to pages 12+ for a quick look) : http://www.ti.com/lit/ml/slup230/slup230.pdf

Someone made an online spreadsheet calculator about it : http://circuitcalculator.com/wordpress/2007/02/16/pcb-thermal-copper-area/

You can also read this Application Note from Fairchild about "Maximum Power Enhancement Techniques for SOT-223 Power MOSFETs", which seems to be exactly what you're looking for :) http://www.fairchildsemi.com/an/AN/AN-1028.pdf

As far I as know, there is no heatsink for SOT223 packages, the only way to dissipate the power is through copper via pin 4.

I doubt you will be able to use your mosfet at full power (1.5W) and at 100°C+ when the Tj max is 150°C, unless you have a quite big copper area and a cooling fan.

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