Minimum ground copper pad size for power loss

[Vindhyachal.takniki]

1. One switcher shows that max power loss is 4W at peak load ( https://www.analog.com/media/en/technical-documentation/data-sheets/lt8645s-2.pdf ) , page 1, figure 2.

2. Now I have to design its ground pad on botton layer while this IC is on top layer. How much gnd pad for heat sink I have to give so that maximum temperature rise goes upto certain degree let say 50C ?

3. Assuming copper thickness is 2oz for PCB design.

4. tried seraching but this shows up, its entirely different thing : https://www.4pcb.com/trace-width-calculator.html

[jmelson]

There's no simple answer.  First, if the pad is on the opposite side of the board from the heat source, it won't do much.  The PC board substrate (fibherglass) has about 1000 X the thermal resistance of copper.  You can solve that by lots of vias.  Second, the thermal pad will have to conduct heat to the environment.  Will there be a case over the board?
That blocks the heat removal.  4 W is a LOT of heat to radiate/conduct from a PC board alone.

Jon

[Vindhyachal.takniki]

I can made dissipation pad both on top layer and bottom. Would remove green masking from both top and bottom layer. But how much pad size calculation is a question

[Momchilo]

I calculated something similar for MOSFETs with high currents.
There is no way to dissipate 4W from a PCB alone. You have to add some heat sinks on the backside with many vias or on top of the chip.

There are some papers with good estimations. For example: https://www.onsemi.com/pub/Collateral/AND9596-D.PDF
Here it is explained more simply: https://www.pcbcart.com/article/content/pcb-theraml-design.html#:~:text=a.,C%20rise%20in%20board%20temperature

They mention 15cm² per Watt. But it depends on many factors like the number of layers.

[bobbydazzler]

fr4 radiates heat better than copper.  I would just leave enough pad exposed with a lot of vias to conduct heat to place a heatsink on.  4w definitely needs a heatsink.