Precautions needed to parallel P-MOSFETs as high side load switches?

Question

I plan to use P-MOSFETs (SI7463) as high side load switches, with a slight soft-start (to be tested and tweaked after PCBs are made).

I'd like to parallel them in order to make the equivalent Rds-on lower, but I can't find simple straightforward recommendations on how to do so.

I'd imagine that due to their positive temperature coefficient they will "automatically" share current "equally", however, I've seen some parallel applications in which a small series resistor is recommended (which defeats the purpose for me).

Does a simple implementation like this lack anything?

The load are LED Panels, up to 8A total. Switching frequency is maybe once or twice a day (10uHz? :D ).

With an advertised Rds-on of ~0.01R and 8A, power dissipation would be 0.6W. Junction to ambient on a 1x1" FR-4 board is about 52C/W typical, so 31C temp rise.

Adding an extra FET would be an easy and cheap way of halving the power dissipation and enlarging the effective dissipation area.

^{simulate this circuit – Schematic created using CircuitLab}

Follow up:

I ended up following Trevors advice and using a single more expensive device with lower Rds-on (BSC030P03NS3). To the touch, it feels like the traces around the device heat up more than the device itself (4 layers, 10mm wide, 35um thickness on external and 17um on internal). (They are probably the same temperature and the copper feels hotter than the plastic). It works fine without heatsinks cause the avg temp over time is much lower than the peak (8 Amps).

Answer 1

Another issue you need to consider is the thermal effects of the heat-sink...

Depending on the airflow and the geometry of the heat-sink, one end may be hotter than the other. Further, if, for example, there are three MOSFETS in a line, the central one can be considerably hotter than the other two.

In addition, there are mechanical considerations that need to be accounted for.

Can you guarantee three or more MOSFETS soldered into a board will all have intimate thermal contact with the heat-sink if it is attached later.

If the components are surface mount, will the thermal expansion of the heat-sink cause undue stress and or failure of the solder joints.

Answer 2

Your FETs have low RdsON, so don't forget to equalize length of copper traces. I have illustrated this:

^{simulate this circuit – Schematic created using CircuitLab}

Notice on top schematic, both FETs have identical "wire" length. In bottom schematic, the one on the right has longer "wires".

If you use 4 layer board, remember copper thickness is different between layers...