How much power dissipation is practical with SMD MOSFETs?

The data sheet for this MOSFET CSD18531Q5A states :
http://www.ti.com/lit/gpn/csd18531q5a
http://www.ti.com/lit/gpn/csd18531q5a
Max. Pd (abs max ratings) 3.1W
with 1sq in, 2 oz copper
RθJC Thermal Resistance Junction to Case 1 °C/W
RθJA Thermal Resistance Junction to Ambient 50 °C/W

Even if the Pd is 1W, then the pcb track or case temperature would be 50 °C above ambient? On a max. ambient of say 45 °C, the case temperature would be 100 °C?
In other words, what is the practical max. temperature allowable on the pcb track or the component?

 

no, the 50 degree c/w is a max rating, the actual is determined by the curent flow. you have to design the pc board with enough copper to use as a heatsink for the current it is designed for.

 

How much power dissipation is practical with SMD MOSFETs?

The data sheet for this MOSFET CSD18531Q5A states :
http://www.ti.com/lit/gpn/csd18531q5a
Max. Pd (abs max ratings) 3.1W
with 1sq in, 2 oz copper
RθJC Thermal Resistance Junction to Case 1 °C/W
RθJA Thermal Resistance Junction to Ambient 50 °C/W

Even if the Pd is 1W, then the pcb track or case temperature would be 50 °C above ambient? On a max. ambient of say 45 °C, the case temperature would be 100 °C?
In other words, what is the practical max. temperature allowable on the pcb track or the component?

Remember, the power dissipation is not the load, but the chip.

 

Yes, it is as you describe. At 45 C ambient the case would be 95C, the chip inside would be 96C.

 

Thank you all.
Yes, I understand that it is not the output power, but the power dissipated by the MOSFET.
Since it is on the pcb, what would be the allowable temperature or practical maximum temperature?
Is it possible to add a heatsink on these SMD devices apart from increasing the copper?

 

Thank you all.
Yes, I understand that it is not the output power, but the power dissipated by the MOSFET.
Since it is on the pcb, what would be the allowable temperature or practical maximum temperature?
Is it possible to add a heatsink on these SMD devices apart from increasing the copper?

Typically standard FR4 circuit boards have UL RTI values around 105 deg C to 130 deg C.. You need to look at the approvals of the company producing the boards for you and the substrate/build up they are using.
Of course any applicable standards your product is being built to may dictate other requirements for temps,etc...

And yes you can add heatsinks too.. I used these in a past project for a SMT mosfet and they worked great. http://www.ohmite.com/cat/sink_d.pdf
I could easily dissipate 9-10W..

 

Some tiny heat sinks are available to be glued on or clipped on.

 

Thank you all.
Yes, I understand that it is not the output power, but the power dissipated by the MOSFET.
Since it is on the pcb, what would be the allowable temperature or practical maximum temperature?
Is it possible to add a heatsink on these SMD devices apart from increasing the copper?

If you're that concerned about power then why are you using an SMD part which is difficult to heat sink?

 

I was intrigued by the small size of a 20A MPPT charger with SMD Components