How to select a MOSFET

Hello Everyone, I have the following Spec

Junction Temperature 125C
Power Consumed:2W
Ambient Temperature=60C

What parameter should i consider in the datsheets of MOSFET such that the selected MOSFET will meet the Spec defined for temperature.

Regards
Santhosh V

You can easily calculate required thermal resistance junction-to ambient

Rthja < (125-60)/2 = 32.5 K/W

Rthja is sum of Rthjc and Rthca, so if you know datasheet Rthjc (junction-to-case) value, you can calculate maximum Rthca (case-to-ambient) thermal resistance. Practically a large copper pour or a small heatsink will be required in this case. Most likely it doesn't depend much on the transistor, at least for similar packages.

Some datasheets have also examples of copper pours with total Rthja numbers.

Yes i agree. PD=Tj-Ta/Rthja. What is that we wanted to see in datasheet in a number of MOSFET Datasheet ,such that i need to select the excat MOSFET.

ghasant said:

Hello Everyone, I have the following Spec

Junction Temperature 125C
Power Consumed:2W
Ambient Temperature=60C

What parameter should i consider in the datsheets of MOSFET such that the selected MOSFET will meet the Spec defined for temperature.

Regards
Santhosh V

Click to expand...

The parameters you consider are mechanical thermal resistance and modifiers for air flow... Restricted, free convection, forced air velocity. Also there is heat velocity for rapid spreading, but I'll ignore that for now. The thermal parameters are either RθJC for packages with heatsink under case or RθJL for ones without. (Not recomended)

Similar to Ohm's Law, each layer has thermal resistance, Rθ, with the heat flux resulting in temperature change, much like the electrical properties of MOSFET of ΔVds=ΔI*RdsOn and P=I²RdsOn ...

You have ΔT=ΔP*Rθja [°C or K]
Rθja=Rθjc+Rθcladding + Rinsulated-coating or heatsink to air

This is appended by lack of airflow Rsink-air and reduced if forced air cooled depends on surface arial density and only 25~50% for simple PCB or 500% for high density heatsink fins.

Then all good designs have a margin and tolerance for aging so you choose 85 to 100°C max for Tj rather than 125°C, so you may have to reduce P with a lower RdsOn.

Remember using no margin, in free air.

ΔT=ΔP* Rθja = (125-60)=2W * Rθja
Rθja=32°/W


Where Rja=R(jcn-amb.)= (Rjc + Rsolder + Rcopper-heatsink)

Each layer of material has a thermal resistance. Copper area, FR4, soldermask, air but for simplicity, we often neglect the obvious and only consider the conductor.

There are website charts for copper thermal resistance vs weight(oz.), area but example 2sq.in.1oz 2 layer Cu is 45°C/W but can be improved to 25°C/W with hundreds of microvias reduced to 20°C/W with 3m/s surface air speed.

Considering cost of copper layout, it is cheaper to reduce RdsOn such as 20mΩ $0.11@3k https://www.digikey.com/product-detail/en/AON7412/785-1582-2-ND/3152539

...
Then all good designs have a margin and tolerance for aging so you choose 85 to 100°C max for Tj rather than 125°C, so you may have to reduce P with a lower RdsOn.

Remember using no margin, in free air.

ΔT=ΔP* Rθja = (125-60)=2W * Rθja
Rθja=32°/W

Where Rja=R(jcn-amb.)= (Rjc + Rsolder + Rcopper-heatsink)

Considering cost of copper layout, it is cheaper to reduce RdsOn such as 20mΩ $0.11@3k https://www.digikey.com/product-detail/en/AON7412/785-1582-2-ND/3152539

Click to expand...

As you mentioned, Power consumed is 2W and you should have maximum current rating of your system, from this you can find out required Rdson for your circuit (Pd=I*I*Rdson). and then from thermal resistance formula Tj = Ta+Pd*Rthja; You can find out maximum junction temperature when maximum current flows in the circuit and this calculated junction temperature should be less than maximum limit-125*C. I recommend you to take margin of 25 to 30*C and your calculated junction temperature should be less that 95*c or 100*c.
Note: Always check Rthja spec, as it is given for a defined thermal pad on PCB.Always take great care of it and check if it is possible to have it in your PCB design or not.
you can prepare a excel sheet and evaluated(compare) different Mosfets by comparing calculated junction temperatures and select one with fit your requirement.

In netshell, you have to see Rdson, Rthja and thermal Pad size for this Rthja spec.

The specification suggests that the transistor is operated in linear mode, not as switch, otherwise the power dissipation won't be fixed as a design parameter.

FvM said:

The specification suggests that the transistor is operated in linear mode, not as switch, otherwise the power dissipation won't be fixed as a design parameter.

Click to expand...

Yes, you are right. if MOSfet is switching @ a frequency then it is completely different scenario. In switching, there are 2 kind of losses, conduction losses and switching losses.

Total losses = Conduction losses + Switching losses;

Switching losses are more than conduction losses; rise time, fall time and freq. plays very vital role in switching losses, usually if rise time is more, switching losses are more.
you have to add switching losses separately to find total losses.