MOSFET THERMAL CALCULATION

In this topic we will see very basic thermal design calculations of MOSFET

Example 1: 

Specifications:

MPN: SQJ457EP

Load Current :5 Amps
Power Dissipation :0.625 W
Rds(on) : 0.0250 Ohms



Power Dissipation : I^2*Rds(on)
Pd = 5*5*0.0250 W
Power dissipation = 0.625 W

Thermal information from the datasheets given below

Thermal resistance example

Junction to ambient temperature =Power dissipation * RthJA

Junction to ambient temperature = 0.625 W * 68 °C/W

Junction to ambient temperature = 42.5 °C 

Junction temperature = Junction to ambient temperature + Ambient

Junction temperature = 42.5°C + 25°C  = 67.5°C

Junction to Case temperature = Power dissipation * RthJC

Junction to Case temperature = 0.625 W * 2.2 °C/W

Junction to Case temperature =1.375°C

Case temperature = Junction temperature - Junction to Case temperature

Case temperature = 67.5°C - 1.375°C = 66.125°C

Lt-spice Example circuit: (PMOS reverse protection)



Result:



We are getting slight difference between calculated power dissipation and simulated power dissipation. Because, Rds resistance will vary based on load current ID. While calculating the Power dissipation, I took the general Rds value mentioned  in the datasheet. For specific Rds on resistance value, we should consider the Id Vs Rds(on) Graph in the datasheet.