# Mosfet control Rds on , and Vgs

I want to know how to calculate Vgs , assuming load current Id is constant , Vd = voltage of battery is constant , and fet is operating in thermal stability region , so there is no thermal runnawy , connecting gate to ground leads us having

Vgs = - Vs , so we need to calculate Vs

Vds = Vd - Vs = V battery - Vs = Ids Rds

so how to measure Rds ? i know it is dependent on temperature

but measuring temperature in thermal stability will be difficult , as temp increase Ids decreases so temp decrease until certain point so no change after that , so the temp that it will settle at is what matters

can i assume max Rds to know min Vgs ? is there anoghter approach to control mosfet ?

update :

https://www.infineon.com/dgdl/Infineon-ApplicationNote_Linear_Mode_Operation_Safe_Operation_Diagram_MOSFETs-AN-v01_00-EN.pdf?fileId=db3a30433e30e4bf013e3646e9381200

i know that there is safe operating area,

assuming Ron on limit i did these calculations below

**is it right? or should i assume that Vgs = 10 Volt at max Rds

when to use Rds on limit and Max power line limit ?**

update 2 :

• As shown, your MOSFET will act like a forward biased diode, not like a transistor. (HINT: Note the orientation of the MOSFET's body diode.) If you want the MOSFET to act like a transistor, you must connect the source to the 30V supply and the drain to the load. – Jim Fischer Feb 17 '20 at 6:51
• @Jim , thanks for your reply ,actually i dont know about making mosfet act as transistor , but i have tested this circuit and it actually works fine for reverse polarity protection , so i wanted to just make right calculations . – Ahmed elmenshawie Feb 17 '20 at 7:00
• Vgs = 10 volts Ahmed and not -Vs. Vs is nearly 30 volts at instant of power supplied. – Andy aka Feb 17 '20 at 8:33
• @Andyaka thanks for your reply , well i have updated my question , asking about is my calculations wrong and i have to assume vgs = 10 , and when to assume Rds on limit , and max power limit . – Ahmed elmenshawie Feb 17 '20 at 8:40
• Have you looked in the data sheet to see what Rds(on) is for 10 volts on the gate-source? – Andy aka Feb 17 '20 at 8:54