i want to make reverse polarity protection for input 48 voltage , load 30 Ampere 2 dc buck converters. i was searching many solutions like diodes , but i wanna make it with p fet , so i came across this design

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so here zener is over voltage protection keep Vgs lower than vgss

high resistor is used to limit current for zener , and bleed resistor for capacitor so discharge capacitor .

i have IRF 4905 download file

first approach : Rds max 0.02 ohm , so worst case power Dissipation = (i load)^2 Rds

Pd= 18 watt . is that a big number ? where max Pd is 200watt .

Temp Rise = Pd* R (junction to ambient) = 18*62=1116 c which i very bad , where max temp 175 c .

so i think of using 1 mosfet for each buck , so current dec to half and pd becomes 4.5 watt, temp rise = 279 c ,

does using heat sink solve the problem or i have to find another mosfet with lower Rds?

does assuming Rds max wrong ? where in datasheet :

Second approach

if Vds is -15v , id=30 A , Vds nearly = 0.3 then Rds= Vds/ids = 0.01 thats make temp rise = 558c .

for id=15 A Vds=0.2v,Rds= 0.0133 ,temp rise becomes 186 c. heat sink will help alot here

i think second approach is better where zener makes Vgs constant , so if load current max is known then Vds is already known.

so which approach is correct ? worst case or second ?

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i have read thermal runaway could happen where as there is Pd temp rise up and R increases ,Ids is constant due to load ,so mosfet temp will keep increasing.

so how to know if temp will balance or not , and how to calculate Rds where temp is not constant ?

is Temp = T room + temp rise , is the temp that i need to know new Rds from this graph ?

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capacitor cant be electrolyte because if power connected wrong it will blown , so ceramic one .

how to choose capacitance ceramic for this application ?

resistor calculation for zener :

for zener :

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for capacitor :

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shall i calculate R for zener , and for capacitor and choose the larger one or the smaller one ?


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