# Mosfet Selection Help needed

Discussion in 'The Projects Forum' started by mishra87, Sep 22, 2016.

1. ### mishra87 Thread Starter Member

Jan 17, 2016
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2
I have boost dc dc converter. I have to give 7A continuous load current to inductive load through mosfet . What should be MOSFEt current rating. Forget about all parameters of except Ids(continuous).

Lets say N Ch MOSFET

Thanks

Jun 4, 2014
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3. ### ronv AAC Fanatic!

Nov 12, 2008
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To make the calculation you need to know input voltage and output voltage. You should know some other stuff but you can guess at them.

4. ### mishra87 Thread Starter Member

Jan 17, 2016
201
2
M
My intention to post this question is if I have to switch 7A current through MOSFET what should be current rating of MOSFET means Id rating.

5. ### ronv AAC Fanatic!

Nov 12, 2008
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It's not quite that simple.
You can use the Id of seven amps, but that is only if you can keep the case temperature at 25C. That would take a cooled heatsink. It's better to use the Rds on and calculate the power. Then you can decide if you want to spend your money on heat sinks or lower Rds on FETs.

6. ### jayanthd Member

Jul 4, 2015
275
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Rds On should be very low like 3 milli Ohms.

7. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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Assuming the mosfet is rated for at least the current and voltage you need, the limiting factor is heat. You might find a tiny mosfet rated at 7A and your voltage, but the conductive heat, e.g., RDS(on) *I^2 may be a watt or two. If that wonderful tiny package had a temp coefficient of 200°/W, then you will need a good heatsink or a different mosfet.

Precise calculations can require information you don't have and be complicated. It seems almost every mosfet manufacturer has an application note to describe how to calculate the heat produced and how to dissipate it. I have found these application notes from Maxim and TI to be among the simpler discussions available:

http://pdfserv.maxim-ic.com/en/an/AN1832.pdf
http://www.ti.com/lit/an/slva462/slva462.pdf

For starters, if the mosfet you are considering has an RDS(on) *I^2 more than its package will dissipate, I would look for a larger package. There are of course, other factors, but it is easier to rule out a candidate than to rule one in.

JOhn

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