# mosfet question

Discussion in 'General Electronics Chat' started by shortbus, Oct 2, 2009.

1. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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hi, i'm, a newbie to electronics and this forum. i have a question about doing calculations to choose a Nmosfet.

my circuit has a open circuit of 100VDC and a working voltage of approx. 35VDC. witch voltage do i use to choose the Nmos? do i average them ?

the circuit will be switching at a frequency between 5 and 50 kHz. the voltage will be at 100VDC only at the start (5% or less) of the time of each on period.

2. ### Audioguru New Member

Dec 20, 2007
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The Mosfet must have a voltage rating that is higher than the max voltage it will see.
It gets 100V so select a 150V (or higher) Mosfet.

3. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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thank you for the reply. i know to use a higher voltage rated mosfet. what i was asking is, what voltage do i use in figuring the other ratings of the mosfet. like the temperature and such?

cary

4. ### Audioguru New Member

Dec 20, 2007
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It takes time for parts to heat up. If the voltage reaches its operating voltage of 35V quickly then use 35V in the power calculation for the load, not for the switching Mosfet.

The Mosfet heats when it is conducting but if it is a switch then its voltage should be very low. Then multiply the low voltage times the current in the Mosfet to determine how much heat it will make.

5. ### shortbus Thread Starter AAC Fanatic!

Sep 30, 2009
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thank you Audioguru! that will help with my calculations. all i've read doesn't give this kind of practical answer.

cary