Do I Understand This Correctly.
Hello everyone. I have two question, one about calculating dissipation in a MOSFET to work out if I need a heatsink, and the other about resistors.
I know this is really basic stuff, and I think I have it right, so prepare your clue-by-fours anyway...
First off, resistors.
I want to drop the voltage across a rail to turn on an LED if power is present. Sounds simple right? Lets say I have two rails, one is +5V the other is +48V.
Take the attached image (resistors.png). If the working voltage of the LED is 2.4V, and I have a 5V supply, Ive calculated:
R = (V
S - V
LED) over I
LEDassuming 20mA for I
LED this gives
(5 - 2.4) / 20 = 130ohm or 150ohm as next best available value
For the 48V supply:
(48 - 2.4) / 20 = 2280ohm or 2.7K as next best available value
The 48V situation is pretty severe in terms of wattage dissipated, which I calculated as 48 * 0.02 = 0.96 or roughly 1W, so a pretty "hefty" resistor needed there.
Is that math sound??
Now for the MOSFET.
I want to switch on/off a load that draws a maximum of 1.5A. Im using a Fairchild FDC855N as an example, datasheet here:
https://www.fairchildsemi.com/datasheets/FD/FDC855N.pdfNow take the second attached diagram (mosfet.png). This is a logic level MOSFET (which I only learned existed yesterday!) which has an R
DS of 0.0036ohm at 4.5V gate voltage. I also learned to fudge this a bit, so lets call it 0.00432ohm.
The formula is R * I
2 to give watts dissipated, so with this I figure:
0.00432 * 1.5
2 is 0.00432 * 2.25 = 0.00972 is 9.7mW
Convert this in to temperature rise:
62.5 * 0.00972 = 0.6075 + 25C ambient = 25.6C
This doesnt seem right to me. Switching 1.5A continuous would only cause a 0.6 degree rise above ambient? Just seems low... I suppose Im trying to work out if I should be adding any kind of heat sinking capability to this little package, or how much, which from the datasheet seems to be mostly via the drain pins connected to a large copper area? But if the dissipation is that low, perhaps not very much is needed?
Any pointers or have I actually got it?
Cheers