I’m currently driving 9V @ 2A with a n channel mosfet in a TO-220 package. I want to switch to a smd version of the mosfet. My questions are;
Is heat going to be an issue with 9V @ 2A?
If so, which package works with a heat sink?
Whats a good SMD version of 60V/30A n channel mosfet? (It doesn’t have to be 60V @ 30A. But enough margin to run at least 30 watts and ~15-20V)
Pick one with very low Rds(on). I remember using a fet designed for 90A with very low Rds and it didn’t even get warm while switching 5Amps of LED strips 
I will look up the type when I get the chance to it.
Make sure that the Rds(on) is as low as possible for your gate drive voltage (it varies enormously). Be generous with your PCB heatsink area and you’ll be fine (you won’t need an external heatsink).
Something like this?
http://www.mouser.com/ProductDetail/ON-Semiconductor/ECH8651R-TL-H/?qs=sGAEpiMZZMsrr1kXJis%252bJ4SgQURsP7knoUI0jRvLhCQ%3D
This is a dual one, I need a tri one; but couldn’t find it.
That would work - I’m sure you can find single FET versions. e.g. http://au.mouser.com/ProductDetail/ON-Semiconductor/NTD5407NT4G/?qs=sGAEpiMZZMvsEea7gdidXrhlK0rh5PYxSh78%252bzFE2Z4%3D
Note the Rds(on) change with gate voltage on the top right of page 3 of the datasheet (graph labelled “RDS(on) - VGS”). You can clearly see that unless your drive voltage is >4.5 or so, the on state resistance will be high, hence you will generate more heat. Full turn on may not be achieved below 2V, which will cause extreme heating and short life…
Nice, thanks Learning something everyday. I’ll see if I can find one with a slightly lower drive voltage.
You can find various versions of “logic level” MOSFETS but they tend to be more expensive.
5V should be sufficient to drive either of those parts, especially of you are not using high frequencies.
I guess I could use dual ones like this; One controlled by 3.3v which then feeds 9v to the gate of the other fet?
I’ll do that if I have heat problems.
Good idea - we actually do this quite a lot - using a small FET to bootstrap a larger one…
Would you do that just to be safe? This is going to be installed outside (with the temp fluctuations associated with that).
At 5V, the double FET you suggested has a Rds(on) of ~10mOhm. Therefore, at 10A load, it will see voltage drop of 0.10V (100.010) and a power dissipation of 1W (0.1010) will be required.
The package itself can safely dissipate 1W at up to 60 degC ambient with no additional heatsinking, so if you use a reasonable copper area under the Drain pins (stitch to a bottom layer heatsink with lots of vias too) and use them in parallel you won’t have any issues.
Nice, thanks Byron. I just noticed however (as I was designing this part in eagle), that it’s 2.9x2.3mm in size with 0.3mm pads. No way in hell I can hand solder that
Time to find a little more reasonable packaging!
Try yourself some home-reflow! Works like a charm, even for seriously tiny components…
This looks like the winner;
Can solder it easier, it’s cheap and turns on at 3V+ with low on resistance.
Be careful - I think that part will be marginal.
At 3V and a junction temperature of 125C you’re looking at 40mOhm, about 4W of power dissipation required at 10A. THe package can only get rid of 1.35W at 0C and the device is only capable of 8.5A at 25C, and gets worse from there…
We’ve used this style of HEXFET from IR before for simple, logic level applications http://www.digikey.com/product-detail/en/IRF7842TRPBF/IRF7842PBFCT-ND/812587
As a general rule with FETs, don’t be afraid to over-specify…
Without a doubt, Byron, you’ve been extremely helpful for your first day on the forums, so welcome welcome WELCOME ! The last person who I remember coming on board with such a bang was Justin. What’s even better is that unlike Justin you seem to be an Aussie ! Yay, another one of us !
Oi ya cheeky bugger… 
@ Byron - Indeed - welcome cousin…
@ Brett - Does it count that half the missus family live on the coast? (And yes they are all employed) 
Byron, thanks for explaining this. I’ve heard about as many explanations to selecting fets as there are people I have asked. I finally now understand what to look for. I guess you thought me how to fish.