# Very quick question about MOSFETs

I have a few projects coming up; they all need to switch higher current (.5 to 6 amps) at about 12 volts.

I think I have picked out a MOSFET that can be driven by the arduino:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=100852985&uq=634427114694274155

But i'd really like it if somebody could take a quick look at the specs to tell me if i am reading them correctly.

FET Feature Logic Level Gate This means that the gate is activated by lower voltages Rds On (Max) @ Id, Vgs 12.8 mOhm @ 30A, 10V This is the amount of internal resistance between the source and load Drain to Source Voltage (Vdss) 25V This is the maximum voltage that this MOSFET can switch Current - Continuous Drain (Id) @ 25° C 30A This is the maximum amount of current that can flow through MOSFET Vgs(th) (Max) @ Id 2V @ 20µA This is amount of voltage (and current) needed to switch on Gate Charge (Qg) @ Vgs 8.3nC @ 5V Amount of capacitance that this MOSFET will introduce into circuit Input Capacitance (Ciss) @ Vds 1043pF @ 15V I don't know what this means Power - Max 46W this is the maximum amount of current and voltage that can pass through the device

things in Yellow - are not clear to me things in Red - are things i do not understand. things in Blue - I think i understand, but would like your feedback

Thanks for your time (yes, i realise this is somewhat like a few posts that exist already, but those were not entirely clear or did not mention all of the measurements i have a question about.)

Post the part number - digikey says no part found with your link.

Yellow comment - this part will handle your current/voltage just fine.

I would personally go for a part with Lower Rds. P = 6A*12mOhm = 72mW, not too warm.

Red comment = Gate has 1043pF of capacitance. Lower capacitance will switch On/Off faster with the arduino driving it.

You need surface mount or thru hole?

Rds On (Max) @ Id, Vgs 12.8 mOhm @ 30A, 10V This is the amount of internal resistance between the source and load

More formally, this is the maximum amount of internal resistance between the Drain and Source terminals of the MOSFET when fully-on (10V from Gate to Source) and carrying 30A of current from Drain to Source. At <30A the resistance should be less.

Current - Continuous Drain (Id) @ 25° C 30A This is the maximum amount of current that can flow through MOSFET

Forget this number. It is a hard physical limit but you will not get even close to this number in practice due to heating issues. Manufacturers like to put this number on the datasheet because it makes the part look big and powerful.

Vgs(th) (Max) @ Id 2V @ 20µA This is amount of voltage (and current) needed to switch on

Kind of. It says that at 2V Gate-to-Source voltage the MOSFET will just start turning on, and ALLOW 20 microamps of current from Drain-to-Source. It is not that the MOSFET NEEDS a certain amount of current to switch on.

Gate Charge (Qg) @ Vgs 8.3nC @ 5V Amount of capacitance that this MOSFET will introduce into circuit

Careful…this is a specification on gate charge, measured in Coulombs, not capacitance. It specifies how much charge you have to pump into the Gate terminal at 5V to turn on the MOSFET. You can kinda sorta derive capacitance from this because Q=CV so C=Q/V=8.3nC/5V=1660pF. But a MOSFET is not a capacitor exactly (it’s actually like a time-varying capacitor).

Input Capacitance (Ciss) @ Vds 1043pF @ 15V I don’t know what this means

Now we’re talking actual capacitance of the Gate-to-Source terminal. Notice this is less than the 1660pF above because once the MOSFET starts to turn on, not only do you have to charge this “Ciss” capacitor but then you have to overcome the Miller effect and dump extra charge into the gate to get it to fully turn on (hence the “time-varying capacitance”).

Power - Max 46W this is the maximum amount of current and voltage that can pass through the device

Forget it…another “look how awesome we are” number that has no basis in reality. Maybe if you dipped the part in liquid nitrogen it could dissipate 46W but otherwise you’re stuck at around 1W with no heatsinking, 10W with a good heatsink, maybe more with some forced-air cooling.

The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O’s fused and protected

Yellow comment - this part will handle your current/voltage just fine.

I would personally go for a part with Lower Rds. P = 6A*12mOhm = 72mW, not too warm.

Red comment = Gate has 1043pF of capacitance. Lower capacitance will switch On/Off faster with the arduino driving it.

You need surface mount or thru hole?

Digi-Key Part Number IPU13N03LAGIN-ND

Thanks for the info about the Rds and the Capacitance. I did not know what property the Capacitance value influenced (although, now that i think about it, what you said does make senese. =) )

I'll look for something with a lower Rds and Capacitance rating.

I need through hole as this portion of the project is not on a circuit board..

RuggedCircuits:

Rds On (Max) @ Id, Vgs 12.8 mOhm @ 30A, 10V This is the amount of internal resistance between the source and load

More formally, this is the maximum amount of internal resistance between the Drain and Source terminals of the MOSFET when fully-on (10V from Gate to Source) and carrying 30A of current from Drain to Source. At <30A the resistance should be less.

Current - Continuous Drain (Id) @ 25° C 30A This is the maximum amount of current that can flow through MOSFET

Forget this number. It is a hard physical limit but you will not get even close to this number in practice due to heating issues. Manufacturers like to put this number on the datasheet because it makes the part look big and powerful.

Vgs(th) (Max) @ Id 2V @ 20µA This is amount of voltage (and current) needed to switch on

Kind of. It says that at 2V Gate-to-Source voltage the MOSFET will just start turning on, and ALLOW 20 microamps of current from Drain-to-Source. It is not that the MOSFET NEEDS a certain amount of current to switch on.

Gate Charge (Qg) @ Vgs 8.3nC @ 5V Amount of capacitance that this MOSFET will introduce into circuit

Careful…this is a specification on gate charge, measured in Coulombs, not capacitance. It specifies how much charge you have to pump into the Gate terminal at 5V to turn on the MOSFET. You can kinda sorta derive capacitance from this because Q=CV so C=Q/V=8.3nC/5V=1660pF. But a MOSFET is not a capacitor exactly (it’s actually like a time-varying capacitor).

Input Capacitance (Ciss) @ Vds 1043pF @ 15V I don’t know what this means

Now we’re talking actual capacitance of the Gate-to-Source terminal. Notice this is less than the 1660pF above because once the MOSFET starts to turn on, not only do you have to charge this “Ciss” capacitor but then you have to overcome the Miller effect and dump extra charge into the gate to get it to fully turn on (hence the “time-varying capacitance”).

Power - Max 46W this is the maximum amount of current and voltage that can pass through the device

Forget it…another “look how awesome we are” number that has no basis in reality. Maybe if you dipped the part in liquid nitrogen it could dissipate 46W but otherwise you’re stuck at around 1W with no heatsinking, 10W with a good heatsink, maybe more with some forced-air cooling.

The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O’s fused and protected

Wow! thanks for the detailed point by point! And this type of answer is EXACTLY what i was looking for! I had no idea that some of the specs were just marketing stuff. Now i know what i (and others who search this thread…) can ignore when picking out a MosFet.

RuggedCircuits:
More formally, this is the maximum amount of internal resistance between the Drain and Source terminals of the MOSFET when fully-on (10V from Gate to Source) and carrying 30A of current from Drain to Source. At <30A the resistance should be less.

If this is a logic level mosfet why is 10 volts when it is fully on? shouldn’t the level be in the 2-5v ballpark? I thought only the standard mosfets operated in the 10-12v ballpark.

RuggedCircuits:
Kind of. It says that at 2V Gate-to-Source voltage the MOSFET will just start turning on, and ALLOW 20 microamps of current from Drain-to-Source. It is not that the MOSFET NEEDS a certain amount of current to switch on.

Thanks for clearing this up! How would i determine the max voltage needed on the gate pin in order to allow 12V @ about 500ma go through? Is it 5V as stated with the gate Gate Charge measurement or is that the bare minimum like it was with the Gate to Source voltage?

Thank you BOTH for your replies. You’re really helping me fill in the gaps in my electronics knowledge (you have no idea how much i appreciate it!)

If this is a logic level mosfet why is 10 volts when it is fully on? shouldn't the level be in the 2-5v ballpark? I thought only the standard mosfets operated in the 10-12v ballpark.

Because the part is actually specified at two gate-to-source voltages. If you look in the datasheet you'll see that they guarantee Rds(on) is no more than 21.9 milliohms when Vgs=4.5V, but ALSO specify that Rds(on) is no more than 12.8 milliohms when Vgs=10V. So you can always turn on a MOSFET "harder" by increasing Vgs (within bounds). The fact that the resistance is "small" at Vgs=4.5V and is actually specified/guaranteed in the datasheet is what makes this a "logic-level" MOSFET.

How would i determine the max voltage needed on the gate pin in order to allow 12V @ about 500ma go through? Is it 5V as stated with the gate Gate Charge measurement or is that the bare minimum like it was with the Gate to Source voltage?

You're really interested in the MINIMUM voltage needed on the gate pin to allow 500mA to go through. Your part will do just fine at 4.5V. With an on-resistance of 21.9milliohms (max) then 500mA will only drop 11mV across the MOSFET and won't heat it up at all.

-- The Flexible MIDI Shield: MIDI IN/OUT, stacking headers, your choice of I/O pins

RuggedCircuits:

If this is a logic level mosfet why is 10 volts when it is fully on? shouldn't the level be in the 2-5v ballpark? I thought only the standard mosfets operated in the 10-12v ballpark.

Because the part is actually specified at two gate-to-source voltages. If you look in the datasheet you'll see that they guarantee Rds(on) is no more than 21.9 milliohms when Vgs=4.5V, but ALSO specify that Rds(on) is no more than 12.8 milliohms when Vgs=10V. So you can always turn on a MOSFET "harder" by increasing Vgs (within bounds). The fact that the resistance is "small" at Vgs=4.5V and is actually specified/guaranteed in the datasheet is what makes this a "logic-level" MOSFET.

Ah! Ok, That clears up the last bit of confusion i had! Thanks!

RuggedCircuits:

How would i determine the max voltage needed on the gate pin in order to allow 12V @ about 500ma go through? Is it 5V as stated with the gate Gate Charge measurement or is that the bare minimum like it was with the Gate to Source voltage?

You're really interested in the MINIMUM voltage needed on the gate pin to allow 500mA to go through. Your part will do just fine at 4.5V. With an on-resistance of 21.9milliohms (max) then 500mA will only drop 11mV across the MOSFET and won't heat it up at all.

That's what i figured; but hearing a pro say it is always nice :P.

Thanks again for the help!

Here's what you do - search for n-channel mosfet click on Fets-Single start filtering: in-stock through hole logic level will bring up 12 pages of results click on view page 1 sort by price Go down the list and find one that is a good compromise between Rds, Input Capacitance, and Power handling. I like this one NTD4960N-35GOS-ND The one you started with wouldn't be bad either now that Iv'e started looking at multiple specs.

I personally have been using these the pre-lead-free version of these (obsoleted right after I bought them too) IRF3707ZPBF-ND to drive a speaker with, difference is just packaging. I had looked it up at International Rectifier first and then went shopping for it, vs a parameter search like above.