# Specification Clarification for FETs

I use a MOSFET in my project, and I don't know what model of MOSFET I need. I do have a list of specs, but I don't know what they would be called on the datasheet.

I need some help translating my list of required specs into something I can put into an interactive search on an electronics store website.

• Switching voltage (The voltage at which the MOSFET lets current flow)
• Maximum current it can pass from the source to the drain. (How many Amps can I run through it)
• Maximum voltage it can hold back when the gate has no current.(How much voltage can it hold back when not switched 'on')
• Gate capacitance (No idea)

Or, would I be able to just use a switching transistor? I feel like it would simplify things if I could. I am using this circuit:

Edit: Would this MOSFET(datasheet) work? I would be supplying the circuit with 11.4V, the current after R2 would be about 10mA, and L1 gets supplied with about 1A.

• What is the bulby thing in series to D1? You want a fly back diode in parallel to the inductor without any extra series resistances, otherwise it'll defeat its goal (killing voltage surges). Aug 24 '14 at 8:29
• Not the best choice. It has unnecesarily high on-resistance. At 1A, you will have 2.8V on MOSFET and 8.6V on the coil. Aug 24 '14 at 8:44
• Also R3 is not needed because inrush current is already limited by R2. Aug 24 '14 at 8:54
• @jippie No idea. My best guess, is a meter, or a fuse of some sort Aug 24 '14 at 22:08

From your other question you need to be able to switch 3A and you need to switch 3A so that when the MOSFET is on it isn't going to get too warm so maybe say "it shouldn't dissipate more than 100 mW at 3A. This means its on resistance ($$\R_{DS(on)}\$$) is a maximum of:

$$\\dfrac{power}{current^2}\$$ = 11 milli ohms.

Next you need to be able to turn that fet on down to 11 miili ohms with the gate voltage you have available - maybe this is 5 volts or maybe it's 10 volts - you have to work that out then look at a the graphs on the data sheet for several FETs and decide if the gate voltage you can supply is ok for switching it. Here's an example of the type of graph to look for: -

I'm not saying this FET will do the job, I'm just pointing out the graph you need to look for. On the graph above, with a gate voltage of 3.3 volts and a drain current of 3 amps, the drain source voltage will be about 0.45 volts meaning, the on-resistance is 0.45/3 = 150 milli ohms.

Yes it's a little tricky for a novice but you will be doing your self a favour finding the FET yourself and gaining that knowledge you can use countless times over (well maybe a few!).

Also make sure your voltage rating is well above what the circuit might generate and don't forget a reverse biased diode across the coil rated at least at 3 amps.

• I was actually looking for translations, not an explanation. However, I will keep in mind what you said, and I shall put it to good use. Thanks! Aug 23 '14 at 18:45
• Well, in my book an explanation will stand you in better stead in the long run. Feel free of course to upvote this and the my previous answer if it proved useful enough. Aug 23 '14 at 18:50
• I cannot upvote. I have 12 reputation, and I need 15 to upvote. Anyway do you know the translations? Aug 23 '14 at 18:53
• Like what would the switching voltage be called on a spec sheet? I have looked, and they certainly do not call it "switching voltage" Aug 23 '14 at 18:53
• @CoilKid 'Maximum current' is I_DS which means drain-source current. 'Maximum voltage' is V_DS which means drain-source voltage. 'Switching voltage' is V_GS(th) which means threshold voltage. In practice the gate voltage must be higher than this, like 10V for 'standard level' or 4.5V for 'logic level' MOSFETs, as shows the above picture. Gate capacitance affects the speed of turn-on and turn-off because you have to fill the gate with electrons or holes and it takes some time. More often it is defined as total gate charge. Aug 23 '14 at 19:31