Help required for choosing a transistor:

Hi guys, I need some advice on how to choose a transistor for a project. The transistor will act as a switch to control a current of about 6A for a very short duration (on the order of a few millisenconds). (as some of you might know, I'm trying to build a coilgun). What kind of transistor should I use? BJT, FET, MOSFET ? (please reasons for selecting a particular type as well). I'd appreciate a specific part number very much indeed...

Thanks

go to digikey.com
search for “logic level n-channel mosfet”
select “single FET transistor”
filter down to find what you need.

Suggest N-channel to sink current to ground. They seem to be more efficien than trying to source that same current from a high voltage source.
Want Low Rds, low input capacitance for low power loss in the device, and fast switching times.
Also search for MOSFET driver to turn that input capacitance on/off fast.

um.. sorry, I'm new to FET terminology. I read up about them in an allaboutcircuits.com text book. Could you explain the last paragraph again?

Suggest N-channel to sink current to ground. They seem to be more efficien than trying to source that same current from a high voltage source.

An N-channel MOSFET will go between your load and ground -- on the "low-side". The gate voltage required to control it is a relatively small voltage near ground (eg: 5V) even if the voltage across your load is 30V or 60V or something. "Logic level" means that it is feasible to use 5V to control the gate.

Want Low Rds,

RdsON is the resistance from drain to source (across the current carrying junction) when the MOSFET is fully "on". This is typically in the few dozen to few hundred milliohm range -- ie: way under one ohm.

low input capacitance for low power loss in the device, and fast switching times.

The gate of a MOSFET acts like a little capacitor that has to be charged up to get the MOSFET to switch on and discharged to make it go off. The larger that capacitance is, the longer it takes to go from on-->off or off-->on.

When the MOSFET is partially on, the resistance from drain to source is high and the part will see a voltage drop and will dissipate heat. A part that can carry 10A fully on, might have trouble with 200mA when partially on. You want to keep the amount of time the MOSFET is partially on to a bare minimum when you're switching current.

Also search for MOSFET driver to turn that input capacitance on/off fast.

Since your puny Arduino logic level output only sources and sinks a dozen or so mA, you may want to employ a driver circuit to supply the larger current needed to charge up a large MOSFET's gate as quickly as you need, to get the on-->off and off-->on transitions very very short.

Well, that brought out a nice bout of questions and answers, one of the best I've seen in a while!

Thanks a lot , Gardner. I really appreciate CrossRoads and your help.
Now I have to read up about driver circuits for MOSFETs.

By the way, is there any particular reason why BJTs aren’t used for this type of application?

Oh.. and I'd also like to know, roughly, the amount of time it takes to charge up the gate 'capacitance'. Would it be on the order of 1 ms (milli second)?

mahela007: By the way, is there any particular reason why BJTs aren't used for this type of application?

Mainly, the issue is that they have a large ON resistance, even when saturated. The on resistance might be 1.5 to 2 ohms. For parts carrying a lot of current, a good deal of your power is coming out of the transistor as heat, requiring large heat sink capacity, bulk weight cost and lowering overall efficiency.

There are BJT based H-bridge and motor driver circuits around, but IMO they are all outdated and should not be used. The L298N, in particular, should be permanently retired.

Thanks for the explanation. I've got one more question, however, and I don't think it deserves it's own thread. For my application (low current i.e 6A coilgun), which would be better? A MOSFET or an IGBT? Thanks in advance.

mahela007: For my application (low current i.e 6A coilgun), which would be better? A MOSFET or an IGBT? Thanks in advance.

I don't know much about IGBTs, really. But I think they are more aimed at high power applications -- thousands of volts and hundreds of amps. Personally, I would not bother with such exotic parts unless you need to switch over about 75V. Paralleling four or five power MOSFETS should get you in the 100A range, if you need larger current than a single part handles easily.

Ok.. thanks. But if my coil gun blows up, I'll hold you responsible.. (Joking, of course :D)