1 Mar 01:15
Re: [EE] Datasheet confusion
Richard Prosser <rhprosser <at> gmail.com>
2007-03-01 00:15:29 GMT
2007-03-01 00:15:29 GMT
Marcell, IIRC the threashold voltage is controlled by doping levels and the like. So there is no real problem getting "logic level" FETs to control small to reasonable currents. The tradeoff is mostly in Ron (so there is a limitation here to some degree) and the maximum voltages the device will withstand. Trying to find a logic level fet for higher voltages becomes difficult and normally you'd have a separate driver chip or circuit and drive the fet (MOSFET) at 10-15V or so. The other thing to watch is the switching time. This is mostly determined by how fast you can charge or discharge the gate capacitance. FETs (particularly MOSFETs) are very capacitive so moving a lump of charge quickly directly via a microcontroller port limits the available on/off times. Again, supply voltage comes into it due to the effect the voltage gain has on "miller" capacitance so again higher voltages are more difficult to switch quickly. If your switching time is too slow, then the device is operating in a linear zone for a while and can generate a lot of heat. RP On 01/03/07, Marcel Birthelmer <marcelb.lists <at> gmail.com> wrote: > On 2/28/07, peter green <plugwash <at> p10link.net> wrote: > > > 1) The HFE is irrelevant. Either the voltage is high enough to turn it > > > on, or not. > > with fets you can still get a gate voltage that is high enough to let the fet conduct some current between source and drain but not high enough to allow your loads full current. > >(Continue reading)
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