Do FETs that could be controlled from PIC output exist?

[nikens]

FET on PIC output

I found this diagram

on this site and whis to know if you don't need a reverse biased diode like you need when using a BJT to eliminate spikes at startup and shutdown operation.

PS author mention that FETs normaly need voltages higher than 10V at the gait to conduct through drain - source. Do FETs that could be controlled directly from PIC output exsist?

 

[erikl]

FET on PIC output

You'd need a logic type n-channel MOSFET for this, i.e. one which already can switch the necessary current with the "high" voltage output of the PIC. Search G00gle for "logic mosfet".
BTW: A gate series resistor of about 1kΩ between the resistor to GND and the MOSFET gate would be welcome!

 

[dick_freebird]

FET on PIC output

PIC outputs are probably ~5mA DC, 50mA peak current
and this will be pretty weak for a power MOSFET's Cgate,
resulting in slow transitions and some excess pulsed power
dissipation in the FET, a low max switching rate and so on.

An option might be a solid state relay (optically isolated
power MOSFET) if you do not need high speed and can find
a DC rated one (AC SSRs are triac based, no good for driving
DC loads).

Of course a power MOSFET driver IC ought to set you back
a whole buck or two, which might be a small price to pay for
"fuggedaboudit".

That's "forget about it", for our friends who don't speak
New Jersey.

Now, at the load (which is inductive), you want that reverse
diode to provide a freewheel current path when the MOSFET
has shut off. Many FETs are not designed for reverse,
body-drain current. A back-diode across the motor will let
the armature current circulate like it wants to without
developing crazy high flyback voltage spikes.

 

[checkmate]

Re: FET on PIC output

Power FETs have their on resistance usually characterized at 10V drive. You can drive drive it at 5V, but with a much higher on resistance and slower slew rates.

Logic FETs can have much lower thresholds, but that will mean lower current capabilities and breakdown voltages. You can string several of them in parallel though if current is your bottleneck.

True that the body diode can be used for recirculation. The question lies in whether the FET can take the additional power dissipation?