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Thread: FET component evaluation
face BY : Russell McMahon email (remove spam text)

My solenoid load is (24VDC, 800mA). The PWM is 20kHz, (pic driven).  The
opto-coupler output is 5 volts.  The FET's high level input is 5 volts.
Will this opto drive the FET properly? (y/n) Cost is not an issue, but
performance is.  I will address filtering later!

FET would need to be a "lohic level" device to be driven by 5 volts. ie look
at Vth spec and rating curves. Device should be fully turned on at Vgs=5v
AMD have a low enough Rdson to meet your need. For gate drives not much
above Vth FETs will suually exhibit a "current source" operation at far less
than their ratewd current. Can be useful if you want it and very annoying if
you are not expecting it.

FET needs essentially zero current when operated but needs current to
charge/discharge gate capacitance. This is typically arouind 1 mA at 29 kHz
foir typical FETS as a VERY ROUGH guide. (See data sheet for gate charge for
your FET to calculate real value).

Why are you PWMing a solenoid?  - it surely will not be responding at 20 KHz
:-) Back EMF will need to be accounted for.


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