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using small signal mosfet without gate resistance

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olemp

olemp

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what happens if we use a smal signal mosfet(like BS170) without gate resistance.
think that we are using it with pic mcu like here:
**broken link removed**
it will probably cause to current spikes...is it reliable? what is the restriction of using it without gate resistance. for example can we use it to control the speed of a small fan in 10khz ??.
 

Speaking for BSS89 small signal nmos, If I do not use gate resistor then the transistor is more likely to be damaged. I throw away tens of these mosfets to find if the problem realy comes from unused gate resistors...and now I am sure....
 

There are no restrictions, you can use it without gate restistance (mosfets are driven by voltage, so you dont need resistanse to limit current).


Regards.


Mr.Cube
 

mrcube_ns said:
There are no restrictions, you can use it without gate restistance (mosfets are driven by voltage, so you dont need resistanse to limit current).
Click to expand...

but their inputs behave like capacitor. so without resistor instantaneous current passes....at least this is true for power mosfets but i don't know small signal mosfets.....
i have simulated it pspice..
also an important parameter is output resistans of the pic mcu i/o... but i could't find this value.
here simulation result. puls with of the current is 100 picosecond:))
 

but their inputs behave like capacitor. so without resistor instantaneous current passes
Click to expand...

Yes, but this capacitance is very small, otherwise mosfet will not be fast as they are.

Resistor are usualy put in gate circuit becouse there are posibility to get oscilations on high frequences, so those resistance and that gate capacitor makes input low pass filter => means that for small mosfets and for normal use you dont need resistace (you can put 10-100Ohm to be 100% sure but everythig will working nice even without resistance).


Regards.


Mr.Cube


BTW. There are lots of other parsit efects which are not included in Pspice simulation, you make, so be very carefuly with pspice results.
 

    olemp

    olemp

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