Two issues concerning P-FET connected to VCC

p fet basics

hello,

I am shorting GATE and SOURCE of P-FET and connected to VCC. And Drain is connected to ground. Pupose of this circuit is when if supply is not there then i would like to ground the VCC pin.
1. Is it required to connect pull down at FET gate?
2. If i do not connect pull down whether FET will be ON when VCC = off?

niks

connect p fet

Hi Niks,

It's a rather unusual question.
You can't turn the fet off when the supply voltage is on because you gonna have
to apply a higher voltage than the supply voltage to the gate of the fet. Same
thing if you would use a N-channel fet connected to gnd, you would have to
apply a negative voltage to the gate of the fet to turn it off.
If gate and source are connected, the current trough the fet will be equal to Idss. If you use a P-channel fet you should connect the gate to Vcc and insert a resistor in series with the source to limit the current when the supply voltage is on, there
will be always that resistor in series with the channel resistance of the fet.
There will never be a short circuit. I (try to) think that it's a bad idea and you
should abandon it.

on1aag.