Decoupling capacitor at the Drain of N-Channel mosfet when using saturation mode?

Question

I always put a decoupling capacitor at the VCC pin of every component on my pcbs and this question derives from my habit to put decoupling capacitors on every power pin of the components I place.

I wonder about using decoupling cap on the Drain pin on N-Channel Mosfet, when using them in saturation mode (fully ON/ fully OFF).

Lets say I have this circuit, to make the question more specific (as this is the circuit I am working now):

^{simulate this circuit – Schematic created using CircuitLab}

Here is my thought: The Drain pin will originally have the voltage of lets say 12V, but when I give a signal to the Gate of the mosfet (driving it to saturation) the Drain pin will have a near-Zero volts. So the decoupling capacitor seems pointless to me at that point, as I do not need to "de-couple" the Drain of Mosfet from anything. I need it to do the opposite actually, short itself to GND and draw current. Using a capacitor (C1) there will only make the "short" delay some nanoseconds more, since it will have to discharge as well.

My thought is, I do not need to use a capacitor at the Drain pin of a mosfet, when Driving it in saturation mode?. Is there a reason I should use a Capacitor there when I simply need to turn on and off a resistive load?

Answer 1

The capacitor is not necessary and can only degrade performance (speed) of the circuit. In an extreme case (if you toggled the FET at a high frequency), the energy stored in the capacitor could overheat the FET when it is discharged into it.

If there is a possibility of higher voltage, high energy spikes on the drain line, then perhaps the capacitor could be considered as filtering those spikes (when the FET is off), but since your FET is already a 30V device, this is unlikely to be a problem with a 12 V supply.

Note that if your '12 V' is really a 12 V car battery, that '12V' can reach 40 V in certain extreme conditions (load dump), and you really should use a 40 V or higher rated FET.

Be aware also that when an incandescent lamp turns on, there can be a very high (10x the normal value) inrush current when the filament is cold. Be sure your FET can handle that surge.