Consider an NMOS transistor which is turned off when there's 0 V on the gate, and turned on when there's 5 V on the gate.
I've been told that a tiny bit of current flows into the gate of a MOSFET transistor... well where does this current go? Does it come out the drain, or the source, or does it split between the two?
The gate has a significant capacitance, any current that flows is either charging or discharging that capacitance.
It comes back out exactly where it went in. Up the gate pin. You have to both charge and discharge the gate.
Graham
The other responders told you what that current was doing, but didn't answer the question.
Unfortunately, the answer you want is not simple. It really depends heavily on the surrounding circuit. Usually, the driver that is charging and discharging the gate is tied in some way to the source pin, so the circuit loop that carries this current passes through that pin.
However, if the drain of the MOSFET has a low impedance path to the same supply as the gate driver circuit, some of the current will flow there, too.
-- Dave Tweed
To add a bit of clarification:
The gate capacitance is between gate and source, so, while you're charging that capacitance, the charging current is going in the gate and coming out the source. Discharge is the reverse.
The actual electrons are going in the gate while charging, and back out the gate on discharge.
------------------------------------------------ Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA
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The current is needed to form/discharge voltages on the circuit capacitances.
Current i = dQ/dt Amps, delta-coulombs delta-seconds Charge q = CV Coulombs Farads Volts
There is also a leakage current, very small, that bleeds through imperfect insulation in the capacitive structures.
Other losses, in series resistance, will affect energy transfer.
RL
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