The turn ON and OFF times of MOSFET gets affected by its internal capacitance and the internal impedance of the gate drive circuit but it does not affect during steady-state operation.
To understand switching characteristics of MOSFET we can take the simple equivalent circuit for an n-type MOSFET is given below :
Turn ON Process :
- The gate voltage is made positive to turn ON the MOSFET. When the gate voltage has applied the gate to source capacitance CGS starts charging.
- When the voltage reached through CGS certain voltage level is called the Threshold voltage ( VGST ) at the same time drain current ID begins to increase.
- The time needed to charge CGS to the threshold voltage level is known as a turn-on delay time.
- The CGS charges from threshold to the full gate (VGSP) voltage. The time required for this charging is known as rising time (tr).
- During this period, the drain current increases to its full value of ID because of that the MOSFET is fully turned ON.
- MOSFET turn-on time is given by,
tON = t[d(on)] + tr
- The turn-on time can be reduced using a low-impedance gate drive source.
Turn OFF Process:
- The MOSFET can be turned off with a negative or zero gate voltage. Due to this, the gate to source voltage decreases from VI to VGSP.
- CGS discharges from VI to VGSP gate voltage. The time required for this discharge is called a turn-off delay time, during which the drain current also begins to decrease.
- The CGS continues to discharging and its voltage equals a threshold voltage (VGST).
- The time required to discharge CGS from VGSP to VGST is called fall time (tf). The drain current will be zero when the voltage VGS < VGST then it is said to have turned off.
- MOSFET turn-off time is given by,
tOFF = t[d(off)] + tf
MOSFET switching waveform is shown in the figure below :
