In a previous video, we mentioned that MOSFETs, unlike bipolar transistors, are voltage-controlled devices. However, in reality, MOSFETs also require current during the transition from off to on due to the parasitic capacitances Cgd, Cgs, and Cds between the various terminals of the MOSFET.

The specifications of a MOSFET correspond to:

Cgd = Crss; Cds = Coss - Crss; Cgs = Ciss - Crss.

Firstly, the conduction condition of a MOSFET is that the Vgs voltage needs to reach at least the threshold voltage Vgs(th), which charges the capacitance Cgs at the gate. Once the MOSFET is fully turned on, it no longer requires current.

So, what is the specific process of MOSFET turn-on?

It is generally divided into four stages:

Stage T1: The drive turn-on pulse charges the input capacitance Cgs of the MOSFET gate-source with Vgs voltage = threshold voltage Vth. Before this, the MOSFET is in the off state. At this time, only a very small current passes through the MOSFET, and the Vds voltage remains unchanged at Vdd.

Stage T2: When the Vgs voltage reaches the threshold voltage Vth, the current ID starts to flow through the drain-source, and the Vgs voltage continues to rise, gradually increasing the current. At this time, the Vds voltage still remains at Vdd.

When the Vds voltage reaches the Miller plateau voltage Vgp, the current ID rises to the maximum value of the load current ID. At this point, the Vds voltage begins to decrease from Vdd.

Stage T3: Now we come to the important Miller plateau.

During the Miller period, the current ID remains constant at the load ID, and the Vds voltage continues to decrease. The height of the Miller plateau is influenced by the load current. The larger the load current, the longer the time it takes for the current ID to reach, leading to a higher Miller plateau voltage Vgp.

Stage T4: When the Miller plateau ends, the Vgs voltage continues to decrease, but at this point, the decrease is very small. Finally, it stabilizes.Generally, after the end of the Miller plateau, it can be considered that the MOSFET is basically turned on.

Why does the Miller plateau occur?

Because when the MOSFET is turned on, Vd > Vg, Vgd first discharges through the MOSFET and then recharges in reverse, taking the current for charging Cgs, causing the Vgs voltage to remain unchanged, thus creating the Miller plateau. In other words, to reduce turn-on losses, it is necessary to minimize the time of the Miller plateau.