In some high-frequency switching circuits, the Miller effect of MOSFETs has annoying drawbacks such as extending the switching frequency, increasing power consumption, and reducing system stability. As shown in the figure below, there is a flat small step between t2 and t3, and the blue line represents the "Miller plateau".

Conduction of MOSFET (Miller effect):

When the MOSFET is turned on, Vds starts to decrease, and Id starts to increase, at which point the MOSFET enters the saturation region. However, due to the Miller effect, Vgs does not continue to rise for a period of time, by which time Id has reached its maximum value, while Vds continues to decrease until the Miller capacitance is charged, causing Vgs to rise again to the value of the driving voltage. At this point, the MOSFET enters the resistive region, and Vds drops completely, ending the conduction.

Miller Effect: The Miller capacitance prevents the rise of Vgs, and consequently the fall of Vgs, thereby prolonging the duration of losses, increasing the losses.

The Miller capacitance shown in the figure is labeled.

Cgs: GS parasitic capacitance

Cgd: GD parasitic capacitance

Input capacitance Ciss = Cgs + Cgd

Output capacitance Coss = Cgd + Cds

Reverse transfer capacitance Crss = Cgd

The Miller effect refers to the amplification effect of the equivalent input capacitance value under the action of reverse amplification between input and output, forming a Miller plateau.

Disadvantages of Miller Effect:

As shown in the first graph, under inductive loads, the switching process of the MOSFET is significantly prolonged due to the Miller effect, resulting in a longer overlap time between the D and S poles of the MOSFET, leading to greater conduction losses.

However, since the Miller capacitance is inevitable due to the manufacturing process of the MOSFET, it cannot be completely eliminated.

But we can reduce the impact of the Miller effect by reducing the gate resistance Rg.

As can be seen, the smaller R1, the faster the charging of gs, and the faster the MOSFET turns on.

However, is the Miller effect really useless?

We know that everything has two sides, and the existence of the Miller effect is no exception.

We can use the Miller effect to achieve the purpose of circuit soft start.

By increasing the gate resistance of the MOSFET and paralleling a large capacitor between the G and D poles of the MOSFET, we artificially extend the Miller step.

The circuit shown in the figure below adds a gate resistance and a parallel capacitor between the G and D poles, increasing the Miller step, and transforming the output waveform into a triangular pulse.

That's all for this issue.