maychang

LuJianchang posted on 2021-2-6 17:25 Does Miller capacitance refer to Cgd?

It does not refer to Cgd, but Cgd multiplied by the voltage gain of that stage.

I once posted a thread about Miller capacitors in this forum, but I don't know if it's still there.

maychang

LuJianchang posted on 2021-2-6 17:25 Does Miller capacitance refer to Cgd?

Miller capacitance is Cgd multiplied by the voltage gain, so the MOS tube source follower (common drain circuit, the voltage gain is approximately 1) does not have any Miller capacitance (multiplying by 1 is the same as not multiplying). However, the Miller capacitance of the common source circuit cannot be ignored.

LuJianchang

maychang published on 2021-2-6 18:17 Miller capacitance is Cgd multiplied by the voltage gain, so the MOS tube source follower (common drain circuit, the voltage gain is approximately 1) does not have any Miller...

After reading the talk about Miller effect you posted, I have a certain understanding that the Miller capacitance in the common-source amplifier circuit seems to increase by K+1 times (K is the amplification factor), but this is analyzed when the input signal is an AC signal, and the input signal of the circuit posted by the OP is a DC signal. Obviously, the Miller capacitance is Cgd multiplied by the voltage amplification factor, which is not applicable in this circuit.

maychang

LuJianchang posted on 2021-2-7 13:25 After reading your article on Miller effect, I have a certain understanding of the Miller capacitance in the common source amplifier circuit, which seems to increase by K+1 times (K is the amplification factor).

"But this is an analysis when the input signal is an AC signal, and the circuit input signal posted by the OP is a DC signal. Obviously, the Miller capacitance is Cgd multiplied by the voltage gain, which is not applicable in this circuit."

This matter needs to be discussed in two steps.

maychang

LuJianchang posted on 2021-2-7 13:25 After reading your article on Miller effect, I have a certain understanding of the Miller capacitance in the common source amplifier circuit, which seems to increase by K+1 times (K is the amplification factor).

First, the original poster is talking about "quickly shutting down the MOS tube". "Shutdown" is an AC process. The MOS tube must pass through the linear state from fully turned on to fully turned off (in the switching power supply, this linear state consumes a lot of power, so it must be "quickly shut down"). The so-called Miller effect occurs in the short process of the linear state. In the linear working process, there must be voltage amplification, and Miller capacitance is generated in this process. There is no Miller capacitance when it is fully turned on, and there is no Miller capacitance when it is fully turned off.

maychang

LuJianchang posted on 2021-2-7 13:25 After reading your article on Miller effect, I have a certain understanding of the Miller capacitance in the common source amplifier circuit, which seems to increase by K+1 times (K is the amplification factor).

Thank you very much for reading my article "Miscellaneous Talks on the Miller Effect".

The attached picture in that post is a straight line with an inclined angle, followed by a straight line that is approximately horizontal, and then another straight line with an inclined angle. The first and last two straight lines indicate that the charge flowing into the gate changes with the input voltage, and the approximately horizontal line indicates that the charge flowing into the gate hardly changes with the input voltage. The charge flowing into the gate hardly changes with the input voltage, which means that the gate-to-ground capacitance is very large. The gate-to-ground capacitance becomes very large, which means that the Miller effect is at work during this period. And this period is exactly when the MOS tube is in an approximately linear working state.

LuJianchang

maychang posted on 2021-2-7 14:27 Thank you very much for reading my article "Miscellaneous Talks on the Miller Effect". The attached picture in that post is a slanted straight line, followed by a nearly horizontal...

The "approximately horizontal straight line" is because during the shutdown process, Vgs is fixed at the Miller platform, the Vd voltage begins to rise, and Cgd discharges to the ground through the gate effective drive resistance, right?

The figure is Figure 8-12 in Chapter 8 of Mastering Switching Power Supply Design (2nd Edition), which shows the second stage of the shutdown process.

maychang

LuJianchang published on 2021-2-7 15:00 The “approximately horizontal straight line” is because during the shutdown process, Vgs is fixed at the Miller platform, the Vd voltage begins to rise, and Cgd is effectively driven through the gate...

The "Miller platform" is first mentioned in Figure 8-8 "The second stage of the conduction process" in Chapter 8 of the book. In fact, the "Miller platform" is the period when the MOS tube is in a linear amplification state, that is, the period when Cgd is amplified to become the Miller capacitor.

If it is from the off-state to the on-state, Cgd and Miller capacitance are charged by the driving signal source. If it is from the on-state to the off-state, Cgd and Miller capacitance are discharged by the driving signal source. If the internal resistance of the driving signal source is large during the falling process (relative to the rising process), then of course it can only be discharged through the driving resistor as you said (this is the case when the driving signal source is an emitter follower).

maychang

LuJianchang published on 2021-2-7 15:00 The “approximately horizontal straight line” is because during the shutdown process, Vgs is fixed at the Miller platform, the Vd voltage begins to rise, and Cgd is effectively driven through the gate...

If the signal source driving the MOS tube is an emitter follower, then the internal resistance of the signal source is very small when the driving tube is turned on, and the internal resistance of the signal source is quite large (equal to the emitter load resistance of the driving tube) when the driving tube is turned off.

QWE4562009

maychang posted on 2021-2-7 14:27 Thank you very much for reading my article "Miscellaneous Talks on the Miller Effect". The attached picture in that post is a slanted straight line, followed by a nearly horizontal...

It's obviously a classic, how can you say it's a cliché?

QWE4562009

maychang posted on 2021-2-7 14:27 Thank you very much for reading my article "Miscellaneous Talks on the Miller Effect". The attached picture in that post is a slanted straight line, followed by a nearly horizontal...

It's obviously a classic, how can you say it's a cliché?

maychang

QWE4562009 posted on 2021-2-23 12:01 It is obviously a classic, how can it be called a setback

"It's obviously a classic, how can you say it's a cliché?"

What is "Cuowen"?

lkh747566933

As a newcomer to the automotive electronics industry, I would like to read the posts of the great masters and actively learn and improve myself.