Hi everyone,
There are a few titles on the form about this topic, but But I did not fully understand this issue especially in the field of power electronics.

What is the common mode voltage?
what is the clear or very simple definition of it?
Why do we need desciribe common mode voltage?
Also why the formula of Common mode voltage is (V1+V2)/2?
Where does this formula come from?
Can I choose any place as a common point? or do I have to choose ground as a common mode point?

Thank you for all the answers
best regards

 

As the name implies "common-mode" voltage is the voltage that's common to more than one line (typically a differential pair) to the circuit common (ground).

Common-mode voltage can disrupt the normal (differential-mode) signal so it's important to know its value and whether the circuit can ignore this voltage (its common-mode rejection).

It should be apparent that the formula (V1+V2)/2 gives the common voltage to the two differential signals V1 and V2.
Thus, for example, if V1 =2V and V2 =3V then the differential-mode (desired signal) voltage is 1V and the average common-mode (undesired signal) voltage is 2.5V.

 

Hi Crutschow,
I am very pleased with your encouragement, thank you so much.
But, there are still I do not understand clearly.
How do I choose the common mode point?
Also why the formula of Common mode voltage is (V1+V2)/2?

thank you so much.

 

How do I choose the common mode point?
Also why the formula of Common mode voltage is (V1+V2)/2?

The common mode voltage is referenced to circuit common as I previously stated.
What do you mean "choose"?

Why not?
The common-mode is the voltage that is common to both V1 and V2 and that is the formula to calculate it.
What formula would you use?

 

Hi Crutschow,
I am very pleased with your encouragement, thank you so much.
But, there are still I do not understand clearly.
How do I choose the common mode point?
Also why the formula of Common mode voltage is (V1+V2)/2?

thank you so much.

From Razavi:

 

The common mode voltage is referenced to circuit common as I previously stated.
What do you mean "choose"?
Why not?
The common-mode is the voltage that is common to both V1 and V2 and that is the formula to calculate it.
What formula would you use?

Thank you for answer.
it is ok so, Is there a common mode voltage in every circuit?
I mean that how can I determine the circuit common because I conufuse that some aplication espacialy inverter aplication I sow the circuit common is mid point of two capacitor or netural point of DC source.

How can I prove the formula?

 

From Razavi:

View attachment 116249

Thank you so much for your reply.
What I understand from your answer is that in every two input or output circuits have a common component and a differantial component. Is it true?

 

Is there a common mode voltage in every circuit?

No, only for circuits with two inputs or outputs.
In switching power supply circuits there can also be common-mode current between the input and/or output and common (that is current flowing in the same direction in both lines).
This is sometimes suppressed (to reduce EMI) with a common-mode choke connected in series with the signal line and the common line, which readily allows differential (normal) currents to flow but has a high inductive impedance to the common-mode current.

 

No, only for circuits with two inputs or outputs.
In switching power supply circuits there can also be common-mode current between the input and/or output and common (that is current flowing in the same direction in both lines).
This is sometimes suppressed (to reduce EMI) with a common-mode choke connected in series with the signal line and the common line, which readily allows differential (normal) currents to flow but has a high inductive impedance to the common-mode current.

Thank you for all the answers you gave me.I think I started to understand

 

enes246: if you attach the schematic here, someone may explain it in detail and it may easier to understand.

 

I add a pic as an example for the common mode analaysis. where is the circuit common for this inverter O1 or O2?