Common Mode Noise - Why the currents have the same direction on the two cables?

[diamadiss]

Hi,
I have a quiry about how can the two currents can exist as long as have the same direction for instance in Line and Neutral in a power supply (they created from an electromagnetic noise/signal which will add the same noise->voltage->current at both wire L & N)?
A current to flow must have to start from a high voltage point and to return to another lower voltage point... if these two currents are the same and have the same direction they will not "repelled" each other?
I read on the internet that they will flow both from a stray/parasitic capacitance to the PE(protective earth) but how a parasitc capacitance can exist from the supply PCB to the PE cable?
Also in order to eliminate the common mode noise they add two capacitors between Line & PE and between Neutral & PE in series with a common mode choke (filter) but if in "theory" the two currents have the same flow and will not have any path to flow why to place these two capacitors across PE? I hope I didn't tire you with my long text.

 

[betwixt]

You are confusing the current paths, they differential (line power) and common mode (noise) are following different routes.

The differential is the main power feed, current passes from 'AC INPUT' Line to 'LOAD' Line and by virtue of being a loop, from 'LOAD' Neutral to 'AC LINE' Neutral. The flow reverses along the same path as the AC polarity changes.

The Common mode noise isn't differential (by definition!) the same noise is on both Line and Neutral at the same time and with the same polarity with respect to reference GND.

To a large degree, differential noise can be cancelled by the choke because the current flows through the windings in such a way that the magnetic fields from each coil cancel each other out. CX1 and CX2 are to help the cancellation by ensuring there is a low impedance to the noise at each side of the choke.

The common mode noise is present at the same time and polarity after the choke and CX2 because the whole circuit before it is carrying noise voltages, there is nowhere across the line to dump it. That's where CY1 and CY2 come into play, there is one on each line to conduct noise to the GND line.

Brian.

 

[KlausST]

Hi,

Just to add what Brian wrote:

The common mode choke (L_CM)
* is high impedance for common mode current (noise)
* is low impedance for differential mode current (load)
Thus it suppresses noise while it can carry high load current.

Klaus

 

[diamadiss]

Thanks for the replies... if we didn't put a PE ground (or is the paracitic capacitor between PCB and PE the probblem?) on the device we will have common mode currents? I am trying to understand what problem will create on the PCB?

 

[FvM]

A common mode noise source needs a finite impedance to ground to have an effect. It can be e.g. formed by the parasitic capacitance of a chassis or ground plane, a real PE line or another external connection.

A circuit of infinitisemal size without external connections can't cause common mode noise.