common mode choke for 25Khz spike common noise
I am trying to calculate how much henrys I would need for a common mode choke to attenuate an spike that happens a 25Khz. It is for sure common mode noise as it appears in Live and Neutral. My understanding is that the bigger the better in terms of common mode choke, but this logic doesn't follow any sort of calculation and it is where I would like to get because there are always space constrains. Thanks
Winsu.
Do you need a choke... or would coupled inductors suit your purpose? It resembles a transformer. You don't necessarily need a large Henry value. Connect the windings in the manner needed to inhibit current at your problem frequency.
Hi,
A total cancelling is impossible.
Thus you need to consider what residual spike value you can tolerate...and you need to know how big the spike was before.
Additionally you need to know the waveform if the spike...and the common mode load behaviour.(*)
But for buying a part you also need to know the expectable max load current.
*****
Some more depictive:
Just imagine a simple circuit:
* a spike generator
* and a choke
With just these two parts: no attenuation happens.
For sure the choke will become high ohmic, but as there is no load_for_the_spike, there will be no current. No current means zero voltage drop in the choke.
So you need some additional parts for this simulation.
One important part is: the reference. Where is the spike (generator) referenced to? Usually earth_GND.
Now youvare able to measure the spike voltage before and after the choke - referenced to earth GND.
This also means: your "real device" needs some earth GND reference, too.
Without this reference your "device" will not recognize the spike at all...means there is no common mode problem at all.
(Let's assume you use your music player in an aeroplane. And the aeroplane gets hit by a thunderbolt. The voltage reference is earth_GND. The aeroplane gets charged to millions volts referenced to earth_GND. And your music player also gets charged to this voltage - referenced to earth_GND. But since your music player has no true reference to earth_GND there is no problem for it. The music player will not even recognize the millions voltage)
For your common mode spike problem this means this earh_GND connection may be just stray capacitance in the pF range, or an installed C, RC, or any other circuit. Usually common mode filters include them. This are the "Y" safety capacitors.
Now you have a closed loop.( Earth_GND --> spike --> choke --> RC --> back to earth_GND)
Now you may rung simulation software on this circuit.
You will see:
* the higher the spike pulse voltage the higher the spike voltage at your load (across the RC)
* the higher the choke value the lower the spike voltage at your load
* the bigger the spike pule_width the higher the spike voltage at your load
....
Klaus