CMC Choke Current specification

[chaitanyab]

Hi,

I am bit confused the DC current specification in Common mode choke in many datasheet, does it refers to Differential mode current the CMC choke can handle? or Common mode current?
For instance see the snapshot for RB series choke.

 

[FvM]

I don't see a DC current specification in the list. Rated currents are differential mode RMS currents. Common mode current isn't specified in the RB6122 datasheet, there's however an application note https://www.schaffner.com/fileadmin...ation_note/Schaffner_AN_RB_common_chockes.pdf

 

[chaitanyab]

Hi FvM,

Thanks.

The reason I am asking is that for designing a custom CMC choke I need to consider pretty high saturation differential current for the CMC choke as compared to the CM mode current. It means my inductance should not drop for the DM mode current, however the DM mode will not see the CM mode inductance, so what is the use of designing inductor with saturation current for DM mode and specifying it in datasheet? I understand DM current should not burn the inductor, so if we select the wire gauge that can handle the DM mode current I think it should be sufficient, the entire CM mode inductance need not remain constant for DM mode current, because I think it can be allowed to drop as the CM mode currents would never be as large as DM mode currents.

The problem is to cater for DM mode current and still not saturate the inductance we need bigger core or bigger air gap which means more turns and DM current handling wire gauge for DM mode current. However in the circuit operation the DM mode will not see any inductance (or very little leakage for that matter) .

So what am I missing here?

PS: Just curious which page exactly in the appnote it says it is Differential current?

 

[FvM]

CM chokes are build with high µr cores and no air gap. Respectively the CM saturation current is rather low, some 100 mA for the discussed RB series. Knowing this, are you seriously asking for a clarification that the rated current spec means differential current?

The interesting point is why DM saturation limits apply and how they are constituted. We use to analyze leakage inductance as primarily determined by the winding geometry, not significantly affected by core permeability, in the first approximation a property of the air core transformer. But due to the asymmetrical geometry with separated windings, the DM field lines are not entirely running in air but also crossing the core. At high inrush or peak DM currents, core saturation can occur.

Saturation won't occur with an ideal toroidal winding geometry. But CM choke design prefers separated windings, to increase DM (leakage) inductance for filtering purposes and to achieve higher voltage strength.

--- Updated Feb 9, 2022 ---

Here's the complete information for the chokes listed in post #1.
Core is MnZn ferrite with µr of about 6000.

CM currents flowing through one or more windings without cancelling the flux to zero (limit ICMmax) or high peak currents running in differential-mode through both windings (limit IDMmax) like an inrush current can cause saturation. Table 1 shows the limits specified at 25 °C and 100 °C ambient temperature:

ICMmax is the maximum allowed current flowing through one winding without causing saturation. IDMmax is the maximum allowed current flowing back and forward through both windings without causing saturation. DC currents or the peak value of AC currents should below this level under all operation modes. For applications with ambient temperatures >125 °C different core types can be optionally used.

 

[Easy peasy]

For flat Dc current - it is the same as RMS, the values in the table are for the diff current - i.e. send and return, max CM current would be that current that sends the core to say 50 - 75% of Bsat - one has to allow for some AC CM currents.