Physical Layer - CAN System Engineering - page 65

Fig. 2.22 Simplified equiva-
lent circuit of a common-
mode choke
or coupling coefficient, which depicts the strength of the impact (coupling) of the
magnetic flow to the magnetic flow of the other signal. Through the normalization
on the rated inductance, the sum of the stray factor and coupling factor is always 1.
This means that the stray factor or coupling factor gives indications for the effec-
tive working inductance in case of common-mode operation. Because the impact of
the CMC on the differential signal shall be as small as possible, the coupling coef-
ficient deals as a quality characteristic of the CMC and shall be as big as possible.
In practice, it must be distinguished between two different types of winding
which shows different coupling coefficients. The chokes with sector winding are
similar to the figures shown above (see Fig. 2.20 ). Each coil has its own part or sec-
tor of the core. The CAN_L and CAN_H signal lines are first twisted and then wind
up together around the core in a CMC with bifilar winding. Through the sector-
based layout with a distance between both coils, sector-based CMCs have a smaller
coupling coefficient (typically c ≈ 0.97) than CMCs with bifilar winding (c ≈ 0.99).
Chokes with bifilar winding have, caused by the small distance of the CAN_H
and CAN_L signal lines, bigger capacitances between the CAN signal lines. Dif-
ferent types of cores (in practice, there are ring cores and I-cores) do not result in
significant differences regarding signal quality aspects in the time domain (working
frequency range).
Figure 2.22 shows a simplified but sufficient equivalent circuit for signal integ-
rity considerations in the time domain.
The coupling of the magnetic flows is mentioned as the coupling arch between
the inductances L 1 and L 2 in the equivalent circuit. The practical values of the main
inductance are in the range of 11 and 100 µH (automotive industry). Typically,
CMCs with 51 µH are used in CAN systems (also the automotive industry) and
100 µH in FlexRay systems.
The resistance in serial to the main inductance describes the ohmic load of the
coil. The capacitance Cn represents the sum of the capacitances between coil input
and output (which should be as small as possible). The particular capacitances occur
by distances between the signal lines to each other, distances between signal lines