Common-mode choke standard ranges

Experience gained from working with customers for over twenty years, to find the best balance between performance and cost, allows us to offer standard ranges of chokes optimised for particular applications, with well defined inductance and current capabilities.

Single-phase

WB range

Low cost, open-construction chokes, with current ratings from 0.3A to 85A. Leadout options are available for most types to allow either vertical or horizontal mounting, and vertical or horizontal mounting bases are available for some types.

VCM range

Similar to the WB range, but mounted on an industry-standard vertical mounting base. Single-layer windings are used to minimise choke self-capacitance, for improved high frequency performance.

Three-phase

WT range

Low cost, open-construction chokes, with current ratings from 2.5A to 76A. Leadout options are available for most types to allow either vertical or horizontal mounting, and horizontal mounting bases are available for some types.

Benefits

Ferrite toroidal cores are used, to give low flux leakage, for improved emc performance. The choke ranges shown on this summary are all based on 6,000 permeability materials, selected to give an optimum balance between low frequency inductance, hold-up of inductance at high frequency, and resistance to saturation effects at high temperature.

Alternative types with different characteristics are also available, based on:-
a).   2,000 or 4,000 permeability ferrites for extended frequency response and improved saturation characteristics.
b).   10,000 permeability ferrites for increased low frequency inductance at the expense of poorer high frequency performance and increased vulnerability to saturation effects.

Common-mode emc chokes

Common-mode noise results from interference currents flowing from line to earth or neutral to earth. A major source of these interference currents is leakage capacitance from direct-off-line power supply semiconductor switches to an earthed heatsink.
The particular nature of this interference, occurring from L-E and N-E but not from L-N, allows the use of phase-cancelling "bucking" windings. These are arranged so that the main phase currents are cancelled in the choke, and therefore give no line frequency magnetisation of the magnetic core.

This allows the use of high permeability ferrite cores, which can then give very large values of common-mode inductance, 0.5mH to 30mH+ according to current rating, in a small size. These large values of inductance are needed to maintain the LxC product required for filtering, where the values of "Y" capacitor that can be connected from L-E and N-E are severely restricted by earth leakage current requirements.
Although the magnitude of the harmonics of the interference generating switching waveform will decrease as the interference frequency increases, the capacitive impedance coupling into the earth line also reduces with frequency, allowing interference currents to be generated at frequencies well into the MHz band. This requires the use of a suppression choke which will not only have a high inductance at around 0.15MHz but which will also maintain its inductance to higher frequencies up to the MHz region.