Chokes are commonly used in electronic circuits to block signal frequencies above a desired range, while at the same time allowing DC or low frequency signals to pass. Thus, chokes have been employed to prevent electromagnetic interference (EMI) from disturbing various electronic devices. EMI is generated, for example, as a byproduct of switching regulators which have current and voltage waveforms with fast rise and fall times. Because switching regulators are typically contained in power supplies, EMI may be transmitted throughout an electronic device via power supply conductors. Excessive EMI can lead to logic errors in a computer and can cause interference with other adjacent electronic components.
A choke is typically provided by a magnetic core through which, or around which, conductors or windings are positioned. Thus, a typical choke defines first and second mutually coupled magnetic paths. A choke may be schematically represented as a low pass filter. For any choke to function as intended, its inductance or inductive reactance, should not fall below a specific minimum, even though the current in a winding rises to a maximum value. Beyond the maximum current value, the reactance falls off appreciably. Thus, the choke's ability to impede interference signals drops, thereby allowing passage of these signals. It is therefore desirable to prevent the choke from being driven into saturation.
Ferrite materials are commonly used as the core material for many chokes because, among other reasons, ferrites have sensitive magnetic-frequency relationships. The ferrite material used to form the choke will determine which signal frequencies the choke will attenuate. Most ferrites having suitable inductance values for choke applications saturate at less than about 4,000 Gauss. Accordingly, when configured differentially, ferrites have a relatively low current carrying capacity at low frequencies before the choke is driven into saturation and its impedance level deteriorates at a desired frequency.
The techniques normally used to prevent this saturation are to provide a core air gap, use a larger cross-section core, or simply limit the allowable current. An example of a choke with a core air gap is illustrated in U.S. Pat. No. 5,115,059 to Covi et al. The choke described by Covi et al. is used for suppressing both differential and common mode noise on DC power supply conductors. The choke includes two complementary E-shaped ferrite halves mutually defining a pair of slots through which the conductors pass.
One style of ferrite noise suppressor includes individual electrical conductors extending through respective individual openings in a ferrite body, as shown, for example, by U.S. Pat. No. 4,758,808 to Sasaki et al. U.S. Pat. No. 4,785,273 to Doty discloses a transformer with a generally cylindrical ferrite body having a pair of opposing slots extending therethrough. A stripline formed by a pair of planar conductors extends through the slots.
If the electrical conductors are configured for common mode operation, then saturation problems can be mitigated or averted. In other words, bringing the high side and ground return through the same core annulus produces opposing fields in the core which tend to cancel. For example, large ferrite sleeves have been installed surrounding parallel input/output conductors and are able to function in this manner to suppress EMI entering or exiting an electronic device. The parallel conductors create electric fields in the ferrite sleeve which tend to cancel each other. In other words, a common mode choke configuration allows a choke to function with high currents which would saturate a differential choke.
Chokes are commonly applied directly to printed circuit (pc) boards. However, it is not feasible to use chokes with large cores or a gapped section for pc board applications, rather, ferrite beads may commonly be used on individual conductors. Similarly, U.S. Pat. No. 4,656,451 to Pomponio discloses two stacked ferrite beads, with the ferrite for each bead selected to impede a different signal frequency. Two parallel passageways extend longitudinally through both beads and a U-shaped conductor is inserted into the two passageways. Such ferrite beads are very effective in pc board applications unless the current becomes greater than a saturation level. This condition is frequently the case in power supplies or converters where individual conductors (differentially) may handle several amperes.
In applications where common mode filtering of high speed signals is required as, for example, in twisted pair Ethernet networking signals, series ferrite beads have no ability to filter signals based on the mode of the signal traveling along a pair of conductors. Intended signal currents are usually defined as in a differential mode, where signal currents are equal in magnitude and opposite in direction on the pair of conductors. Unintended or noise currents are generally common mode, that is, the currents are equal in magnitude and flow in the same direction. Because of their construction, series ferrite beads cannot distinguish between these intended and unintended signals. Instead, both the intended differential mode signal current and the unintended common mode noise current encounter an impedance.
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