The addition of both U.S. and International regulations has increased the need to effectively filter the main power line. In order to accomplish this, both the common-mode and differential-mode (normal-mode) noise must be controlled. Common-mode noise is interference that is common to both the positive and neutral lines in relation to earth ground and is usually a result of capacitive coupling. Differential-mode noise is the interference that is present between the positive and neutral lines and is typically generated by switching devices such as transistors, SCRs and triacs. This type of noise is more readily filtered when the choke is in close proximity to the noise source.
Common-mode filtering requires capacitors to earth ground. Safety regulations limit these capacitors to a relatively low value. This mandated low value of capacitance for common-mode filtering makes a high value of inductance essential for effective filtering. Common-mode inductors typically require a minimum inductance of 1000 mH and are most often wound in a balun configuration on a 5000 or higher permeability ferrite core. The balun winding allows the 60 Hz flux density generated by each line to cancel in the core, thus avoiding saturation. Lower permeability materials like iron powder are useful for common-mode applications involving significant line imbalance. Otherwise, for most common-mode applications, the increased core size necessary to accommodate the number of turns needed to achieve the required inductance makes this alternative less attractive.
Differential-mode chokes usually have a single winding, though it is possible to put more than one differential-mode choke on a core by connecting the windings in the additive configuration rather than in the balun configuration. This type of choke must be able to support significant 60 Hz flux density without saturating and at the same time respond to the high frequency noise. The distributed air-gap of iron powder in addition to its high saturation flux density of greater than 12,000 gauss (1.2 T) make it well-suited for this requirement.
Iron powder experiences magnetostriction. This means that as the material is magnetized it experiences a very slight change in dimensions. In applications above audible frequencies (>20 kHz) this is of no concern. In certain 60 Hz applications, however, core buzzing can be noticeable. This condition will be more noticeable with E Cores than with toroids. It wil also be more significant with signals which have been chopped (light dimmers, motor controllers) than with normal sinewaves. It is also dependent on operating AC flux density.
Energy storage inductor design is limited by temperature rise resulting from the combined copper and core loss, and core saturation. While the -8, -18 and -52 Materials have lower core losses at 60 Hz.
Further, the higher core loss characteristics of the -26 and -40 Materials at frequencies above 25 KHz will produce a coil with low Q at high frequency. This characteristic is an additional benefit in helping to suppress the unwanted signals. (see pages 27-33).
The -26 and -40 Materials maintain good permeability versus AC flux density characteristics as illustrated in the graph Percent Initial Permeability vs. Peak AC Flux Density. The significant increase in percent permeability for these materials can be a considerable advantage. It appears that this increase in permeability is experienced in applications such as light dimmers.
Tests performed with a low-level 10 KHz sinewave superimposed over a 60 Hz signal of increasing level did indicate that the high frequency signal experienced an increase in inductance as the 60 Hz signal was increased. While this may be the case for a continuous time averaged signal, it is not clear if this is the case for instantaneous noise signals.
View Energy Storage Curves for 60 Hz filtering applications for -26 and Energy Storage Curves for 60 Hz filtering applications for -40 Materials. These curves take into account the increase in permeability illustrated by the curves shown in the percent permeability versus peak AC flux density graph. The AC flux density levels have been referenced. These flux density references can be useful in approximating core loss as well as determining how the inductance will bary with current.
The representation of percent permeability versus peak AC flux density shows how the permeability (inductance) will change with voltage across the coil, but it does not provide a clear view of how the inductance will vary with current. The graphs shown in 60 Hz Inductor Design are an attempt to illustrate (in relative terms only) how the relative inductance will change with changes in current.
Energy storage limits for temperature rises of 10C°, 25C° and 40 C° are also listed for 60 Hz applications for a number of different core sizes. For the same temperature rises, all core sizes operate at a similar flux density, but the loss distribution differs. With physically large cores, the majority of the loss is due to the core losses, while with the physically small cores the majority of the loss is due to the losses in winding. This phenomenon is not unique to iron powder.
A design example can be found at the bottom of 60 Hz Inductor Design. In addition, there is a table of 60 Hz Inductor Examples.
For applications where it is unclear if the high frequency signal will experience the same increase in permeability as the 60 Hz signal, it is recommended that the 60 Hz signal be treated as DC current. This will produce a significantly different result but will be the most conservative approach.
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