In recent times there has been a growing need for energy storage inductors for noise filtering in both mains line filters and dc chokes used in switched-mode power supplies.
In mains line filter applications the filtering falls into two categories: (1) common-mode noise and (2) differential-mode noise. Depending upon the application the filter may be required to suppress frequencies anywhere from several kHz up to several Mhz. The usual filter arrangement consists of two sections between the mains supply and the equipment. Note that the common-mode filter is wound on a single core and the differential mode filter consists of two indiviual wound cores. The common-mode noise is in relation to ground and is common to both lines. Differential mode noise is the noise between the two lines. Both types of noise are usually present to varying degrees. Refer to filter options to see some suggested methods. Small currents can be suppressed with individual cores (or beads) fitted to each line. HF noise suppression is often accomplished by passing all lines through a single core which has material selected to suppress the noise frequencies.
Common-mode filters are usually constructed on ferrite cores with both wires (supply and return) passing together through the core. This type of winding allows the mains 50/60 Hz flux generated by each line to cancel within the core, so avoiding core saturation, but providing suppression of the common interferring signal. These filters are often built into mains input sockets in equipment and can be small in size. The previous diagram (note 1) shows two windings on a toroid core for large currents. They are seperated by a insulating buffer due to mains voltage existing between the two windings, but the magnetic flux cancels out due to the differential windings.
Differential-mode filters must be able to support a significant amount of 50/60 Hz flux without saturating. For this reason an iron powder core is often used. The 26 material is ideally suited to this application. The table gives data for ac line filter applications for various size toroid cores using 26 material. The previous diagram (note 2) shows a single winding on a toroid core for large currents.
In typical DC chokes the AC ripple flux is normally small in comparison to the DC component. These days most DC supplies come from switched mode converters which operate at 50-250kHz, which means that any ripple on the DC is at the same frequency. This allows for much smaller filter component values than used for 50/60Hz mains supplies. Since the DC flux does not generate core loss, the primary concern is saturation and copper loss. For this reason an iron powder core is often used. A table shows the choke data for various size toroid cores. These chokes take the form of the single wound toroid (note 2) shown in the diagram. The data is applicable to 26 and 52 material. If the ripple frequency is greater than 250kHz then the 52 material would be the best choice.
20/04/07
