Filter chokes

Designing the Mutually Coupled Forward-mode Filter Choke... 

Within multiple-output forward converters, it is possible to easily combine the filter chokes of complementary outputs (i.e., +/- 5V, etc.) together on the same core (see Figure 3-2f). This offers several advantages it saves space, vastly improves cross-regulation of those outputs, and exliibits superior ripple voltage levels on both outputs. 

First the core style and material should be selected. This is done in an identical fashion as the single-output filter choke. Either a mopermalloy (MPP) toroid (refer to Section 3.5.5) or a ferrite bobbin core (refer to Section 3.5.2)... 

More windings could be added to the mutually coupled filter choke core, but I highly recommend against this temptation. If the windings are not exact (to the turn), the supply will loose approximately 1 percent in efficiency for each turn in error on each output. Instead, use a mutually coupled filter choke for each complementary set of outputs, and use the output cross-sensing technique described in Section 3.9. 

The DC filter choke is used for ripple voltage and current reduction on the output of the switching power supply immediately following the existing filters. It is also used as an EMI filter on switching power supplies that have a single power line on the input, such as battery and distributed power systems. 

Designing the output filter choke La) in a forward-mode converter is done first. This simple procedure can be seen in Section 3.5.5. A key design factor is to design the inductor to operate in the continuous current mode. The typical value of peak inductor current is 150 percent of the rated output current. The typical valley (minimum) current is about 50 percent of the rated output current. 

Using one of the core materials listed in Table D-1, the designer can feel reasonably confident that he or she has made the best choice for a ferrite. Mopermalloy is a ferrite alloy that has nonmagnetic molybdenum mixed with it. The molybdenum acts as a distributed air-gap within the material, which makes the material excellent for dc biased or unipolar applications. Unfortunately, it is only available in toroid core styles, and it typically used for output filter chokes. 

Once the component values have been calculated, the physical construction of the transformer and the PCB layout become critical for the effectiveness of the filter stage. Magnetic coupling due to stray inductive pick-up of high-frequency noise by the traces and components can circumvent the filter all together. Added to this is the fact that the common-mode filter choke looks more and more capacitive above its self-resonance frequency. The net result is the designer needs to be concerned about the high-frequency behavior of the filter typically above 20 to 40 MHz. 

There are certain things we may do unintentionally at the input of the converter that can have a major impact on the performance of the EMI filter, and also the converter itself. If we don t know the rules of the game, we can end up saturating our filter chokes and even inducing instability. 

However, it is nevertheless clear that introducing power factor correction in off-line power supplies always leads to much smaller EMI filter chokes. 

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