As shown above, attempts to satisfy all the voltage averaging and noise rejection requirements in a single LC filter would require the selection of expensive components, particularly in flyback converters. Even then, only mediocre high-frequency performance would be obtained.
Figure 1.20.2 shows how a far more cost-effective wideband filter can be produced, using a second-stage, much smaller, LC filter to reject the high-frequency noise. The second stage L2, C2, may be quite small and inexpensive because only small inductance and capacitance values are required in this second stage. At the same time, much lower cost standard electrolytic capacitors and inductors may be used in the first stage (LI, CI), thus reducing the overall cost and improving the performance.
In Fig. 1.20.2, the first capacitor CI is selected for the required ripple current
rating and energy storage needs. (This depends on the load current and the operating frequency.) Cl will often be quite large, but does not need to be a low-ESR type when a two-stage filter is used.
The first inductor LI is designed to carry the maximum load current with minimum loss and without saturation. To obtain the maximum inductance and minimum resistance in the smallest size, LI will have a multiple-turn multilayer winding. Although this gives the maximum inductance, it results in a relatively large interwinding capacitance and low self-resonant frequency. Suitable core materials for LI include gapped femtes, Permalloy, iron-dust toroids, or gapped silicon iron in "E I" shapes. Ll will have the majority of the inductance required for energy storage considerations.
The second inductor L2 is designed to have the maximum impedance at high frequency, and requires a low interwinding capacitance. This will provide a high self-resonant frequency. L2 may take the form of a small femte rod, afemte bobbin, small iron-dust toroids, or even an air-cored coil. Since the AC voltage across L2 is small (of the order of 500 mV), the magnetic radiation from an incomplete magnetic path will be quite small and should not present an EMI problem. Normal femte materials may be used for a ferrite rod inductor, as the large air gap will prevent DC saturation of the core.
The second capacitor C2 is much smaller than Cl. It is selected for low impedance at the switching and noise frequencies (rather than for its energy storage ability). In many cases C2 will consist of a small electrolytic shunted by a low-inductance foil or ceramic capacitor. Since Ll and L2 conduct a large DC current component, the term "choke" is more correctly applied to these items. A design example follows.
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