The common mode filter implements a relatively simple and straightforward design process

“The selection of the component values ​​of the common mode filter need not be a difficult and confusing process. Although this alignment can be easily modified to take advantage of predefined component values, the use of standard filter alignment can be used to achieve a relatively simple and straightforward design process.

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The selection of the component values ​​of the common mode filter need not be a difficult and confusing process. Although this alignment can be easily modified to take advantage of predefined component values, the use of standard filter alignment can be used to achieve a relatively simple and straightforward design process.

Line filters prevent excessive noise from being conducted between the Electronic equipment and the AC line; usually, the focus is on protecting the AC line. Figure 1 shows the use of a common-mode filter between the AC line (through an impedance matching Circuit) and a (noisy) power converter. The direction of common mode noise (noise appearing on the two lines at the same time is called the earth) comes from the load and enters the filter, where the noise shared by the two lines is sufficiently attenuated. In this way, the final common-mode output of the filter in the AC line (through the impedance matching circuit) can be ignored.

The design of the common mode filter is essentially the design of two identical differential filters, each of the two polar lines corresponds to one, and the inductors on each side are coupled by a single core:

For differential input currents (from (A) to (B) to L1, from (B) to (A) to L2), the net magnetic flux coupling between the two inductors is zero.

Any inductance encountered by a differential signal is the result of poor coupling between the two chokes. They function as independent components whose leakage inductance is responsive to the differential signal: the leakage inductance attenuates the differential signal.

When the inductors L1 and L2 encounter the same signal with the same polarity (common mode signal), they contribute a net, non-zero magnetic flux in the shared core; therefore, the Inductor functions as an independent component, Their mutual inductance responds to the common signal: the mutual inductance attenuates the common signal.

First-order filter

The simplest design and cheapest filter is a first-order filter. This type of filter uses a single reactive component to store certain bands of spectral energy without transferring that energy to the load. In the case of a low-pass common-mode filter, a common-mode choke is the reactive component used.

The inductance required by the choke is simply the load in ohms divided by the radian frequency, above or above the radian frequency, the signal will be attenuated. For example, attenuation to a 50Ω load at a frequency of 4000 Hz or higher will require 1.99 mH (50 / (2pi x 4000)) inductance. The resulting common mode filter configuration is as follows:

Second order filter

The second-order filter uses two reactive components and has two advantages over the first-order filter: 1) Ideally, the second-order filter provides an attenuation of 12 dB per octave after the cut-off point (the fourth-order filter of the first-order filter). Times) and 2) provide greater attenuation at frequencies higher than the inductor’s self-resonance.

The transfer function coefficient (component value) of the filter can be manipulated to achieve a specific damping coefficient, thereby calculating a specific filter alignment.

The step-by-step design process can utilize standard filter alignment, eliminating the need to directly calculate damping factors for critical filtering. Line filters have their unique requirements, but they have non-critical characteristics and can be easily designed with the smallest allowable damping coefficient.

Standard filter calibration assumes an ideal filter assembly; this is not necessarily true, especially at higher frequencies. For a discussion of the non-ideal characteristics of common mode filter inductors, see the application note “Common Mode Filter Inductance Analysis”.