Circuit designers must deal with many types of noise: internal, external, RF, line frequency and more. Regardless of type or source, noise can be a limiting factor in system performance and so must be addressed and minimized. The noise-reduction challenge usually comes down to this: “at what effort and cost?”
Even the ubiquitous switched-mode power supply (SMPS) has noise issues. Due to its efficiency and small size, this architecture is widely used in applications including LED drivers and electronic ballasts. Unfortunately, SMPS units also are subject to differential mode (DM) noise and common mode (CM) noise, both of which must be suppressed for both performance and regulatory reasons.
Understand the Noise Mechanisms and Solutions
Differential mode and common mode noise have different causes and thus different solutions. Differential mode noise is noise that is conducted on the line and neutral in opposite directions (Figure 1, left). The basic DM filter uses a single-winding choke (inductor) inserted into the line path, along with a capacitor from line to neutral, thus blocking noise from propagating through the system (Figure 1, right).
Since the DM inductor is in the line path, it handles both the noise and the DC offset due to the current being supplied to load. Therefore, it must be designed to provide the needed inductance but do so with low DC resistance (DCR) to handle both the RMS current and the peak line current without saturating.
CM noise is conducted on both the line and neutral (ground) in the same direction (Figure 2, left). The basic CM filter uses a dual-winding inductor in both line and neutral paths, plus a capacitor from line to ground (Figure 2, right).
Since the line and neutral currents pass through the CM windings in opposite directions, there is no net DC flux and therefore no possibility of saturating the CM choke’s magnetic core. The CM filter choke only needs to have the required inductance along with sufficiently low DCR for the RMS current.
A Better Implementation from Triad Magnetics
Since the DM and CM noise mechanisms are largely unrelated, their solutions require two different chokes and arrangements. It would be fortunate if the two noise-suppressing approaches could be implemented by a single choke — saving space, simplifying the bill of materials (BOM) and reducing cost. Unfortunately, that is not possible due to their differing technical requirements.
However, a new component series from Triad Magnetics combines both chokes into a dual-function, open-frame design that provides the features of both chokes in a single, smaller, more cost-effective package. These CMF Series Common Mode Chokes (Figure 3) are more than a simplistic co-packaging of two distinct devices into a single enclosure. Instead, their mechanical design enhances the combined electrical performance, while yielding tangible savings in size and cost.
There are 21 unique models in the CMF Series with current ratings ranging from 0.45 to 2.3 A with inductances from 10 to 100 mH, and stray inductances from 200 to 2100 mH. DC resistances are between 188 to 2930 mΩ, depending on the specific model. They are available in horizontal packages (from 13.5 h × 15 × 24.5 mm, to 14 h × 25 × 29 mm) and vertical packages (from 27 h × 15 × 29 mm) to fit tight clearance situations.
Despite their small size, their creepage and clearance parameter is greater than 3 mm and they are rated for 300 VAC operation. They are an excellent choice for most designs, unless the CM and DM filter inductance values differ significantly.