Circuit designers must deal with many types of noise: internal, external, RF, line frequency and more. Noise can be a limiting factor in system performance and so must be addressed and minimized. The challenge is, 'at what effort and cost?'
Even the ubiquitous switched-mode power supply (SMPS) has noise issues. Due to its efficiency and small size, this device is used in applications including LED drivers and electronic ballasts, etc. Unfortunately, SMPS units also are subject to Differential Mode (DM) noise and Common Mode (CM) noise, both of which must be suppressed.
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, as shown in Figure 1. Common mode noise is conducted on the line and neutral in the same direction returning through ground, also Figure 1.
The basic DM filter uses a single-winding choke (inductor) inserted in series with the line path, along with a capacitor from line to neutral, thus blocking noise propagation through the system.
Since the DM inductor is in series with the line path, it handles the noise and DC offset current being supplied to load.
Therefore, it must be designed to provide the needed inductance, but do so with low DC resistance to handle the RMS current and peak line current without saturating, as shown in Figure 2.
The basic CM filter uses a dual-winding inductor in both the line and neutral paths, plus a capacitor from line to ground, as shown in Figure 3.
Since the line and neutral currents pass through the CM windings in opposite directions, there is no net magnetic flux and therefore no possibility of saturating the CM choke. The CM filter choke only needs to have the required inductance along with sufficiently low DCR for the RMS current.
A Better Idea from Triad Magnetics
Since the DM and CM noise mechanisms are largely unrelated, their solutions require two different chokes. It would be fortunate if the two noise-suppressing approaches could be implemented by one choke – saving space, simplifying the Bill of Materials (BOM) and reducing cost.
Fortunately, a new solution from Triad Magnetics combines both chokes into a dual-function, openframe design that provides the features of both chokes in a single, smaller, more cost-effective package. CMF Series Dual Mode Chokes, as shown in Figure 4, are more than a simplistic co-packaging of two distinct devices into a single component. Instead, their design enhances the electrical performance, while yielding savings in size and cost.
There are 21 CMF models with current ratings from 0.45 to 2.3A; inductances from 10 to 100mH; stray inductances from 200 to 2100mH. DC resistances are 188 to 2930mΩ, depending on model. They are available in horizontal packages (13.5h × 15 × 24.5mm to 14h × 25 × 29mm) and vertical packages (27h × 15 × 29mm) to fit tight clearance situations.
Despite their small size, the creepage and clearance parameter is greater than 3mm and they are rated for 300VAC operation. They are an excellent solution for most designs.