Issue link: https://resources.mouser.com/i/1447260
Würth Elektronik 2022 11 A suitable WE-CMB, size XS with 39mH of inductance in Figure 5 shows the characteristic curve of its attenuation in the 50Ω system. A distinction is always made in the attenuation between the common mode (black line) and differential mode (red, dashed line) suppression. In common-mode operation, the WE-CMB line choke reaches its maximum attenuation at 150kHz. However, the attenuation drops with increasing frequency. Other X/Y capacitors are required because the line filter should suppress interference up to 30MHz. An X capacitor is placed both before as well as after the line filter to block differential mode interference from the mains side and the switch-mode power supply. With its leakage inductance, the WE-CMB, in combination with the X capacitor, forms a low-pass filter that reduces the differential mode interference and subsequent common mode interference. Two X capacitors with a value of 330nF have been selected here as an example. Their SRF is approximately 2MHz. For safety reasons, a resistor must be placed on the mains side in parallel with the X capacitor to discharge the capacitor if the switch mode power supply is disconnected from the mains. A varistor should also be placed before the line filter so that transient overvoltage from the mains are short-circuited. Würth Elektronik disk varistors from the WE-VD series are ideally suitable for this. Overload protection such as a fuse must also be considered and this should always be placed before the varistor. The protection trips in the case of a short-circuit by the varistor. Y capacitors are required for further suppression of the common mode interference. In combination with the WE-CMB, they form a corner frequency "f 0 " which is defined by the "Thomson" oscillation equation: Figure 3: Single-phase line filter (Source: Würth Elektronik) Figure 4: Design of the WE-CMB (Source: Würth Elektronik) Figure 5: Attenuation of the WE-CMB XS 39mH (Source: Würth Elektronik) For safety reasons, a resistor must be placed on the mains side in parallel with the X capacitor to discharge the capacitor if the switch mode power supply is disconnected from the mains. "