Information

3 Technology, construction and regulation technology

3.3 Filtering

Fig. 3.10: Placement of thermal vias

3.3 Filtering

3.3.1 Input filter

Depending on the circuit topology, switching regulators generate conducted EMI noise

related to the alternating current components in their leads. Würth Elektronik MagI³C

power modules are optimized for low conducted noise and radiated interference and

the ripple is negligible. An output filter is therefore not absolutely essential for most

applications.

The AC component of the input current can cause radio frequency interference

depending on the areas of application and use. In the above case, it is the designer’s

responsibility to check whether they intend to have an input filter on the power module

or at another point in their circuit.

How does the noise arise and what can you do to reduce it? Due to the switching

of a buck converter, a pulsating current flows in the input circuit with the switching

frequency of the module clock. The AC component of this current is delivered from

the input capacitor. Depending on the size of the equivalent series resistance (ESR) of

the capacitor, a ripple is superimposed on the DC current that results in an unwanted

voltage drop at the capacitor, as well as at the lead impedances. This differential mode

interference voltage is partly visible in the conducted interference emission measure-

ment.

The more critical interference signal in EMI is common mode noise or CM. The

reason for this is the rapid change in the switch node potential in relation to the earth

potential. Together with parasitic capacitances, such as between the heat sink of the

switching transistor and the device case and on to earth, the voltage change is the

Input filter

Switching frequency

ESR

Differential mode

voltage

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3 Technology, construction and regulation technology 3.3 Filtering Fig. 3.10: Placement of thermal vias 3.3 Filtering Input filter 3.3.1 Input filter Depending on the circuit topology, switching regulators generate conducted EMI noise related to the alternating current components in their leads. Würth Elektronik MagI³C power modules are optimized for low conducted noise and radiated interference and the ripple is negligible. An output filter is therefore not absolutely essential for most applications.
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3 Technology, construction and regulation technology 3.3 Filtering source of a common mode current and can be measured (see chapter 3.6). The faster the semiconductor components in the switching converter switch (blocking/conducting), the higher is the interference potential. Common mode voltage The unwanted noise level of the input current can be reduced with a LC filter at the input of the converter to a level below the limit set by international standard.
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Technology, construction and regulation technology 3.4 Heat management of its magnitude. The quality factor of the input filter can be reduced by connecting a network of resistance (Rd) and Capacitance (Cd) in series. Hence the output impedance can be reduced. The attenuation resistance (Rd) is calculated as follows: Rd = Lf Cf (3.4) The capacitance of the attenuation capacitor should be many times that of the filter capacitor.