The resistor reduces the peak input impedance of the filter at the cut-off frequency without affecting the cut-off frequency as long as the component values for the resistor and second capacitor are chosen correctly. Damped LC FiltersĪdding a second capacitor in series with a resistor across the input to the switching power supply immediately after the first main filter capacitor will change the filter to act as a parallel damped low-pass filter. A good rule of thumb is to aim for a minimum damping factor of 1/√2. Therefore, the designer needs to ensure that the values are selected such that a poor damping factor cannot result in oscillations within the switching circuit control loop. Component values will determine the cut-off frequency and the damping factor of the filter, the damping factor determining the attenuation characteristics at the cut-off frequency. The designer can trade-off performance against component size and cost to create an optimal solution.Īn inductor in series between the supply voltage and the input to the switching power supply combined with a capacitor across the input to the switching power supply will act as an undamped low pass filter. Passive LC filters can achieve the required results for input filtering in a relatively simple solution. However, an LC filter will offer considerable advantages that limit RC filter usage in performance and efficiency. It has an application in low-power solutions where cost and component size are significant constraints. The most basic input filter available is the RC filter. Thus, switching mode power supplies pose the most questions and test the designer’s skills the most regarding power supply filter design. Third, the input filter must prevent the switching circuitry’s noise from being conducted back through the power supply to the input power lines. Second, the input filter must prevent noise generated by the switching circuitry from appearing at the output. First, the input filter must prevent noise on the input power lines from being conducted through the power supply to the output. Switching power suppliers present the designer with a challenge when it comes to effective filtering.
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