In power supplies, capacitors are used to smooth (filter) the pulsating DC output after rectification so that a nearly constant DC voltage is supplied to the load. In a filter circuit the capacitor is charged to the peak of the rectified input voltage during the positive portion of the input.

The smoothing works because the capacitor charges up when the voltage from the rectifier rises above that of the capacitor and then as the rectifier voltage falls, the capacitor provides the required current from its stored charge.

Thus, if you’re smoothing a 30mV waveform, a 10µF capacitor may suffice to smooth out the signal. However, if you’re dealing with a much greater signal, you will need a much larger capacitor, say, maybe 3300µF in order to smooth it out to a near DC level.

In order to suppress the output voltage ripple and noise, the most common and simple way is add the capacitor. Figure 15 shows the output voltage ripple measured by an external 22uF MLCC. From the figure, the ripple voltage reduced from 445.9mV to about 30mV.

Reducing ripple in power supplies The ripple can be reduced by smoothing capacitors which converts the ripple voltage into a smoother dc voltage. Aluminum electrolytic capacitors are widely used for this and have capacitances of 100uF or more. The repeated dc pulses charges the capacitor to the peak voltage.

You can use a filter to remove noise from a power supply just like you use filters to remove noise from a signal. Indeed, you can consider the output capacitors part of a filter that reacts against the output impedance of the power-supply circuit. Increasing the value of the output capacitance will reduce noise.

How to Reduce Ripple of Switching Power Supply?

1. Reducing EMI – diode back connection inductance.
2. Adopting capacitance with good high-frequency performance and low ESR on the output terminal.
3. Adopting a switching power supply in synchronous frequency with the system.
4. Avoiding the mutual interference among several module power supplies.
5. Adding LC filter in power supply.

Stabilized power supply circuit A bridge rectifier converts the AC to pulsating DC with a peak voltage level of 21V (15×1.4142). LED1 lights up to indicate the availability of output from the rectifier. Resistor R1 (2.2-kilo-ohm) limits the current through LED1 to a safe value below 10 mA.

Which of the following would make the best filter for a power supply? Parallel with the filter output, forward-biased. Parallel with the filter output, reverse-biased. Series with the filter output, forward-biased.

7. Which of the following is an advantage of using a L C low pass filter rather than a RC low pass filter in a power supply? a) The reactance of L will be much lower than the resistance of R at mains frequency.

A capacitive filter smooths additional pulses in the output stage so that an almost constant DC voltage is supplied to the load. The output filter charges up to the peak of the input voltage as seen across CF (the positive portion of the input).

The C-filter is the simplest and most economical filter available. On the other hand, RC-filter is used to reduce the amount of ripple voltage across a capacitor filter.

The filtering stage of a power supply circuit smoothes out the ripples in the rectified DC to produce a smooth direct current that’s suitable for even the most sensitive of circuits. Filtering is usually accomplished by introducing a capacitor into the power supply circuit.

Capacitive Filters

CLC or Pie Filter The input capacitor C1 is selected to offer very low reactance to the repel frequency hence major parts of filtering is done by C1. Most of the remaining repels are removed by the combining action of L and C2. This circuit gives much better filter then LC filter.