Power Supply Filter Capacitor Design

For example a 10 ripple in a 5 v power supply means that the actual output voltage varies by 0 5 v.

Power supply filter capacitor design.

The bigger the capacitor the lower the resulting ripple voltage. Because these capacitors have a dc value they are actually storing a lot of energy that never gets used. The lc filter shown in fig. L c filters power supply filters.

The inductor passes dc. Why the capacitor in your power supply filter is too big january 21 2016 by david williams the job of the capacitor in the output filter of a dc power supply is to maintain a constant dc value by removing as much power ripple as possible. The filter capacitor must usually be large to provide an acceptable level of filtering. Regardless of the frequency with which the input voltage is applied a capacitor is used in order to reduce the remaining resistance after rectification.

Assume that we want to make a 9v 500ma power supply using the lm7809 voltage regulator device 12v transformer bridge rectifier and filter capacitor. Simple shunt capacitor filter reduces the ripple voltage but increases the current through the diode. Either lc or rc low pass filters can be used to remove the ripple remaining after the reservoir capacitor. Correct design of the decoupling filter ensures that it won t cause more problems than it solves.

A simple power supply decoupling filter made from an inductor and capacitor covers the gap between about 10khz and 1mhz. This large current may damage the diode and at the same time it causes greater heating of the power transformer resulting in decreased efficiency. Since a single diode will rectify only one half cycle of the ac signal this type of configuration requires a large output filter capacitor for compensating the above limitation. This is the most basic form of power supply design which uses a single diode and a capacitor.

Depending on the power supply the smoothing capacitor is combined with other circuits. 1 2 3 is more efficient and gives better results than the rc filter shown in fig. Putting it to work a practical example. 1 2 4 but for basic power supplies lc designs are less popular than rc as the inductors needed for the filter to work efficiently at 50 to 120hz need to be large and expensive.

In addition to the natural output capacitance of the power supply you might add a series inductor and another filter capacitor to further reduce output noise fig. For a typical 5 v power supply a 2 200 μf electrolytic capacitor will do the job.