Power Supply Filter Capacitor Design

Regardless of the frequency with which the input voltage is applied a capacitor is used in order to reduce the remaining resistance after rectification.

Power supply filter capacitor design.

For a typical 5 v power supply a 2 200 μf electrolytic capacitor will do the job. A simple power supply decoupling filter made from an inductor and capacitor covers the gap between about 10khz and 1mhz. For example a 10 ripple in a 5 v power supply means that the actual output voltage varies by 0 5 v. The chart above shows typical power supply filtering frequency ranges.

This is the most basic form of power supply design which uses a single diode and a capacitor. The filter capacitor must usually be large to provide an acceptable level of filtering. 1 2 3 is more efficient and gives better results than the rc filter shown in fig. This is an assumption in cases where the capacitor is oversize it will be perhaps 80 but this is close enough for government work and not very critical.

Simple shunt capacitor filter reduces the ripple voltage but increases the current through the diode. 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. Either lc or rc low pass filters can be used to remove the ripple remaining after the reservoir capacitor. Assume that we want to make a 9v 500ma power supply using the lm7809 voltage regulator device 12v transformer bridge rectifier and filter capacitor.

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. The bigger the capacitor the lower the resulting ripple voltage. This large current may damage the diode and at the same time it causes greater heating of the power transformer resulting in decreased efficiency. Depending on the power supply the smoothing capacitor is combined with other circuits.

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. Probably the most widely used application of smoothing capacitors is the construction of power supplies. L c filters power supply filters.

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. The inductor passes dc.