Power Supply Filters

Definition of Filter:

A filter is used in power supplies to eliminates the AC fluctuation present in the Rectification process of a Power Supply. This filter may be a capacitor.

Power Supply Filter:

A power supply filter ideally eliminates the fluctuations in the output voltage of a half wave or full wave rectifier and produces a constant level DC voltage.  Filtering is necessary because electronic circuits require a constant source of DC voltage and current to provide power and biasing for proper operation.

power supply filter, capacitor filter

Power Supply Filters

Filters are implemented with capacitors. Voltage regulation in power supplies is usually done with integrated circuit voltage regulation.  A voltage regulator prevents changes in the filtered DC voltage due to variations in input voltage or load. In most power supply applications, the standard 50 Hz AC power line voltage must be converted to an approximately constant DC Voltage.  The 50 Hz pulsating DC output of a half wave rectifier or the 100 Hz pulsating output of a full wave rectifier must be filtered to reduce the large voltage variations. These large or small variations are very dangerous for sensitive electronics components, because they are not capable to bear the fluctuations, they will burn in short time.

Filtering Concepts with Diagrams:

power supply filter, capacitor filter

Rectification without Filter

power supply filter, capacitor filter

Rectifier with a Filter Output

Capacitor Input Filter:

A half wave rectifier with a capacitor input filter is shown in above figure, The filter is simply a capacitor connected from the rectifier output to ground. RL represents the equivalent resistance of a load. We will use the half wave rectifier to illustrate the basic principle and then expand the concept to full wave rectification.

power supply filter, capacitor filter

(a) Initial charging of the capacitor (diode is forward biased) happens only once when poer is tuned on

Figure(a)

Operation of a half wave rectifier with a capacitor input filter.  The current indicates chargin or discharging of the capacitor.

power supply filter, capacitor filter

Figure (b)

In this figure, the capacitor discharging through RL after peak of positive alternation when the diode is reverse biased.  This discharging occurs during the portion of the input voltage indicated by the solid curve.

power supply filter, capacitor filter
Figure(c) 

The capacitor charges back to peak of input when the diode become forward biased.  This charging occurs during the portion of the input voltage indicated by the solid curve.

During the positive first quarter cycle of the input, the diode is forward biased allowing the capacitor to charge to within 0.7 V of the input peak, as shown in figure (a).  When the input begins to decrease below its peak, as shown in figure (b), the capacitor retains its charge and the diode becomes reverse biased because the cathode is more positive than the anode.  During the remaining part of the cycle, the capacitor can discharge only through the load resistance at a rate determined by the RLC time constant, which is normally long compared to the period of the input.  The larger the input constant, the less the capacitor will discharge.  During the first quarter of the next cycle, as shown in figure (c), the diode will again become forward biased when the input voltage exceeds the capacitor voltage by approximately 0.7 V.

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