Raffaele Ilardo
Power supplies and ripple voltage
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What we mean as "Power supply"
Generally speaking, a power supply means a device which, connected to the mains voltage, provides for electronic circuits the adequate voltage and current they require.This voltage should be as much as possible a continuous voltage, stable and free from any kind of disturbance. Unfortunately, in most of the cases, the voltage supplied by a power supply maintains "traces" of the network voltage, in the form of more or less extensive undulations that are superimposed, or "on horseback", of the continuous voltage.
Continuous voltage VM affected by ripple (VRpp)
The image on the left shows a DC voltage with an average value of VM; around this value VM, the voltage has an amplitude ripple VRpp (i.e. from peak to peak). As can be seen, these undulations have the appearance of a sawtooth and a period which, for the double half-wave feeders, is of 10 ms (and therefore the frequency is 100 Hz).
The ripple voltage
The ondulations, generally indicated as "ripple", are due to the current absorbed by the load connected to the output, and that, between a half-wave and the other, determines the phase of descent (or discharge); at the arrival of the following half-wave, the leveling capacitor returns to charge, with one rise of more or less steep voltage (charging phase).Naturally, the lower the current absorbed by the load and the greater the capacitance of the capacitor, the lower the voltage drop in the discharge phase. The ripple voltage therefore results lower
In the simplest power supplies the only element that provides for leveling the output voltage is the capacitor which follows the rectifier bridge; since a high capaciance is required, the capacitor is always an electrolytic. Regardless of the constructive particularities that characterize a good one electrolytic capacitor, the need always arises to determine the value that this condenser must have, in order to suppress in an acceptable quantity the undulations from alternating voltage.
Filter capacitor calculation
To determine this value one can resort to various more or less empirical methods of calculation.The criterion set out below is based on the evaluation of two quantities: the current i0 which will absorb the output load and the VRIP value that we want to accept as ripple voltage. Based on these two values, we can define the Cp, that is the "performance coefficient" corresponding to our needs.
In detail, the value of Cp is obtained by dividing the value of the current i0 (in mA) by the value of the ripple voltage VRIP(in mV).
performance coefficient
If, for example, an output current of 300 mA is envisaged and the ripple is to be kept within 500 mV, the coefficient will have a value Cp = 300/500 = 0.6.
You can use this value to enter in the attached graph and, as shown, a capacitance value of about 5000 µF is obtained for the capacitor.
As a general criterion, it could in any case be argued that, at bottom, the greater the capacitor's capacitance, the better it is!
Current in rectifier diodes
X axis: 10ms/div
Y axis: 1A/div
Among other things, it is interesting to note that, while the load absorbs a constant current, the diodes provide to load the capacitor of leveling only for short periods of time; for this reason, to reach the average value required by the load, the diodes are crossed at each half-wave from significantly higher current peaks.
As can be seen in the figure at the side, in the case of a power supply called to supply a direct current of 1 A, the diodes are crossed by a succession of current peaks of about 4 A.
Circuit solutions to reduce ripple voltage
Actually, the capacitance values obtained with the described method may appear to be excessive in relation to the corresponding ripple voltage; for this reason, in most cases, it is preferable to resort to circuit solutions that increase the leveling effect of the capacitor. One solution is to perform the filtering using a transistor (figure above): the filter cell, composed of one or more capacitors, is applied on the base of TR1, while the load is connected to the emitter circuit; in this way the effect of the capacitor is multiplied by the amplification factor of the transitor.However, one wonders why choose to create a more or less elaborate circuit by an approximate functioning, when so many are available integrated voltage regulators which guarantee the best results with maximum simplicity.