Rectifiers

Rectification

Rectification can be defined rather broadly as the process that modifies the direct component of a voltage or current. Since rectification can occur in any circuit that contains a nonlinear element, this process plays a very important role in electronic circuits. In some cases the effect is desired, as in converting an alternating current to a direct current. In others the effect is undesirable: for example, the distortion of voltage or current waveforms in circuits that are supposedly linear but contain unavoidable nonlinearities.

Rectifier circuits usually include energy-storage elements to smooth the ripples in the output voltage, thus producing a more effective conversion of alternating current to direct current. Such rectifier-filter circuits are used in many electronic systems. They serve to convert AC voltages from the power distribution system to DC voltages required for the operation of most electronic devices.

In rectifier circuits used for power conversion, the input AC voltage amplitude is fixed, but the output power varies with the changing demands of the DC load. Important design considerations for this case are: the ratio of DC load power to AC input power (efficiency), variation of output voltage with load current (regulation), and residual AC fluctuations present in the output (ripple).

Ripple Factor

The pulsations, or ripple, caused by the AC components in the rectifier output, appearing as audible hum in sound amplifiers, are very undesirable in the supply to many electronic circuits. The amount of ripple, compared with the direct component of current or voltage, is a measure of the purity of rectifier output and is called the ripple factor.

The load current of a rectifier is composed of the direct component I_DC and an effective value for all the AC components I_AC. By definition the effective value I_RMS of the total load current is then

from which

This supplies the numerator for the ripple definition equation, so that in general

It can be seen that ripple factor is a function of the waveform of the current in the rectifier load. If I_DC = I_RMS, the ripple would be zero.