21-06-2012, 02:05 PM
Half Wave Rectifier
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Notes for Semiconductor Diode Rectification
Preferably (in general) electronic circuitry is energized from a voltage source, i.e., a
source for which ideally no current flows under open-circuit conditions. Absent the
need to maintain a stand-by load current power dissipation will be minimal. In addition
a DC (unipolar constant voltage) power supply virtually always is preferable over an AC
power supply for electronic equipment. An AC source invariably couples power supply
dynamics with circuit signal dynamics, whereas (except for a start-up transient) a DC
supply introduces no special time relationship.
However for economic as well as technical reasons commercial electrical energy
generally is generated by rotating machinery with a sinusoidal waveform, and
distributed as such from a primary generating station to substations, and ultimately to a
consumer. An electronic ‘power supply’, as the term usually is interpreted, accepts a
sinusoidal input voltage and converts it (ideally) to a constant DC output .
In this note basic concepts used for making the conversion are examined in the context
of a semiconductor diode rectifier. There are typically two steps to the conversion; a bidirectional
(sinusoidal) AC input voltage is ‘rectified’ to form a unidirectional waveform
and this waveform is ‘filtered’ to provide a (nearly) constant magnitude.
Half-Wave Rectifier
Assume, for the moment, that the capacitor in the diode
circuit diagram to the right is absent. One cycle of a
sinusoidal source voltage is sketched to the left of the
circuit; the positive half-cycle is filled simply to
distinguish it clearly from the negative half-cycle.. The
diode conducts only in forward bias, i.e., only during
the positive half cycle. Current flowing through the
diode produces a voltage across the load resistance
during this half-cycle. Since there is negligible
conduction during the other half-cycle the load voltage during this period is zero. The waveform
sketched on the right illustrates this rectification.
Full-Wave Bridge Rectifier
In the half-wave rectifier circuit source power is provided from the source only during a small
conduction angle near the peak of the positive half-cycle. Observe however that by reversing the
diode orientation power would be supplied during the negative half-cycle rather than the positive halfcycle.
This suggests powering the load using two half-wave rectifiers, one conducting during the
positive half-cycle and the other during the negative half-cycle.
Full-Wave Transformer-Coupled Rectifier
Transformers also are used to implement full-wave rectification circuits. The use of transformers also
provides an additional voltage step-up or step-down capability, as well as DC electrical isolation
between primary and secondary sides, albeit at the cost of bulk and weight compared to the bridge
rectifier. Two transformers, often conveniently assembled as a single package with a continuous
center-tapped secondary winding on a single magnetic core, are used to provide two secondary
voltages 180° out of phase with each other. Hence, while both diodes in the circuit shown below
conduct on a positive (forward bias) half-cycle of the respective secondary voltages these correspond
to opposite half-cycles of the primary voltage. As the diagram illustrates, the two rectifiers extract
power from the primary during opposite half-cycles but both rectifiers feed current through the load
resistor in the same direction to produce a full-wave rectified unipolar output waveform.