______________________________________________________________ Solid State Power Supplies
the voltages at the opposite ends of the secondary windings are 180 degrees out of phase
with each other. So, when the voltage at point A is positive with respect to ground, the
voltage at point B is negative with respect to ground. Let us examine the operation of the
circuit during one complete cycle.
4-17. During the first half cycle (indicated by the solid arrows), the anode of Dl is
positive with respect to ground and the anode of D2 is negative. Figure 4-6, view (A)
shows that current flows from ground (center tap), up through the load resistor (RL),
through diode Dl to point A. In the transformer, current flows from point A, through the
upper winding, and back to ground (center tap). When Dl conducts, it acts like a closed
switch so that the positive half cycle is felt across the load (RL).
Figure 4-6. Practical Full-wave Rectifier
4-18. During the second half cycle (indicated by the dotted lines), the polarity of the
applied voltage has reversed. Now the anode of D2 is positive with respect to ground and
the anode of Dl is negative. Now only D2 can conduct. Current now flows, as shown, from
ground (center tap), up through the load resistor (RL), through diode D2 to point B of T1.
In the transformer, current flows from point B up through the lower windings and back to
ground (center tap). Notice that the current flows across the load resistor (RL) in the SAME
DIRECTION for both halves of the input cycle.
4-19. Figure 4-6, view (B) represents the output waveform from the full-wave rectifier.
The waveform consists of two pulses of current (or voltage) for each cycle of input
voltage. The ripple frequency at the output of the full-wave rectifier is therefore TWICE
THE LINE FREQUENCY.
4-20. The higher frequency at the output of a full-wave rectifier offers a distinct
advantage. Because of the higher ripple frequency, the output is closely approximate to
pure DC. The higher frequency also makes filtering much easier than it is for the output of
the half-wave rectifier.
4-21. In terms of peak value, the average value of current and voltage at the output of the
full-wave rectifier is twice as great as that at the output of the half-wave rectifier. Figure 4
7 shows the relationship between the peak value and the average value. Since the output
waveform is essentially a sine wave with both alternations at the same polarity, the average
current or voltage is 63.7 percent (or 0.637) of the peak current or voltage. This is shown in
the following equation:
23 June 2005