Rectifier Circuits—Single-Phase
The simplest rectifier is the half-wave circuit shown in Fig. 9.1. This circuit is often used for low-power rectifiers operating directly from the AC power line. It can deliver power to a resistive or capacitive load. The half-wave rectifier draws DC and even-order harmonic currents from the source, in addition to the usual odd-order harmonics that characterize most nonlinear loads. If a transformer is used, the DC component may saturate the iron, but a gapped core can allow flux reset and prevent this.
Half-wave rectifiers are often used with filter capacitors to provide a low-ripple DC output. The capacitive load makes the line current conduction angle decrease as shown in Fig. 9.1. High-frequency harmonic currents increase, and the apparent power factor, P/VA, decreases. The rectifier in this case is subjected to a peak reverse voltage equal to the capacitor voltage plus the peak line voltage. Since the capacitor usually is charged to near peak line voltage, the diode must be rated for repetitive operation at twice peak line voltage. Although the half-wave circuit is widely used in switch-mode power supplies where the poor current waveform may not cause a problem, an aggregation of such power supplies can have serious effects on the power system in, for example, a data processing center. Single-phase loads with such current distortion have a high percentage of triplen harmonics, both even and odd order—3, 6, 9, 12, 15, 18, 21, …. These triplen harmonic currents are additive in the neutral of a three-phase distribution system, and the neutral current can approach twice the line current. Oversized neutral conductors are required. Equipments that cause DC components in the supply lines of electric utilities are not permitted by IEEE 519, but the DC components of half-wave power supplies do not pass beyond the first transformer they encounter. However, they may cause the core of the transformer to saturate.
The full-wave, center-tapped rectifier shown in Fig. 9.2 was widely used in the days of radio, because the two rectifying elements could be contained in the same vacuum tube. It is still popular for low-voltage supplies, since there is only a single rectifier element in series with the load. However, many of these simple rectifiers have been superseded by switch-mode units that are much smaller and lighter because of their high-frequency transformer. These are described in Chapter 13.
The secondary sections in this circuit carry half-wave currents with an rms value of 50% of peak over a full cycle. With a primary voltage of 1.0 Vrms and a secondary voltage of 1.0 Vrms each side of center tap and a resistive load of 1.0 Ω, each secondary current is 0.707 Arms, and the primary current is 1.0 A rms. Circuit voltamperes are 1.0 VA. The transformer has a primary rating of 1.0 VA, but each secondary must be rated at 0.707 VA. Thus, the transformer must have a VA rating of (1 + 0.707 + 0.707)/2 = 1.207 VA. It must be some 20% oversized for the circuit rating because of the half-wave currents in the secondary sections.
Figure 9.3 shows the familiar bridge circuit. Here, the transformer is fully utilized, since both windings carry sinusoidal currents. The bridge circuit has twice the diode losses of the center tap circuit, because there are two diodes in series with the load. However, the transformer losses are lower because of reduced harmonic currents. Encapsulated diode assemblies are available for use in any of the above circuits.
