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1. Introduction
In low-voltage output switching power supply circuits such as push-pull, bridge, and half-bridge topologies, the secondary side of the transformer typically employs a conventional full-wave rectifier configuration. However, this design often leads to a complex transformer structure and requires a relatively large transformer capacity. The improved double-half-wave rectifier circuit proposed in this paper aims to simplify the overall circuit design and enhance the utilization efficiency of the isolation transformer. This paper provides a detailed comparison of the working principles between the traditional and improved rectifier circuits.
2. Traditional Double Half-Wave Rectifier Circuit
In the traditional double-half-wave rectification setup, the center tap of the secondary winding is connected to the negative output terminal. This divides the secondary winding into two opposite polarity windings with leakage inductance. However, due to imperfections in the magnetic structure, the leakage inductances are not perfectly balanced, leading to initial imbalances that affect the circuit's performance. Additionally, during the freewheeling period, the primary winding of the isolation transformer experiences a short-circuit effect, which influences the current distribution on the secondary side. These factors must be carefully considered when designing and operating the traditional double-half-wave rectifier.
The operation of the circuit can be described as follows: During the positive half-cycle of each duty cycle, diode VD1 is forward-biased while VD2 is reverse-biased. Current ILO flows through N21, and no current passes through N22. During the freewheeling phase, the voltage across N21 and N22 drops to zero. Ideally, the output current should be evenly split between N21 and N22. In practice, however, due to differences in leakage inductance caused by the actual magnetic structure, the current distribution becomes uneven. N21 discharges most of its energy, causing I21 to decrease, while N22 increases its current through the leakage inductance and effective voltage, completing the freewheeling process.
During the negative half-cycle, VD1 is turned off, and VD2 conducts, allowing all the current from LO to flow through N22 and VD2. In the next freewheeling interval, the voltages across N21 and N22 return to zero, and N22 releases its current as described earlier (the main waveform is shown in Figure 2). From this analysis, it’s clear that both N21 and N22 contribute to the load current during both the conduction and freewheeling phases. This results in an increase in the RMS current of the coil, thereby increasing the transformer’s required capacity. Moreover, the magnetic structure of the two coils introduces significant deviations from the theoretical values.
3. Improved Double Half-Wave Rectifier Circuit
To transition from the traditional double-half-wave rectification to the improved version, we can leverage the working principle of the single-ended forward converter. By dividing the transformer’s operation into two distinct phases — the positive and negative half-cycles — we arrive at the improved dual half-wave rectifier circuit shown in Figure 3. This circuit eliminates the need for a center-tapped secondary winding and instead uses two identical rectifier diodes, two identical filter inductors, and a common filter capacitor, similar to the traditional design but with enhanced performance.
Figure 1: Traditional double half-wave rectifier circuit
Figure 2: Traditional double half-wave rectifier circuit waveform
Figure 3: Improved double half-wave rectifier circuit
Figure 4: Improved double half-wave rectifier circuit waveform
The operation of the improved circuit proceeds as follows: During the positive half-cycle of each working cycle, the secondary winding voltage U2 is positive, forward biasing VD1 and reverse biasing VD2. VD1 simultaneously performs rectification and freewheeling, allowing L1 to discharge continuously through VD1 and the path. Meanwhile, the current from L2 flows through the secondary winding, forming a loop with VD1 and C. The total output current is the sum of the DC components of IL1 and IL2. During the positive half-cycle, the transformer only carries half of the load current. As the cycle progresses, UL1 becomes negative, causing IL1 to gradually decrease, while UL2 becomes positive, and IL2 increases. After the positive half-cycle ends, a freewheeling interval begins, and U2 drops to zero, causing I2 to rapidly decline to zero. At this point, VD2 turns on to provide a freewheeling path for L2, allowing IL1 to continue decreasing while UL2 becomes negative and IL2 starts to drop.
When the negative half-cycle begins, the secondary winding voltage U2 becomes negative, forward biasing VD2 and reverse biasing VD1. VD2 again serves both rectification and freewheeling functions. The current on the secondary winding changes direction rapidly, matching the magnitude of IL1. During VD2’s rectification phase, the secondary winding, VD2, C, and L1 form a closed loop. UL1 becomes positive, enabling L1 to store energy, and IL1 begins to rise. Meanwhile, L2 releases stored energy through the VD2 freewheeling current, causing IL2 to decrease. When U2 returns to zero, another freewheeling cycle begins, and UL1 becomes negative, causing IL1 to decrease while the current in L2 continues to fall. This process then repeats (as shown in Figure 4).
4. Comparative Analysis
By comparing the working principles of the traditional and improved double-half-wave rectifier circuits, it is evident that the latter eliminates the need for a center tap. This reduces the number of turns and mitigates imbalances in the magnetic structure, effectively solving the DC magnetization issue caused by asymmetry in the secondary winding. Furthermore, the improved design reduces the transformer’s capacity, simplifies the circuit structure, lowers power consumption, and decreases the physical size of the transformer. The stress on the diodes and filter capacitors remains similar to that in the traditional circuit, except for the addition of a filter inductor. Each filter inductor carries only half of the output current, and the ripple currents on the two inductors cancel or partially cancel each other. This allows for a reduction in the filter capacitor’s size or the inductor’s inductance. Since the secondary winding carries approximately half the output current, its current capacity can be reduced by about 7% when considering the freewheeling effect. Overall, the improved dual half-wave rectification technique offers a simpler and more efficient solution for use in push-pull, bridge, and half-bridge circuits.