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1 Introduction
In low-voltage output switching power supply circuits such as push-pull, bridge, and half-bridge configurations, the secondary side of the transformer typically employs a conventional full-wave rectifier circuit. However, this design often leads to a complex transformer structure and requires a larger transformer capacity. The improved double-half-wave rectifier circuit introduced in this paper offers a more simplified configuration for these topologies while enhancing the utilization efficiency of the isolation transformer. This paper provides a comparative analysis of the working principles between the traditional and the improved double-half-wave rectifier circuits.
2 Traditional Double-Half-Wave Rectifier Circuit
In the traditional double-half-wave rectification setup, the center tap of the transformer’s secondary winding is connected to the negative output terminal. This arrangement divides the secondary winding into two parts with opposite polarities and leakage inductances. However, due to imperfections in the magnetic structure, the initial imbalance caused by unequal leakage inductances significantly affects the circuit's normal operation. Additionally, during the freewheeling phase, the primary winding of the isolation transformer experiences a short-circuit condition, which further influences the current distribution on the secondary side. These issues are critical in traditional double-half-wave rectification systems.
The operation of the circuit follows a specific sequence: during the positive half-cycle of each duty cycle, diode VD1 is forward-biased while VD2 remains reverse-biased. Current flows through the inductor LO and the N21 winding, while no current flows through N22. During the freewheeling period, the voltage across both windings becomes zero. Ideally, the current should be evenly distributed between N21 and N22, but in practice, due to differences in leakage inductance caused by the actual magnetic circuit, this balance is not achieved.
N21 releases most of its energy, causing I21 to decrease, while N22 experiences an increase in current due to its leakage inductance and effective voltage. During the negative half-cycle, VD1 is turned off, and VD2 conducts, allowing the load current to flow through N22 and VD2. In the subsequent freewheeling phase, the voltages across N21 and N22 return to zero, and N22 continues to release its stored energy (as shown in Figure 2).
This analysis highlights that both N21 and N22 contribute to the load current during both the conduction and freewheeling phases. As a result, the effective current in the coils increases, leading to higher transformer capacity requirements. Moreover, the magnetic structure of the two windings introduces discrepancies between the theoretical and actual current 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 a single-ended forward converter. This approach allows us to divide the transformer’s operation into two distinct phases: the positive half-cycle and the negative half-cycle. The resulting circuit, shown in Figure 3, eliminates the need for a center tap on the secondary winding and uses two identical rectifier diodes, two identical filter inductors, and a single filter capacitor—similar to the conventional setup.
Figure 1: Traditional double-half-wave rectifier circuit
Figure 2: Traditional double-half-wave rectifier waveform
Figure 3: Improved double-half-wave rectifier circuit
Figure 4: Improved double-half-wave rectifier waveform
The operation of the improved circuit proceeds as follows: during the positive half-cycle, the secondary winding voltage U2 is positive, forward-biasing VD1 and reverse-biasing VD2. VD1 operates in both rectification and freewheeling modes, allowing L1 to discharge continuously through VD1 and the loop formed with C. Meanwhile, the current in L2 flows through the secondary winding, completing a loop with VD1 and C. The total output current is the sum of the DC components of IL1 and IL2. During this phase, the transformer only carries half of the load current.
As the positive half-cycle ends, a freewheeling interval begins. The U2 voltage drops to zero, and I2 rapidly decreases. VD2 turns on, creating a freewheeling path for L2. At the same time, UL1 becomes negative, causing IL1 to decrease, while UL2 is positive, increasing IL2. When the negative half-cycle starts, U2 becomes negative, forward-biasing VD2 and reverse-biasing VD1. VD2 again performs both rectification and freewheeling functions. The secondary current reverses direction, matching the magnitude of IL1. During this phase, the transformer, VD2, C, and L1 form a closed loop, allowing L1 to store energy while L2 discharges through VD2. When U2 returns to zero, another freewheeling cycle begins, repeating the process (as illustrated in Figure 4).
4 Comparative Analysis
A comparison of the working principles of the traditional and improved double-half-wave rectifier circuits reveals several key advantages of the latter. The improved design eliminates the need for a center tap, reducing the number of winding turns and minimizing magnetic imbalance. This helps prevent DC magnetization caused by asymmetry in the magnetic structure. Additionally, the transformer’s capacity is reduced, the circuit structure is simplified, and overall power consumption is lowered, resulting in a smaller transformer size.
The stress on the diodes and filter capacitors in the improved circuit is similar to that of the traditional version, except for the addition of a filter inductor. Each filter inductor carries only half of the output current, and the ripple currents from the two inductors tend to cancel or partially cancel each other. This reduces the required capacitance or inductance of the filter components. Furthermore, the secondary coil only carries about half of the output current, and with proper consideration of the freewheeling effect, the current can be reduced by approximately 7%. Therefore, the improved double-half-wave rectification technique offers a more compact and efficient solution for use in push-pull, bridge, and half-bridge circuits.