**1. Overview**
The DC-DC converter is a fundamental component in switching power supplies, with forward and flyback topologies being the most commonly used. In traditional forward converters, the power processing circuit typically consists of a single stage, which leads to high voltage stress on the MOSFET switch. This becomes particularly problematic when the secondary side uses self-biased synchronous rectification, especially under wide input voltage variations—such as when the input voltage reaches 75V. In such cases, the gate bias voltage may become too high, risking damage to the synchronous rectification MOSFET. Additionally, when the output current is large, the losses on the output inductor can significantly increase, negatively impacting overall efficiency.
By implementing a cross-cascade forward synchronous rectification topology, the need for an output filter inductor can be eliminated. This not only improves efficiency but also enhances reliability, allowing for optimal synchronous rectification performance. The design ensures stable operation across a wide input voltage range while maintaining high efficiency and minimizing losses.
**2. Basic Technology**
**2.1 Cross-Cascade Forward Conversion Principle**
The cross-cascade forward topology is illustrated in Figure 1. It consists of two stages: the first stage handles voltage regulation, while the second stage functions as a synchronous buck converter. This configuration allows for a constant input voltage over a wide range, with isolation provided by the first forward converter. Both stages operate at their optimal points, ensuring that the entire 35–75V input range is converted into a tightly regulated intermediate 25V bus voltage.
The actual intermediate bus voltage is determined by the isolation ratio of the first stage. When the intermediate voltage is higher, a smaller inductor value and lower inductor current can be used, resulting in reduced losses. The duty cycle of the buck stage is maintained between 30% and 60%, balancing the losses between the two forward conversion stages. A typical switching frequency of 240–300kHz is used to minimize switching losses, and low RDS(on) MOSFETs are employed to further reduce conduction losses.
In contrast, a single-stage converter requires a MOSFET rated for at least 200V or higher, which increases both RDS(on) and overall losses, reducing efficiency. A simplified schematic of the cross-cascade forward topology is shown in Figure 1.
**2.2 Synchronous Rectification Technology**
Synchronous rectification (SR) is a technique that uses active devices like MOSFETs instead of diodes for rectification. Unlike standard diodes, which have a forward voltage drop of about 1V, or Schottky diodes with around 0.5V, MOSFETs can achieve much lower voltage drops when properly driven. When the gate-source voltage exceeds the threshold, the MOSFET turns on, allowing current to flow with minimal resistance. The voltage drop is proportional to the channel resistance, and connecting multiple MOSFETs in parallel can further reduce this drop.
The main losses in SR include conduction loss, switching loss, and drive loss. Conduction loss depends on RDS(on), while switching loss is influenced by factors such as switching frequency, input capacitance, and output capacitance. Drive loss occurs due to the charging and discharging of the gate capacitance and is proportional to the square of the drive voltage.
Using a two-stage converter allows for optimized transformer isolation, enabling the use of low-voltage MOSFETs with lower RDS(on) and reduced switching losses. In a cross-cascade forward topology, the synchronous rectification MOSFET only needs to withstand twice the output voltage plus a safety factor, significantly reducing the required voltage rating compared to a single-stage solution. This enables the use of low-cost, high-performance MOSFETs, improving efficiency and reliability. By paralleling MOSFETs, the total RDS(on) can be minimized, further lowering losses. With proper thermal management, the system’s lifespan and reliability are greatly enhanced.
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