**1. Overview**
The DC-DC converter is a crucial part of any switching power supply system, and it commonly uses forward or flyback topologies. In traditional forward converters, the power processing circuit usually has only one stage, which leads to high voltage stress on the MOSFET switch. This becomes particularly problematic when the secondary side employs self-biased synchronous rectification, especially under varying input voltages. For example, when the input voltage reaches 75V, there's a risk that the gate bias voltage could be too high, potentially damaging the synchronous rectification MOSFET. Additionally, when the output current is large, the losses in the output inductor can significantly increase, which negatively affects overall efficiency. By implementing a cross-cascade forward synchronous rectification topology, the output filter inductor can be eliminated, resulting in a more efficient and reliable DC-DC converter with improved synchronous rectification performance.
**2. Basic Technology**
**2.1 Cross-Cascade Forward Conversion Principle**
The cross-cascade forward topology is illustrated in Figure 1. The first stage is responsible for voltage regulation, while the second stage functions as a synchronous buck converter. This two-stage configuration allows for a stable input voltage across a wide range and provides isolation. Both stages are designed to 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 transformer. A higher intermediate voltage allows for smaller inductors and lower inductor currents, reducing losses. The duty cycle of the entire buck stage is typically maintained between 30% and 60%, balancing the losses between the two forward stages. To achieve maximum performance and minimize switching losses, the operating frequency is generally set between 240kHz and 300kHz. The use of low RDS(on) MOSFETs further reduces conduction losses. In contrast, a single-stage converter would require a MOSFET rated for at least 200V, leading to higher losses and reduced 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 to reduce conduction losses. While a standard diode has a forward voltage drop of about 1V and a Schottky diode around 0.5V, a MOSFET can have a much lower voltage drop, proportional to its channel resistance. When multiple MOSFETs are connected in parallel, the voltage drop can be reduced significantly, lowering overall losses. The main losses associated with SR using MOSFETs include conduction loss, switching loss (turn-on and turn-off), and drive loss. These depend on factors such as the RMS forward current, RDS(on), switching frequency, input capacitance (CGSS), output capacitance (Coss), and duty cycle. It's clear that lower RDS(on) values lead to lower conduction losses. Low-voltage MOSFETs, such as those rated at 100V, typically have lower RDS(on) and Coss compared to higher-voltage ones, making them more efficient. The two-stage approach in cross-cascade conversion helps optimize the transformer's isolation ratio, as the regulator stage stabilizes the input voltage to a fixed intermediate bus. This results in lower voltage stress on the MOSFETs used in the synchronous rectification stage. In this configuration, the MOSFETs only need to withstand twice the output voltage plus a safety factor, which is significantly less than what is required in a single-stage solution. As a result, low-voltage, low-RDS(on) MOSFETs can be used, minimizing conduction and drive losses. Additionally, paralleling MOSFETs further reduces RDS(on) and improves efficiency. With proper thermal management, the reliability and lifespan of the system are greatly enhanced.
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