The DC-DC converter is the core component of the switching power supply, and the commonly used forward and flyback circuit topologies. The power processing circuit of the conventional forward converter has only one stage, and there is a large voltage stress of the MOSFET power switch, especially when the secondary side adopts a self-biased synchronous rectification method, and the input voltage varies widely, such as when the input voltage is 75V. There is a risk that the gate bias voltage is too high, and there is a possibility that the synchronous rectification MOSFET is damaged due to the gate voltage being too high. Moreover, when the output current is large, the loss on the output inductor will be greatly increased, which seriously affects the efficiency improvement. By using a cross-cascade forward synchronous rectification conversion circuit, not only the output filter inductor coil can be omitted, but also a high-efficiency, high-reliability DC-DC converter can be realized to achieve the best synchronous rectification effect.
2 Basic technology 2.1 Cross-cascade forward transformation principleThe topology of the cross-cascade transform is shown in Figure 1. The pre-stage is used for voltage regulation, and the latter stage is used for a synchronous buck converter composed of two-stage cross-concatenated forward converters. In order to achieve a constant input voltage over a wide input voltage range and isolation stage, both front and rear forward conversions should operate at the optimum target, ensuring that a high efficiency synchronous buck converter consisting of it can receive the entire 35-75V. The input voltage range for communication is converted to a tightly adjusted intermediate 25V bus voltage. The actual intermediate bus voltage is preset by the isolation stage and depends on the isolation ratio of the isolation stage. When the intermediate voltage is high, a smaller buck inductor value and a lower inductor current can be used, resulting in less loss. The duty cycle of the entire buck stage is maintained at 30^'60%, which helps to balance the loss of the two-stage forward conversion before and after. To maximize performance and minimize switching losses, the switching frequency is typically 240k-300kHz; due to the low on-resistance (RDS(on)) MOSFET, the conduction losses are small. The conventional single-stage converter main switch must use a MOSFET of at least 200V or more, and its RDS(on) and other parameters are significantly increased, which necessarily means that the loss increases and the efficiency decreases. A simplified schematic diagram of the cross-cascade forward transform topology is shown in Figure 1.
It is well known that the forward voltage drop of a common diode is 1V, the forward voltage drop of a Schottky diode is 0.5V, and a common diode and a Schottky diode are used as rectifying elements. At high current, the power consumption of the rectifying element itself is very considerable. . In contrast, if a power MOSFET is used as the rectifying element, when the driving voltage applied to the gate and source of the MOSFET exceeds its threshold voltage, the MOSFET enters an on state, from drain to source or from source to drain. Extremely, both can conduct current. The voltage drop across the MOSFET is only proportional to the channel resistance of the MOSFET. When n MOSFETs are connected in parallel, the voltage drop can be reduced to 1/n of a single MOSFET. Therefore, the loss theoretically caused by the voltage drop of the rectifying element can be artificially minimized. Synchronous recTIfy (SR) is a technique that uses this characteristic of active devices such as MOSFETs for rectification.
The main losses of implementing SR with power MOSFETs are:
Conduction loss:
Opening loss:
Turn-off loss:
Drive loss:
Where I is the forward current rms value, RDS(on) is the on-state resistance, fS is the switching frequency, CGSS is the input capacitance, Coss is the output capacitance, and D is the duty cycle. It can be seen that the forward conduction loss is proportional to RDS(on). RDS(on) of different VDS MOSFETs can often differ by several orders of magnitude, so the loss of a 100V MOSFET in the same circuit topology is significantly lower than that of a 200V MOSFET. Considering that the low VDS MOSFET is smaller than the high VDS MOSFET Coss, according to the turn-off loss type, the turn-off loss of the low VDSMOSFET is also small. The driving loss type is the loss caused by the charging and discharging of the input capacitor during the switching process, and the loss is proportional to the square of the gate-source driving voltage. Due to the use of a two-stage converter, the isolation ratio of the transformer can be optimized because the regulator stage has stabilized the wider input voltage to a fixed intermediate bus voltage.
The forward on-resistance RDS(on) of the MOSFET and the input capacitance are fixed, and the drive loss is only proportional to the square of the drive voltage. In summary, the use of a two-stage converter minimizes forward conduction losses, drive losses, and the like. In addition, in the cross-cascade forward conversion circuit topology, the output stage synchronous rectification MOSFET requires only twice the output voltage, plus 1.2 times the fuse factor, the device withstand voltage is only 2.4 times the output voltage, much smaller than Traditional single-stage converter solutions require 4-10 times the output voltage. In this way, a two-stage converter with a cross-cascade forward conversion circuit topology can use a low-voltage, low-RDS (on, MOSFET) to achieve very low output-level conduction losses. The two-stage converter also uses the output of a parallel MOSFET. , get lower RDS (on) and lower loss. In the overall design of the system, as long as the component heat distribution is reasonable, the service life and reliability of the device will be greatly improved.
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