[align=left] In recent years, power electronics technology has developed rapidly, and DC switching power supplies have been widely used in computer, aerospace and other fields. In the past, bulky and inefficient power supply devices have been replaced by small and efficient power supplies. However, to achieve high performance, high efficiency, high reliability and reduce size and weight of power supply devices, it is necessary to realize the high frequency of switching power supplies. The high frequency of switching power supplies not only reduces the size of power converters, increases the power density and performance-price ratio of converters, but also greatly improves the instantaneous response speed and suppresses the audio noise generated by the power supply, thus becoming a new development trend. However, further increases in the switching frequency of power converters (the switching devices in traditional PWM converters operate in hard switching state) are limited by the following factors: (1) large turn-on and turn-off losses; (2) inductive turn-off problem; (3) capacitive turn-on problem; (4) diode reverse recovery problem; (5) severe di/d and d/d shocks and the electromagnetic interference (EMI) they generate. Soft switching technology is one of the important technologies that enables power converters to be used at high frequencies. It applies the principle of resonance to make the current (or voltage) in the switching device change according to a sinusoidal or quasi-sinusoidal law. When the current naturally crosses zero, the device is turned off (or turned on when the voltage is zero), thereby reducing switching losses. This not only solves the problems of hard switching losses, capacitive turn-on, inductive turn-off, and diode reverse recovery in hard-switching converters, but also solves problems such as EM caused by hard switching. In summary, traditional hard-switching PWM-based switching power supplies have shortcomings. To study new and more effective power control strategies, soft-switching PWM technology is used to design switching power supplies. This paper uses a novel zero-current conversion (ZCT) full-bridge DC/DC converter as the main circuit topology, and analyzes and studies a DC/DC converter composed of UC3875 chips. [b]1 UC3875 Chip[/b] The UC3875 monolithic integrated circuit is a high-performance dedicated soft-switching power supply phase-shift PWM controller manufactured by Untrode Corporation. It has four independent output drive terminals that can directly drive four power switching transistors, each with its own turn-on delay (i.e., dead time) adjustment. Auxiliary switching transistors can be controlled by processing and adjusting the corresponding main switching transistor signal. The UC3875 phase-shift controller is characterized by excellent control performance across both ends of the phase-shift range, and its ability to effectively control, protect, and drive. Its protection functions include: undervoltage lockout (all four outputs remain low until the bias voltage reaches a threshold of 10.75V), with a built-in 1.5V hysteresis ensuring reliable operation); overcurrent protection (shutting down all outputs within 70ns in case of a fault); and fault recovery with full-cycle restart capability. Furthermore, the chip integrates an error amplifier with a bandwidth exceeding 7MHz, a 5V reference voltage, soft start, a ramp voltage generator, and slope compensation circuitry. The device also features undervoltage lockout. During undervoltage lockout, all outputs remain low until the supply voltage reaches the 10.75V threshold. To improve the reliability of undervoltage lockout, a 1.5V hysteresis is typically applied to the undervoltage lockout threshold, meaning the undervoltage lockout circuit continues to operate even when the supply voltage drops to 9.25V. This device also features overcurrent protection. Within 70ns after an overcurrent fault occurs, all output stages can enter a judgment state. After the overcurrent fault is cleared, the device can resume operation. Table 1 shows the pinout of this chip: [align=center] Figure 1 Overall Framework Diagram [/align] When the output signal at pin 2 reaches a certain high value, the internal RS flip-flop and gate circuits cause the C output to be out of phase with the A output, i.e., the A and C output signals are phase-shifted by 180 degrees. Similarly, when the output signal at pin 2 is below 1V, the internal RS flip-flop and gate circuits cause the C output to be in phase with the A output, i.e., the A and C output signals are phase-shifted by 0 degrees. It can be seen that by controlling the output at pin 2, the phase between A and C can be controlled to vary between 0 and 180 degrees. The working principle of B and D is similar to that of A and C. 2 Design of DC/DC full-bridge converter (1) Block diagram of switching power supply As shown in Figure 1, the main components of the switching power supply circuit include input rectifier and filter circuit; detection and protection circuit; single-phase bridge inverter circuit; high-frequency transformer; auxiliary circuit; output rectifier and filter circuit, etc. The overall block diagram is shown in Figure 1. Its main function is to convert the 220V/50Hz AC power supply into DC power through the transformer and the three-phase rectifier bridge, and then filter the DC power through the filter formed by the inductor and capacitor. On the one hand, it can make the DC voltage smooth, and on the other hand, it can improve the input power factor. After rectification, a DC voltage of 310V can be obtained, and then this 310V voltage is used as the DC input of the DC/DC converter. After the DC/DC converter, a DC voltage of 120V can be obtained as the output. (2) Main circuit of a full-bridge DC/DC converter Figure 2 shows the main circuit of a new type of zero current conversion (ZCT) full-bridge DC/DC converter. It uses a simple auxiliary circuit on the secondary side, which not only enables the main switch and auxiliary switch to achieve ZCS throughout the entire load range, but also achieves soft switching of the output rectifier diode. At the same time, the resonant inductor in the auxiliary circuit can also help the main switch and auxiliary switch to achieve soft turn-on. Its simple auxiliary circuit consists of a resonant inductor, a resonant capacitor, an auxiliary clamping diode, and an auxiliary switch, which are added to the secondary side of the main transformer to achieve zero-current switching of the main switch. In the topology, Lk is the leakage inductance of the transformer, is the magnetic induction inductance of the transformer, and D and D are the output rectifier diodes. -n, . and, are the DC input, output voltage and output current, respectively, and the transformation ratio of the primary side to the secondary side of the transformer is. Figure 3.5 shows the working waveform when the steady state is working. (3) Design of the external circuit The system control circuit is implemented using UC3875, and its peripheral circuit is shown in Figure 3. The chip is powered by a +15V/2A power supply. In Figure 3, the scales are shown. The switching frequency is determined; the dead time settings for OUTA and OUTB are completed by R and C; the dead time settings for OUTC and OUTD are completed by R and C; R and C set the slope and amplitude of the sawtooth wave; TEST is connected to an internal current comparator and an external DC input detection terminal of the main power supply circuit, serving as a fault protection circuit; C sets the soft-start time; the chip operates in voltage control mode. The voltage regulator utilizes the error amplifier within the UC3875. The output voltage is divided by the adjustable resistor RV and then sent to the inverting input of the error amplifier via R. Adjusting RV can adjust the feedback coefficient of the output voltage, thereby regulating the output voltage. The 5V reference voltage is divided by R and sent to the non-inverting input of the error amplifier as the voltage setpoint signal. R, R, and C are connected to the inverting input and output of the error amplifier as a compensation network, forming a PI regulator. 3. Simulation Analysis Figure 4 shows the simulation diagram of the main circuit under rated load. Figure 5 shows the driving waveforms of switching transistors S1 and S2. The switching time is one switching cycle of the converter, 10us. It can be seen from Figure 5 that the waveforms are complementary. In order to prevent the S1 and S2 transistors from shooting through, a dead time of 0.5us is provided. 4. Conclusion This paper introduces the UC3875 chip and a new type of DC/DC full-bridge ZCT soft-switching power supply controlled by it. Experiments were conducted, and the following conclusions were drawn. (1) The soft-switching DC/DC converter control circuit adopts the UC3875 phase-shift control chip, which is convenient to control and has excellent performance. (2) The phase-shift control zero-voltage PWM converter operates under zero-voltage switching conditions, which greatly reduces switching losses, which is conducive to increasing the switching frequency and reducing the size and weight of the converter. Click to download: Simulation Study of a New Type of DC/DC Full-Bridge ZCT Soft-Switching Power Supply Editor: Chen Dong