Soft-switching power supply design of phase-shifted full-bridge main circuit

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Overview:
The phase-shifted full-bridge converter can greatly reduce the switching voltage, current stress and spike interference of the power tube, reduce the loss and increase the switching frequency. How to design a switching power supply based on PWM soft switching mode with UC3875 as the core? Please refer to the following for details.

Main Circuit Analysis This soft switching power supply uses a full-bridge converter structure using a MOSFET as a switching transistor with a parameter of 1000V/24A. ZCS is realized by phase-shifting ZVZCSPWM control, that is, the super-forearm switch tube realizes ZVS and lag arm switch tube. The schematic diagram of the circuit structure is shown in Figure 1. VT1~VT4 are the four MOSFET switching tubes of the full-bridge converter. VD1 and VD2 are the reverse super-fast recovery diodes of the super-forearm switching tubes VT1 and VT2 respectively. C1 and C2 are respectively implemented. VTl, VT2 ZVS set high frequency capacitor, VD3, VD4 is a reverse current blocking diode, used to achieve ZCS of the lag arms VT3, VT4, Llk is the transformer leakage inductance, Cb is the blocking capacitor, T is the main transformer, The secondary side is composed of a high-frequency rectifier circuit composed of VD5~VD8 and filter elements such as Lf, C3, and C4.

Figure 1 Schematic diagram of 1.2kw soft-switching DC power supply circuit

The basic working principle is as follows:
When the switching tubes VT1, VT4 or VT2, VT3 are simultaneously turned on, the circuit operates in the same manner as the hard switching mode of the full-bridge converter, and the primary side of the main transformer supplies energy to the load. Through phase shift control, VT4 is not turned off immediately when VT1 is turned off, but the phase shift angle is determined according to the output feedback signal. After a certain period of time, VT4 is turned off. Before VT1 is turned off, VT1 is turned on in parallel. The voltage on capacitor C1 is equal to the turn-on voltage drop of VT1. Under ideal conditions, its value is zero. When VT1 is turned off, C1 starts to charge. Because the capacitor voltage cannot be abrupt, VT1 is zero voltage turn-off.
Due to the leakage inductance L1k of the transformer and the secondary rectifying filter inductor, after the VT1 is turned off, the primary current cannot be abruptly changed, and the Cb is continuously charged. At the same time, the C2 is also discharged through the primary side. When the C2 voltage drops to zero, the VD2 is naturally turned on. At this time, VT2 is turned on, and VT2 is turned on at zero voltage.
When C1 is fully charged and C2 is discharged, since VD2 is turned on, the voltage applied to the primary winding and leakage inductance of the transformer is the voltage across the blocking capacitor Cb, and the primary current begins to decrease, but continues to Cb is charged until the primary current is zero. At this time, due to the blocking effect of VD4, the capacitor Cb cannot be discharged through VT2, VT4, VD4, and the voltage across Cb remains unchanged. At this time, the current flowing through VT4 is zero, and the shutdown is performed. VT4 is zero current shutdown.
(Please read the PDF for details)