Switching power supply programmable feedback

Project Description: This project describes
a digital control switching power supply feedback system based on the STC8G microcontroller, licensed under
CC
-BY-NC-SA-4.0. 
The system
controls the output voltage of the switching power supply via serial port data transmission and reception, and transmits real-time voltage and current data back to the host computer via serial port. Data can be directly connected to a computer for power supply output control, or connected to other microcontroller devices as a host computer. 
Project Attributes:
This is the first public release of this project, and it is the author's original work. This project has not won any awards in other competitions.
Design Principles:
The circuit framework of this verification board
 is divided into a main power supply section,
a digital feedback section, isolated power supply, and
a host computer communication section with safety isolation.
Working Principle:
The output voltage and current values ​​are sampled in real-time by the MCU's built-in ADC channel. The voltage and current values ​​control the MCU to output a PWM signal with varying duty cycle, which is then fed back to the primary side of the main power supply. (Optical isolation)
Current sampling uses the LT199G1, modified from the original official circuit to low-side detection, making it suitable for current detection at higher voltages. The voltage signal across the sampling resistor is amplified by the LT199G1 and then input to the STC8G-ADC pin. The output of the LT199G1 is clamped by a Zener diode to prevent the output voltage signal from being too high and damaging the MCU.
By switching the feedback type, it can also be used as a regular adjustable output voltage switching power supply.
After switching the jumper cap to regular feedback, the output voltage can be adjusted via potentiometer R9 on the board
(reserved for ordinary feedback - AP4313).
Software instructions:
The microcontroller program is based on the STC8G microcontroller (see attached program project).
Serial port transmit and receive data format
: Note: When downloading the program, the MCU clock frequency is set to 27MHz via the programmer.
The host computer uses LabVIEW 2018 to write
the communication baud rate, which needs to be changed to 115200bit before startup.
Physical demonstration:
PCB power-on test.
Design considerations:
 Currently, all key parts of the overall PCB have been successfully tested, but there are still problems with the main power supply section (I hope an expert in switching power supply design can help me look at the problem; I am only a beginner in power supply design and do not recommend using this part of the circuit for now). After powering on, the NCP1342 has a control signal output, but the output frequency is always only 25KHz. Adjusting the frequency setting resistor has no change in the switching frequency. According to the datasheet, it may be running in low power mode (light load mode), but I don't know how to get out of this mode. There is relatively little public information about the NCP1342 online. The domestically produced alternative, PN8213, is still unavailable.
Soldering the power transistor will cause both the power transistor and NCP1342 to break down. During the overall testing, it operated stably once, but after restarting, both the power transistor and NCP1342 exploded.
Transformer parameters: Primary: 0.5mm enameled wire 40T+40T, Feedback: 0.5mm enameled wire 4T, Primary: 1mm*4 enameled wire 4T (4 parallel). Transformer core is ATQ2516.
Other
accessories include a video demonstration of the programmable feedback section (feedback control boost DC-DC-fb pin), output acquisition of real-time voltage and current values, and physical power-on status detection testing.