Use TOP250YN to design 14 or the 28V/10A switching power supply

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I attempted recently use TOP250YN to design 14 or the 28V/10A switching power supply.

1, I have to pin the function S feet, use three-terminal mode, but the results of switching power supply can not afford to vibration. Why?

2, and later changed to the L foot by R2, Detect DC voltage circuit, an increase of under-voltage over-voltage protection. They began to use the 4.7M, input 230V, switching power supply will not start-up. Changed to 2.4M, the input 95 ~ 230VAC , switching power supply only job, transformers began to have "嘀嘀" voices.

3, I Potentiometers PO1, adjust the slow adjustment to output +28 V, transformer sound began to disappear. What sort of sound it? Noise after the disappearance of no-load switching power supply is the normal work. I change the input power, be able to maintain basic +28 V.

4, At this point, the external load of 2.5 in Europe, switching power supply output voltage to 0.3V immediately following. Again "嘀嘀" voices. Removable load only after the restoration slowly. Why can not afford to load it with?

5, I will try to reduce the output voltage, set to +22 V, stabilized, 2.5 in Europe once again access load, the output voltage stability. And, I am going steady load, re-connected to switching power supply. Switching power supply and if the first 4:00 of the question, Why Can not load boot and then it?

6, of which, I used the feedback loop TL431, I will try to reduce the voltage to +14 V, but you can not adjust. Oscilloscope measured using one of the TL431 and 2 feet and found that its voltage amplitude has been greater than 2.5V, and a lot of corrugated . regulating the importation of AC voltage, the output voltage can follow the changes. It is obvious optocoupler's LED turn-on has been, but why can not the output voltage drop down?

Thank you everyone support and help!