Thanks Paul,
As you mentioned feedback loop issue, I check waveform again by combine them together, that is answer......
See below.
Yellow is output voltage, blue is UC3845 pin2(feedback), purple is UC3845 pin6(PWM output), green is UC3845 pin1(compare output)
output voltage ripple is from 13V to 16.5V, when voltage rise up, feedback is no change along with output. But when PWM output stop, feedback start to rise up to as much as 6.5V........then drop down.
I geuss some wrong in feedback path, and other protect circuit force output off when detect voltage up to 16.5V, if output drop to 13V, it will work again...
So I need check feedback path. It a big project.