Hi
I finally solve the problem, its because of the spike on the sensed voltage of the resonance current. I use one CT for current sensing and connect it to the pin6 of l6699 with the minimum track lenght.
Now I can change the output voltage from 14kv to 30kv by changing the reference voltage on the opto-coupler primary.
I have a problem that is related to the high voltage. at the output voltage above 17kv when I touch a secondary ground , it spark to my hand. I measure the voltage of the secondary ground via one high voltage probe. I connect the positive probe to the secondary ground and the negative to the primary ground. It is about 1000 volts. I connect the secondary ground to the middle of the 2 series capacitors that they connect to the phase and null of the input line(220v-ac) . but it doesnt have any effect.
please help me to solve this problem
Regards