I realized this board from my schematic based on the AL8853, a boost LED controller. My circuit is calculated for a high power design (735 V LED output, 350 mA LED current). Before powering my board with a 600 VDC power supply, I tried to test the board with low voltage. J2 is powered with 12 V on pin 1 and 52 V on pin 2. The PWM controller's pin is at 3.85 V forced by R2 and R4 in order to not exceed 5 V. The FB controller's pin is at 150 mV. According the controller's datasheet, I don't understand why there is no oscillation on the GATE controller's pin and Vled is still equal to Vin (52 V).