This phenomenon indicates that the switching power supply is not working or entering the protected status. First, measure whether the power supply control chip startup feet start voltage, without starting voltage or starting voltage is too low, will have to check whether the start-up resistor and start-pin external components leakage, this time as a normal power supply control chip, then after the inspection, it can be quickly found fault. If a start-up voltage, then measuring the output of the control chip in the boot instantly if there is a high and low level of the jump, if not jump, show control chip, peripheral oscillation circuit element, or bad protection circuit has a problem, can be substitution control chip, first check the peripheral components; If a jump, generally for switch tube bad or damaged.