Preheating circuit applied to fluorescent lamp electronic ballast

The invention relates to a preheating circuit applied to a fluorescent lamp electronic ballast, which comprises a diode and a first capacitor which are connected in series and are then connected with a secondary winding of a magnetic ring in parallel, a third resistor and a second capacitor which are connected in series and are then connected with the first capacitor in parallel, a first controlswitch which are connected with the diode and the serial-connecting branch of the first capacitor in parallel, and a second control switch which is connected with the third resistor and the serial-connecting branch of the second capacitor in parallel, when a self-exciting magnetic ring driving circuit to start, the first capacitor and the second capacitor are sequentially charged, and the first control switch and the second control switch are alternatively closed and opened. In the preheating circuit, the control switch drives the secondary side current of the magnetic ring to change the working efficiency of a half-bridge line, so that the preheating start to a lamp tube is realized by controlling the voltages at the two ends of the lamp tube, no extra energy consumption is needed, and the efficiency of the ballast is improved.