METHOD FOR DETECTING THE CUTTING OFF SIGNAL OF BJT IN AN ELECTRONIC BALLAST AND THE ELECTRONIC BALLAST

摘要

This invention relates to a method for detecting the real cutting off signal of BJT (bipolar junction transistor) in an electronic ballast and the corresponding electronic ballast. Said electronic ballast comprises a half-bridge circuit composed of two BJTs, the voltage output signal of said half- bridge circuit is on the one hand provided to a lamp and on the other hand is grounded via a capacitor and a backward diode. In order to accurately and quickly detect the real cutting off signal of said BJT, the HB control unit measures the sharp slope of voltage at the connection point (N) between the capacitor and the diode. By measuring the time between the flank of the control pulse for cutting off BJT and the corresponding sharp slope of voltage appearing accordingly at the connection point, the storage times (Ts1, Ts2) of the two BJTs are obtained. Said storage times are compared with each other, wherein the shorter storage time is used to adjust the amplitude of IC output for base current regulation, and the longer storage time is used to adjust the deadtime of IC outputs to make sure that there is always enough and appropriate deadtime. Thereby an electronic ballast operating more safely is provided.