PURPOSE: To surely synchronize even with a large control power, by cutting off a pulse width controlling transistor (TR) controlling a switching TR at a trigger signal period.

CONSTITUTION: A TRQ1 is a transistor for switching output and a TRQ2 for pulse width control is connected between the emitter and base. A diode D10 cuts off the TRQ2 at the period of trigger signal and inserted between the anode side of a diode D5 and the TRQ2. The diode D10 zeroes a current i2 at triggering to flow a trigger current i1 to the base of the TRQ1 as it is, allowing to obtain sure synchronism with the small trigger current i1.