q 2 are connected to a cathode of the diode d 7 and the second terminal of the secondary driving winding t 13 respectively and then the anode of the diode d 7 is connected to the first terminal of the secondary driving winding t 13 in the examples shown in fig1 a and fig1 b the drive circuit 32 includes two resistors r 3 r 4 and an interlocked switch circuit the interlocked switch circuit includes a diode d s and a pnp bipolar transistor q 3 wherein the anode and the cathode of the diode d s are connected to the base and emitter of q 3 the base of q 3 is connected to the dotted terminal first terminal of the slave secondary driving winding t 13 the collector and the emitter of q 3 are connected to the second terminal of the slave secondary driving winding t 13 and the gate of the slave forward transistor m 7 the resistor r 4 is connected between the collector and the emitter of the q 3 table 2 lists the polarities of the voltage across t 13 and the gate voltage of m 7 within one switching period according to the embodiments in fig1 a fig1 b fig1 a and fig1 b in the examples shown in fig1 a and fig1 b the drive circuit 32 merely includes two resistors r 1 and r 2 connected with each other which are respectively connected to the first terminal and the second terminal of the secondary driving.