Self-excited push-pull DCAC boost conversion circuit

In fact, it is a self-excited push-pull DCAC boost converter circuit, which consists of a freewheeling diode, a damping inductor, two PNP transistors, two base resistors, a resonant capacitor and a transformer with three windings. L1
provides , R4 and R5 provide base DC bias for Q6 and Q7, and the transformer T1 has three windings. The primary main winding is wound in parallel with two wires, and the center axis head 3 and 4 pins are connected to the inductor L1, and the 2nd and 5th pins are connected to the collectors of the power transistors Q6 and Q7. The 1st and 6th pins of the feedback winding are connected to the bases of Q6 and Q7. Since the performance of the switch tubes Q6 and Q7 cannot be the same, at the moment of power-on, the current injected by Vcc into the base of the switch tube cannot be absolutely balanced, and the current flowing through the collectors of the two switch tubes cannot be completely consistent. Assuming i1>i2, the size and direction of the transformer's magnetic flux are determined by i1, and the change in magnetic flux also generates a corresponding induced potential in the feedback winding, which makes the "." (1 foot) of the transformer's feedback winding negative. Due to the induced potential of the feedback winding, the potential of the base of Q7 decreases and the base potential of Q6 increases, thus forming negative feedback to Q7, making the collector current i2 of Q7 smaller, forming positive feedback to Q6, making the collector current i1 of Q6 larger, and the synthetic magnetic flux larger. The change in magnetic flux and the interaction of induced potential make Q6 saturated and turned on, and Q7 cut off. At this time, the magnetic flux reaches its maximum value, and the induced potential, which is proportional to the rate of change of magnetic flux, is also zero.
Feedback The disappearance of the induced potential on the winding causes the potential of the base of Q6 to drop, and the collector current of Q6 also drops. The reverse rate of change of the current causes the reverse rate of change of the magnetic flux, which leads to the reverse induced potential of the winding, that is, the "." (pin 1) of the feedback winding is positive, which causes the base potential of Q7 to rise and the base potential of Q6 to drop, thereby forming negative feedback to Q6, making the collector current i1 of Q6 smaller, forming positive feedback to Q7, making the collector current i2 of Q7 larger, and the synthetic magnetic flux larger. The interaction between the change of magnetic flux and the induced potential causes Q7 to saturate and conduct, and Q6 to cut off. At this time, the magnetic flux reaches its maximum value, and the induced potential, which is proportional to the rate of change of magnetic flux, is also zero.

For more detailed principles and analysis, please see: Royer line principle analysis and parameter calculation