Self-excited push-pull circuit based on switching power supply

Since the performances of the switches V1 and V2 cannot be absolutely consistent, the current injected by the voltage divider of R1 and R2 into the bases of the switches V1 and V2 cannot be absolutely balanced at the moment when the power is turned on, and flows through the two switches. The currents of the collectors cannot be exactly the same.

Assuming I1>I2, the magnitude and direction of the magnetic flux of the transformer are determined by the direction of the conduction current of V1. The induced electromotive force of the feedback winding makes the potential of the base of V2 drop, and the potential of the base of Vl rises, thus forming a negative feedback to V2, making V2 The collector current of Vl is getting smaller and smaller; the positive feedback is formed on Vl, which makes the collector current of Vl larger and larger. The change of the magnetic flux and the interaction of the induced electromotive force make V1 turn on in saturation and V2 turn off. At this time, the magnetic flux reaches the maximum value, and the induced electromotive force proportional to the rate of change of the magnetic flux is zero.

As the induced electromotive force on the feedback winding is zero, the base potential of Vl decreases, the collector current of Vl also decreases, and the rate of change of the current is reversed, causing the rate of change of the magnetic flux to be reversed, resulting in the reverse of the induced electromotive force of the winding. , which causes the base potential of V2 to rise and the potential of Vl to drop, thus forming a negative feedback to Vl, making the collector current of Vl smaller and smaller; forming a positive feedback to V2, making the collector current of V2 larger and larger . The resultant magnetic flux increases, and the interaction of the change of the magnetic flux and the induced electromotive force makes V2 turn on at saturation and Vl is cut off. At this time, the magnetic flux reaches the maximum value, and the induced electromotive force, which is proportional to the rate of change of the magnetic flux, is zero.

The above two processes are continuously cycled, so that the collectors of the two power tubes form square wave electricity, and the purpose of the resonant capacitor C1 is to make the oscillation circuit perform simple resonance oscillation according to a specific frequency. The stage winding, capacitor C2 and load RL form a resonant circuit.