Operation principle of the push-pull switch mode power supply
The rectified output push-pull transformer switching power supply, because the two switching tubes work alternately, is equivalent to the output power of two switching power supplies at the same time, and its output power is approximately twice the output power of a single switching power supply. Therefore, the push-pull transformer switching power supply has a large output power and high working efficiency. After bridge rectification or full-wave rectification, only a small filter inductor and capacitor are needed, and the output voltage ripple can be very small.
In the push-pull circuit, the two switches S1 and S2 are turned on alternately, forming AC voltages with opposite phases at both ends of the windings N1 and N'1. Changing the duty cycle can change the output voltage. When S1 is turned on, the diode VD1 is in the on state, and the current of the inductor L gradually increases. When S2 is turned on, the diode VD2 is in the on state, and the current of the inductor L also gradually increases. When both switches are turned off, both VD1 and VD2 are in the on state, each sharing half of the current. The peak voltage endured by S1 and S2 in the off-state is 2 times Ui. S1 and S2 are turned on at the same time, which is equivalent to a short circuit on the primary side winding of the transformer, so the two switches should be avoided to be turned on at the same time. The duty cycle of each switch cannot exceed 50%, and there must be a dead zone.
Since the two control switches K1 and K2 in the push-pull transformer switching power supply work alternately, its output voltage waveform is very symmetrical, and the switching power supply provides power output to the load throughout the entire working cycle. Therefore, its output current responds instantaneously. The speed is very high and the voltage output characteristics are very good. Push-pull transformer switching power supply is the switching power supply with the highest voltage utilization among all switching power supplies. It can still maintain a large power output even when the input voltage is very low, so push-pull transformer switching power supply is widely used in low-voltage applications. Input voltage to DC/AC inverter, or DC/DC converter circuit.
After the push-pull switching power supply undergoes bridge rectification or full-wave rectification, the voltage pulsation coefficient Sv and current pulsation coefficient Si of the output voltage are very small, and only a small value energy storage filter capacitor or energy storage filter inductor is needed. An output voltage with small voltage ripple and current ripple can be obtained. Therefore, the push-pull switching power supply is a switching power supply with very good output voltage characteristics.
In addition, the transformer of the push-pull switching power supply has bipolar magnetic polarization. The range of magnetic induction change is more than twice that of unipolar magnetic polarization, and the core of the transformer does not need to leave an air gap. Therefore, the conduction of the core of the push-pull switching power supply transformer is The magnetic rate is many times higher than the magnetic permeability of the core of a unipolar magnetically polarized forward or reverse switching power supply transformer; in this way, the number of primary and secondary coil turns of a push-pull switching power supply transformer is comparable to that of a unipolar magnetically polarized transformer. The number of coil turns of the stage is more than twice less. Therefore, the leakage inductance and copper resistance loss of the push-pull switching power supply transformer are much smaller than that of the unipolar magnetic polarization transformer, and the switching power supply has high working efficiency.
In the push-pull switching conversion circuit, the energy conversion is controlled alternately by two tubes. When the same power is output, the current is only half of that of the single-ended switching power supply tube, so the switching loss is reduced and the efficiency is improved.
