Introduction to the working principle of push-pull switching power supply
The rectifier output push-pull transformer switching power supply, due to the alternating operation of two switching tubes, is equivalent to two switching power supplies simultaneously outputting power, and its output power is about twice the output power of a single switching power supply. Therefore, the push-pull transformer switching power supply has a high 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, two switches S1 and S2 alternate conduction, forming alternating voltages of opposite phases at the ends of windings N1 and N'1, respectively. Changing the duty cycle can change the output voltage. When S1 is conducting, diode VD1 is in the on state, and the current of inductor L gradually increases. When S2 is conducting, diode VD2 is in the on state, and the current of inductor L gradually increases. When both switches are turned off, VD1 and VD2 are both in the on state, sharing half of the current each. The peak voltage borne by S1 and S2 when they are in the off state is twice Ui. S1 and S2 conduct simultaneously, which is equivalent to a short circuit in the primary winding of the transformer. Therefore, it is necessary to avoid both switches conducting simultaneously. The duty cycle of each switch should not exceed 50%, and there should be a dead zone.
Due to the alternating operation of the two control switches K1 and K2 in the push-pull transformer switching power supply, the 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 has a high instantaneous response speed and good voltage output characteristics. Push pull transformer switching power supply is the most voltage efficient switching power supply among all switching power supplies. It can maintain a large power output even when the input voltage is very low. Therefore, push-pull transformer switching power supply is widely used in low input voltage DC/AC inverters or DC/DC converter circuits.
After bridge rectification or full wave rectification, the voltage ripple coefficient Sv and current ripple coefficient Si of the output voltage of the push-pull switching power supply are very small. Only a small value of energy storage filter capacitor or energy storage filter inductor is needed to obtain an output voltage with very small voltage ripple and current ripple. Therefore, push-pull switching power supply is a switching power supply with excellent output voltage characteristics.
In addition, the transformer of push-pull switching power supply belongs to bipolar magnetic polarization, and the range of magnetic induction change is more than twice that of unipolar magnetic polarization. Moreover, the transformer core does not need to leave an air gap. Therefore, the magnetic permeability of the transformer core of push-pull switching power supply is many times higher than that of the transformer core of forward or reverse switching power supply with unipolar magnetic polarization; In this way, the number of turns in the primary and secondary coils of a push-pull switching power supply transformer can be more than twice that of a unipolar magnetic polarization transformer. So, the leakage inductance and copper resistance loss of push-pull switching power supply transformers are much smaller than those of unipolar magnetic polarization transformers, and the working efficiency of switching power supplies is very high.
In the push-pull switch conversion circuit, energy conversion is alternately controlled by two transistors. When the output power is the same, the current is only half of that of the single ended switch power supply transistor, so the switching loss is reduced and the efficiency is improved.
