Working principle of push-pull switching power supply
The rectifier output push-pull transformer switching power supply, due to the alternating operation of two switch tubes, is equivalent to two switching power supplies simultaneously outputting power, 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 high output power and work efficiency. After bridge rectification or full wave rectification, only a small filtering inductance and capacitor are required, and the output voltage ripple can be very small.
In the push-pull circuit, two switches S1 and S2 are alternately connected, forming opposite phase AC voltages at both ends of windings N1 and N'1. Changing the duty cycle can change the output voltage. When S1 is on, diode VD1 is in the on state, and the current of inductor L gradually increases. When S2 is on, 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 on, sharing half of the current. The peak voltage withstood by S1 and S2 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 should be avoided that both switches conduct simultaneously. The duty cycle of each switch should not exceed 50%, and there should be a dead zone left.
Due to the alternating operation of the two control switches K1 and K2 in the push-pull transformer switching power supply, 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 instantaneous response speed is very high, and the voltage output characteristics are very good. Push-pull transformer switching power supply is the highest voltage utilization 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 the push-pull switching power supply belongs to bipolar magnetic polarization, with a magnetic induction range that is more than twice that of unipolar magnetic polarization, and the transformer core does not need to leave an air gap. Therefore, the magnetic permeability of the push-pull switching power supply transformer core is many times higher than that of the forward or reverse switching power supply transformer core with unipolar magnetic polarization; In this way, the number of coil turns in the primary and secondary windings of a push-pull switching power transformer can be more than twice that of a unipolar magnetic polarization transformer. Therefore, the leakage inductance and copper resistance losses of push-pull switching power transformers are much smaller than those of unipolar magnetic polarization transformers, and the working efficiency of switching power supplies is very high.
In a push-pull switch conversion circuit, energy conversion is controlled alternately by two transistors. When the same power is output, the current is only half of that of a single ended switch power supply transistor, resulting in a reduction in switch losses and an improvement in efficiency.
