What is a push-pull circuit? Analysis of the advantages and disadvantages of push-pull switching power supply

　　What is a push-pull circuit? Analysis of the advantages and disadvantages of push-pull switching power supply - Push-pull circuit is an amplification circuit. It can be divided into single-ended amplifier and push-pull amplifier according to the number of amplification components in the output stage of the power amplifier.

　　What is a push-pull circuit?

　　Push-pull circuit is an amplification circuit, which can be divided into single-ended amplifier and push-pull amplifier according to the number of amplification components in the output stage of the power amplifier.

　　The output stage of a single-ended amplifier consists of one amplifying element (or multiple elements but connected in parallel as a group) to amplify the positive and negative half cycles of the signal. Single-ended amplification machines can only adopt Class A working conditions.

　　The output stage of a push-pull amplifier has two arms (two sets of amplifying elements). When the current of one arm increases, the current of the other arm decreases, and the states of the two change in turn. For the load, it seems that one arm is pushing and the other is pulling, jointly completing the current output task. Although Class A amplifiers can use push-pull amplification, it is more common to use push-pull amplification to form Class B or Class A and B amplifiers.

　　Advantages of push-pull switching power supply

　　①The voltage output characteristics are very good

　　Since its two control switches K1 and K2 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. Its output current instantaneous response speed is very high, and it is all The switching power supply has the highest voltage utilization rate among switching power supplies. It can still maintain a large power output when the input voltage is very low, so it is widely used in low input voltage DC/AC inverters, or DC/DC in the converter circuit.

　　②It is a switching power supply with very good output voltage characteristics.

　　After it undergoes bridge rectification or full-wave rectification, the voltage pulsation coefficient Sv and current pulsation coefficient Si of the output voltage are very small, so only a very small value of energy storage filter capacitor or energy storage filter inductor is needed to obtain a The output voltage has very small voltage ripple and current ripple.

　　③The working efficiency of switching power supply is very high

　　Its transformer belongs to bipolar magnetic polarization, and the range of magnetic induction change 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 its transformer core is higher than that of unipolar magnetic polarization. The magnetic permeability of the core of the excited or reverse switching power supply transformer is many times higher; in this way, the number of primary and secondary coil turns of the transformer can be more than twice less than the number of primary and secondary coil turns of the unipolar magnetically polarized transformer, and the leakage inductance and copper resistance The losses are much smaller than those of unipolar magnetic polarization transformers.

　　④Simple drive circuit

　　Its two switching devices have a common ground terminal. Compared with half-bridge or full-bridge switching power supplies, the driving circuit is much simpler, which is also an advantage.

　　Let’s learn about the advantages and disadvantages of half-bridge and full-bridge switching power supplies and push-pull switching power supplies.

　　Half-bridge and full-bridge switching power supplies

　　They all have a common shortcoming, that is, when the two control switches K1 and K2 are in the alternating switching state, a semi-conducting area will appear on the two switching devices at the same time, that is, the two control switches are in the on state at the same time; this is Because when the switching device starts to turn on, it is equivalent to charging the capacitor, and it requires a transition process from the off state to the fully on state; and when the switching device transitions from the on state to the off state, it is equivalent to discharging the capacitor. It also requires a transition process from the on state to the completely off state; when the two switching devices are in the transition period of on and off respectively, a semi-conducting state will appear at the same time. At this time, it is equivalent to two control switches being connected at the same time. If connected, a short circuit will occur in the power supply voltage, and a large current will appear in the series circuit of the two control switches, and this current will not pass through the transformer load. Therefore, during the transition period when the two control switches K1 and K2 are on and off respectively, the two switching devices will generate great power loss.

　　Push-pull switching power supply

　　There will be no such loss. Because, when the control switch K1 is about to turn off, the two primary coils N1 winding and N2 winding of the switching transformer will generate counter electromotive force, and the counter electromotive force generated by the N2 winding is exactly opposite to the direction of the input current; at this time, even K2 The switching device is in a semi-conducting or fully conductive state. In a short period of time, no large operating current will appear in the circuit composed of K2, and in the circuit, there is no direct series connection loop between the two control switches; Therefore, unlike the half-bridge and full-bridge switching power supplies, there is no possibility of two control switches colluding at the same time. This is also an advantage of the push-pull switching power supply.

　　The main disadvantages of push-pull switching power supplies

　　Its two switching devices require a high withstand voltage, which must be greater than twice the working voltage, so it is rarely used in 220V AC power supply equipment. In addition, the adjustment range of the output voltage of the push-pull switching power supply with adjustable DC output voltage is much smaller than that of the flyback switching power supply, and it requires an energy storage filter inductor; therefore, it is not suitable for applications that require a large change range in load voltage. occasions, especially those with very light loads or frequent open circuits.

　　Its transformer has two sets of primary coils, which is a disadvantage for low-power output and an advantage for high-power output. Because the coil windings of high-power transformers are generally wound with multi-strand wires, the two sets of primary coils are not fundamentally different from those wound with two-strand wires, and the two coils can reduce the current density by half compared with a single coil.

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