What is a push-pull circuit?

The push-pull circuit is an amplifying circuit which can be divided into a single-ended amplifier and a push-pull amplifier according to the number of amplifying components of the power amplifier output stage.

The output stage of a single-ended amplifier is amplified by two a half cycles of positive and negative signals by an amplifying element (or multiple components but in parallel). Single-ended amplification machines can only take Class A work status.

The output stage of the 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 are alternately switched. For the load, it seems that an "arm" is pushing, and an "arm" is pulling to jointly complete the current output task. Although Class A amplifiers can be push-pull amplifiers, it is more common to use a push-pull amplifier to form a Class B or Class A amplifier.

Advantages of push-pull switching power supply

1 voltage output characteristics are very good

Since its two control switches K1 and K2 alternately work in turn, the output voltage waveform is very symmetrical, and the switching power supply provides power output to the load throughout the working period, the output current instantaneous response speed is high, and it is all The switching power supply with the highest voltage utilization among the switching power supplies can maintain a large power output even when the input voltage is low, so it is widely used in DC/AC inverters with low input voltage, or DC/DC. In the converter circuit.

2 is a switching power supply with excellent output voltage characteristics.

After 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 small value of the energy storage filter capacitor or the energy storage filter inductor is needed, and a Both voltage ripple and current ripple are small output voltages.

3 switching power supply works very efficiently

Its transformer belongs to bipolar magnetic polarization, the magnetic induction range 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 more positive than that of unipolar magnetic polarization. The magnetic permeability of the core of the excitation or reverse switching power supply transformer is many times higher; thus, the number of turns of the primary and secondary coils of the transformer can be more than double that of the primary and secondary coils of the unipolar magnetic polarization transformer, leakage inductance and copper resistance. Losses are much smaller than unipolar magnetically polarized transformers.

4 drive circuit is simple

Its two switching devices have a common ground terminal, which is much simpler than a half-bridge or full-bridge switching power supply. This is also an advantage.

Let's take a look at the advantages and disadvantages of the lower half-bridge and full-bridge switching power supplies and push-pull switching power supplies.

Half bridge and full bridge switching power supply

They all have a common disadvantage. When the two control switches K1 and K2 are in the alternate switching state, the two switching devices will simultaneously have a semi-conducting region, that is, the two control switches are simultaneously turned on; this is Because the switching device is equivalent to charging the capacitor when it starts to conduct, it needs a transition process from the off state to the fully on state; when the switching device switches from the on state to the off state, it is equivalent to discharging the capacitor. It also needs a transition process from the on state to the fully off state; when the two switching devices are in the transition between on and off, respectively, the semi-conducting state occurs at the same time. At this time, the two control switches are connected at the same time. Passing, short circuit to the supply voltage, a large current will appear in the series circuit of the two control switches, and this current does not pass through the transformer load. Therefore, during the transition between the two control switches K1 and K2 being turned on and off, respectively, the two switching devices will generate a large power loss.

Push-pull switching power supply

It does not have this loss. Because, when the control switch K1 is about to be turned off, the two primary windings N1 winding and N2 winding of the switching transformer will generate back electromotive force, and the back electromotive force generated by the N2 winding is exactly opposite to the input current; at this time, even K2 The switching device is in a semi-conducting or fully-conducting state, and in a short time, a large operating current does not occur in the circuit composed of K2, and in the circuit, there is no direct collusion loop between the two control switches; Therefore, there is no possibility of two control switches colluding at the same time as the half-bridge type and the full-bridge switching power supply, which 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 high voltage withstand voltages that must be greater than twice the operating voltage and are therefore rarely used in 220V AC powered equipment. In addition, the adjustment range of the output voltage of the DC output voltage adjustable push-pull switching power supply is much smaller than the adjustment range of the output voltage of the flyback switching power supply, and a storage filter inductor is required; therefore, it is not suitable for requiring the load voltage to vary widely. In occasions, especially when the load is very light or often open.

Its transformer has two sets of primary coils, which is a disadvantage for low power output and an advantage for high power output. Since the coil windings of high power transformers are typically wound with multiple strands, the two sets of primary coils are not fundamentally different from twin strand windings, and the two coils can reduce the current density by half compared to a single coil.

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