Switching power supply principle and design (serial 26) double-excitation transformer switching power supply (part 1)

1-8. Dual-excitation transformer switching power supply

The so-called dual-excitation transformer switching power supply means that within one working cycle, the primary coil of the transformer is excited twice by the DC voltage, positively and negatively. Unlike the single-excitation transformer switching power supply, the dual-excitation transformer switching power supply generally provides power output to the load within the entire working cycle. The output power of the dual-excitation transformer switching power supply is generally very large, so the dual-excitation transformer switching power supply is widely used in some medium and large electronic equipment. The maximum output power of this high-power dual-excitation transformer switching power supply can reach more than 300 watts, and can even exceed 1000 watts.

Push-pull, half-bridge, full-bridge and other transformer switching power supplies are all dual-excitation transformer switching power supplies.

1-8-1. Working principle of push-pull transformer switching power supply

Among the dual-excitation transformer switching power supplies, the push-pull transformer switching power supply is the most commonly used switching power supply. 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 instantaneous response speed is very high and the voltage output characteristics are also very good.
The push-pull transformer switching power supply is the switching power supply with the highest voltage utilization rate among all switching power supplies. It can still maintain a large power output when the input voltage is very low, so the push-pull transformer switching power supply is widely used in DC/AC inverters or DC/DC converter circuits.

1-8-1-1. AC output push-pull transformer switching power supply

Most common DC/AC inverters, such as AC uninterruptible power supplies (UPS), use push-pull transformer switching power supply circuits. This type of DC/AC inverter has a high operating frequency, so the volume can be made very small; due to this feature, push-pull transformer switching power supplies are also often used in AC/AC conversion circuits to reduce the volume of the power transformer.

Figure 1-27 is a simple schematic diagram of a push-pull transformer switching power supply with AC output and pure resistance load. In the figure, K1 and K2 are two control switches. When they are working, one is turned on and the other is turned off. The two switches are turned on and off alternately, working alternately with each other; T is a switching transformer, N1 and N2 are the primary coils of the transformer, and N3 is the secondary coil of the transformer; Ui is the DC input voltage, R is the load resistance; uo is the output voltage, and io is the current flowing through the load.

In Figure 1-27, when the control switch K1 is turned on, the power supply voltage Ui is added to both ends of the transformer primary coil N1 winding through the control switch K1. Through the effect of electromagnetic induction, a voltage proportional to the input voltage of the N1 winding is also output at both ends of the transformer secondary coil N3 winding, and added to both ends of the load R, so that the switching power supply outputs a positive half-cycle voltage. When the control switch K1 is turned from on to off, the control switch K2 is turned from off to on. At this time, the power supply voltage Ui is added to both ends of the transformer primary coil N2 winding. Through mutual inductance, a voltage uo proportional to the input voltage of the N2 winding is also output at both ends of the transformer secondary coil N3 winding, and added to both ends of the load R, so that the switching power supply outputs a negative half-cycle voltage.

Since the power supply voltage Ui is applied to the transformer primary coil N1 winding and N2, the direction of the magnetic flux generated is exactly opposite, so a positive and negative polarity voltage uo corresponding to the voltage applied to the coil N1 and N2 windings can be obtained on the load. The positive half cycle corresponds to the output voltage induced by the N1 winding and the N3 winding when K1 is turned on; the negative half cycle corresponds to the output voltage induced by the N2 winding and the N3 winding when K2 is turned on.

Below we further analyze the working principle of the push-pull transformer switching power supply in detail.

In Figure 1-27, when the control switch K1 is turned on, the input power supply Ui starts to energize the transformer primary coil N1 winding, and the current passes through the two ends of the transformer primary coil N1 winding. Through electromagnetic induction, a magnetic field and magnetic lines of force are generated in the iron core of the transformer; at the same time, a self-induced electromotive force e1 is generated at both ends of the primary coil N1 winding, and an induced electromotive force e3 is also generated at both ends of the secondary coil N3 winding; the induced electromotive force e3 acts on both ends of the load R, thereby generating a load current. Therefore, under the joint action of the primary and secondary currents, a synthetic magnetic field generated by the current flowing through the primary and secondary coils of the transformer will be generated in the iron core of the transformer. The size of this magnetic field can be expressed by the magnetic flux (abbreviated as magnetic flux), that is, the number of magnetic lines of force Φ.

If Φ1 is used to represent the magnetic flux generated by the current of the transformer primary coil N1 winding, and Φ3 is used to represent the magnetic flux generated by the current of the transformer secondary coil, since the directions of the magnetic fields generated by the transformer primary and secondary coil currents are always opposite, the total magnetic flux of the synthetic magnetic field generated by the current flowing through the transformer primary and secondary coils in the transformer core during the period when the control switch K1 is turned on is:

Φ = Φ1－Φ3——K1 on-time (1-125)

The magnetic flux Φ1 generated by the transformer primary coil current can also be divided into two parts, one part is used to offset the magnetic flux Φ3 generated by the transformer secondary coil current, denoted as 10, and the other part is the magnetic flux generated by the excitation current, denoted as ΔΦ 1. Obviously Φ10 = - Φ3, ΔΦ 1 = Φ. That is: the magnetic flux generated in the transformer core is only related to the excitation current flowing through the transformer primary coil, and has nothing to do with the current flowing through the transformer secondary coil; the magnetic flux generated by the current flowing through the transformer secondary coil is completely offset by the magnetic flux generated by another part of the current flowing through the transformer primary coil.

According to the law of electromagnetic induction, the equation for the transformer primary coil N1 winding loop can be listed as follows:

e1 = N1dΦ/dt = Ui —— K1 on-time (1-126)

Similarly, the equation for the transformer secondary coil N3 winding loop can be listed as:

e3 = N3 dΦ/dt = (Up) —— K1 on period (1-127)

In the above formula, (Up) is the amplitude of the forward output voltage of the secondary winding N3 of the switching transformer, which is enclosed in brackets. Since the excitation current flowing through the primary winding N1 of the switching transformer changes linearly, we can consider the forward output voltage of the secondary winding N3 of the switching transformer to be a square wave. The amplitude Up of the square wave is completely equal to the half-wave average value Upa and the effective value Uo.
According to (1-126) and (1-127), we can get:

(Up) = e3 = ne1 = nUi —— K1 is on (1-128)

Equation (1-128) is the voltage relationship equation for the forward output of the push-pull transformer switching power supply. In the above equation, (Up) is the amplitude of the forward output voltage of the secondary coil N3 winding of the switching transformer, Ui is the input voltage of the primary coil N1 winding of the switching power supply transformer; n is the transformation ratio, that is, the ratio of the output voltage of the secondary coil of the switching transformer to the input voltage of the primary coil. n can also be regarded as the turns ratio of the secondary coil N3 winding of the switching transformer to the primary coil N1 winding, that is, n = N3/N1.

It can be seen from this that during the period when the control switch K1 is turned on, the amplitude of the secondary forward output voltage of the push-pull transformer switching power supply transformer is only related to the input voltage and the secondary/primary transformation ratio of the transformer.

Similarly, we can also find that when the control switch K2 is turned on, the amplitude (Up-) of the forward output voltage of the switching transformer N3 coil winding is:

(Up-) = -e3 = -ne2 = -nUi —— K2 on period (1-129)

The negative sign in the above formula indicates that the sign of e3 is opposite to that in formula (1-128), and (Up-) indicates the opposite polarity to (Up).
It should also be pointed out that the calculation results listed in formulas (1-128) and (1-129) do not take into account the influence of the energy stored in the excitation current at the moment when the control switch K1 or K2 is turned off, which will also generate a back electromotive force (flyback output) through the secondary coil N3 winding of the transformer, that is, the push-pull transformer switching power supply has both positive and flyback voltage outputs.
The reason for the generation of the flyback voltage is that the initial value of the current in the primary or secondary coil of the transformer is not equal to zero, or the initial value of the magnetic flux is not equal to zero at the moment when K1 or K2 is turned on. That is, the generation of the flyback voltage in the push-pull transformer switching power supply is generated by the energy stored in the transformer excitation current.