A circuit design scheme for a single-excitation transformer switching power supply

　　The biggest advantage of the transformer switching power supply is that the transformer can output multiple sets of voltages of different values ​​at the same time. It is easy to change the output voltage and output current by simply changing the transformer's turns ratio and the size of the enameled wire cross-sectional area. In addition, the primary and secondary of the transformer are isolated from each other and do not need to share the same ground. Therefore, some people also call the transformer switching power supply an offline switching power supply. The offline here does not mean that there is no need for input power, but that there is no wire connection between the input power supply and the output power supply, and the energy is transmitted entirely through magnetic field coupling.

　　The biggest advantage of using a transformer to electrically isolate the input and output of a transformer switching power supply is that it improves the insulation strength of the equipment and reduces safety risks. It can also reduce EMI interference and make power matching easier.

　　Transformer switching power supplies are divided into single-excitation transformer switching power supplies and dual-excitation transformer switching power supplies. Single-excitation transformer switching power supplies are commonly used in low-power electronic equipment. Therefore, single-excitation transformer switching power supplies are widely used. Dual-excitation transformer switching power supplies are generally used in electronic equipment with higher power, and the circuits are generally more complicated.

　　The disadvantage of the single-excitation transformer switching power supply is that the volume of the transformer is larger than that of the dual-excitation transformer switching power supply, because the magnetic core of the transformer of the single-excitation switching power supply only works at a single end of the magnetic loop curve, and the area of ​​the magnetic loop curve change is very small.

　　Working principle of single-excitation transformer switching power supply

　　Figure 1-16-a is the simplest working principle diagram of a single-excitation transformer switching power supply . In Figure 1-16-a, Ui is the input voltage of the switching power supply, T is the switching transformer, K is the control switch, and R is the load resistance.

　　When the control switch K is turned on, the DC input voltage Ui first supplies power to the primary coil N1 winding of the transformer T, and the current will generate a self-induced electromotive force e1 at both ends of the primary coil N1 winding of the transformer; at the same time, through the action of the mutual inductance M, an induced electromotive force e2 will also be generated at both ends of the secondary coil N2 winding of the transformer; when the control switch K is suddenly turned from the on state to the off state, the energy (magnetic energy) stored in the primary coil N1 winding of the transformer will also generate a back electromotive force e1; at the same time, through the action of the mutual inductance M, an induced electromotive force e2 will also be generated in the secondary coil N2 winding of the transformer.

　　Therefore, the directions of the electromotive forces induced in the primary and secondary coils of the transformer are different before and after the control switch K is turned on.

　　The so-called single-excitation transformer switching power supply refers to a switching power supply in which the primary coil of the transformer is only excited by the DC voltage once within one working cycle. Generally, a single-excitation transformer switching power supply only provides power (or voltage) output to the load for half a cycle within one working cycle. When the primary coil of the transformer is excited by the DC voltage, the secondary coil of the transformer also provides power output to the load. This transformer switching power supply is called a forward switching power supply; when the primary coil of the transformer is excited by the DC voltage, the secondary coil of the transformer does not provide power output to the load, but only provides power output to the load after the excitation voltage of the primary coil of the transformer is turned off. This transformer switching power supply is called a flyback switching power supply.

　　Figure 1-16-b is the waveform of the output voltage of a single-excitation transformer switching power supply . Since the output voltage is output from the secondary of the transformer, there is no DC component in the output voltage uo. The area of ​​the positive half-wave of the output voltage is exactly equal to the area of ​​the negative half-wave. This is the characteristic of the output voltage waveform of a single-excitation transformer switching power supply. In Figure 1-16-b, when only the positive half-wave voltage is output, it is a forward switching power supply; conversely, when only the negative half-wave voltage is output, it is a flyback switching power supply.

　　By the way, the positive and negative polarity of the transformer output voltage waveform in Figure 1-16-b can be changed by adjusting the winding direction (phase) of the transformer coil. Strictly speaking, only when the duty cycle of the control switch is equal to 0.5, the output voltage of the switching power supply can be called positive and negative half-cycle voltage, but because people are accustomed to the term positive and negative half-cycle, so as long as there is a power supply with positive and negative voltage output, we are still accustomed to calling them positive and negative half-cycle. But in order to distinguish it from the voltage waveform when the duty cycle is not equal to 0.5, we sometimes specifically call the voltage waveform when the duty cycle is not equal to 0.5 positive and negative half-waves. Therefore, in some occasions, when it does not affect the understanding of the positive and negative half-wave voltages, or when the duty cycle is uncertain, we are also accustomed to calling the positive and negative half-waves positive and negative half-cycles.

　　In Figure 1-16-a, during the Ton period, the control switch K is turned on, and the input power supply Ui begins to energize the transformer primary coil N1 winding. The current passes through the two ends of the transformer primary coil N1 winding, and a magnetic field is generated in the iron core of the transformer through electromagnetic induction, and magnetic lines of force are generated; 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 e2 is also generated at both ends of the secondary coil N2 winding; the induced electromotive force e2 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 ф.

　　ф= ф 1-ф2 —— K on-time (1-60)

　　The magnetic flux ф1 generated by the transformer primary coil current can also be divided into two parts, one part is used to offset the magnetic flux ф2 generated by the transformer secondary coil current, recorded as ф10, and the other part is the magnetic flux generated by the excitation current, recorded as фΔ 1. Obviously ф10 =- ф2, фΔ 1 =ф. 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 = N1*dф/dt = Ui —— K on-time (1-61)

　　Similarly, the equation for the transformer secondary coil N2 winding loop can be listed as:

　　e2 = N2 *dф/dt = Up —— K on-time (1-62)

　　According to (1-61) and (1-62), we can obtain:

　　Up = e2 = n*E1 = n*Ui —— K on period (1-63)

　　In the above formula, Up is the amplitude of the secondary output voltage of the forward switching power supply transformer (positive half cycle in Figure 1-16-b); Ui is the input voltage of the primary coil N1 winding of the forward 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 ratio of the number of turns of the secondary coil N2 winding of the switching transformer to the primary coil N1 winding, that is: n = N2/N1.

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