Principle and Design of switching power supply for single-excited Transformer (11)

1-5. Single-excited transformer Switching Power Supply
The biggest advantage of transformer switching power supply is that the transformer can output multiple sets of different values at the same time, which makes it easy to change the output voltage and output current, you only need to change the turns ratio of the transformer and the cross-sectional area of the enamelled wire. In addition, the transformer is isolated from each other at the beginning and the secondary nodes without sharing the same location. Therefore, some people call the transformer switch power supply an offline switch power supply. In this example, there is no wire connection between the input power supply and the output power supply, instead of the input power supply, which completely transmits energy through magnetic field coupling.
The biggest advantage of using transformer to isolate the input and output of transformer switching power supply is to improve the insulation strength of the equipment, reduce security risks, reduce EMI interference, and easily perform power matching.
The switch power supply of transformer includes single-excited transformer switch power supply and dual-excited transformer switch power supply, single-excited transformer switching power supply is widely used. The dual-excited transformer switching power supply is generally used in electronic devices with high power, and the circuit is generally more complex.
The disadvantage of Single-excited transformer switching power supply is that the transformer volume is larger than that of double-excited transformer switching power supply, because the core of the transformer of the single-excited switch power supply only works on the single-ended magnetic circuit curve, the area of the magnetic circuit curve change is very small.
1-5-1. Working Principle of Single-excited transformer Switching Power Supply
 

Figure 1-16-A is the simplest working principle of single-excited transformer switching power supply. In Figure 1-16-a, the UI is the input voltage of the switching power supply, T is the switch transformer, k is the control switch, and r is the load resistance.
When the control switch K is switched on, the DC input voltage UI first supplies power to the N1 winding of the primary coil of the transformer t, and the current at both ends of the N1 winding of the primary coil of the transformer will generate the self-inductive potential E1. At the same time, through the action of mutual m, the induction EMF E2 will also be generated at both ends of the N2 winding of the transformer secondary coil. When the control switch K is suddenly switched from the on-off state to the off state, the energy (magnetic energy) stored in the N1 winding of the primary coil of the transformer will also generate the back-EMR E1, in the N2 winding of the transformer secondary coil, the induction potential force E2 is also generated.
Therefore, before the control switch K is switched on and after the switch is switched on, the direction of EMR generated in the first and second coils of the transformer is different.
The so-called single-excited transformer Switching Power Supply refers to the switch power supply within a working cycle, the primary coil of the transformer is limited to DC voltage once. Generally, the single-excited transformer Switching Power Supply provides power (or voltage) output to the load within one working cycle in only half a cycle. When the primary coil of the transformer is stimulated by DC voltage, the secondary coil of the transformer also provides power output to the load; when the primary coil of the transformer is stimulated by the DC voltage, the secondary coil of the transformer does not provide power output to the load, the power output is provided to the load only after the excitation voltage of the primary coil of the transformer is turned off. This type of transformer switching power supply is called the Reverse type switching power supply.
Figure 1-16-B is the waveform of the output voltage of the switch power supply of a Single-excited transformer. Because the output voltage is the secondary output of the transformer, there is no DC component in the output voltage uo. The area of the output positive half wave is exactly the same as that of the negative half wave, which is characteristic of the output voltage waveform of the switch power supply of the single-excited transformer. In Figure 1-16-B, when only the positive half-wave voltage is output, the positive half-wave switching power supply is used. If only the negative half-wave voltage is output, the reverse switching power supply is used.
By the way, in Figure 1-16-B, the positive and negative polarity of the transformer output voltage waveform can be changed by adjusting the Rao line (phase) of the transformer coil. Strictly speaking, only when the duty cycle of the control switch is 0.5, the output voltage of the switch power supply can be called the positive and negative half-week voltage, but because people are used to the positive and negative half-week naming, as long as the power supply is positive and negative voltage output, we are still used to calling them positive and negative half-week. However, in order to be different from the voltage waveform when the duty cycle is not equal to 0.5, we sometimes call the voltage waveform when the duty cycle is not equal to 0.5 positive and negative half wave. Therefore, in some cases, when we do not affect the understanding of positive and negative half-wave voltage, or the duty cycle is not sure, we are also used to calling positive and negative half-wave as positive and negative half-week.
In Figure 1-16-a, during the ton period, the control switch K is connected, and the input power UI starts to power on the N1 winding of the primary coil of the transformer. The current passes through the two ends of the N1 winding of the primary coil of the transformer, electromagnetic Induction will generate a magnetic field in the iron heart of the transformer, and generate a magnetic line. At the same time, at the two ends of the N1 winding of the primary coil, it will generate a self-induced potential force E1, at the two ends of the N2 winding of the secondary coil, the induction EMF E2 is also generated. The induction EMF E2 acts on the two ends of the load R to generate load current. Therefore, under the Joint Action of 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 heart of the transformer, the size of this magnetic field can be expressed by the magnetic line flux, that is, the number of magnetic lines.
Duration = period 1-period 2 -- k connection period (1-60)
Among them, the first core coil current of the transformer can be divided into two parts, one part is used to offset the second core coil current of the transformer produced by the magnetic flux limit 2, recorded as limit 10, the other part is the magnetic flux generated by the excitation current, which is recorded as 1_△1. Apparently limit 10 =-limit 2, limit △1 = limit. That is, the magnetic flux produced by the transformer iron is only related to the excitation current in the primary coil of the transformer, and is irrelevant to the current in the secondary coil of the transformer; the magnetic flux produced by the current in the secondary coil of the transformer, it is completely offset by the magnetic flux generated by another part of the current flowing through the primary coil of the transformer.
According to the Law of electromagnetic induction, the following equations can be listed for the N1 winding circuit of the primary coil of the transformer:
E1 = N1 * D restart/dt = UI -- k During connection (1-61)
Similarly, you can list the equations for the N2 winding circuit of the transformer secondary coil:
E2 = n2 * D duration/dt = up -- k During connection (1-62)
The following values can be obtained based on (1-61) and (1-62:
Up = e2 = N * e1 = N * UI -- k is connected during (1-63)
In the above formula, the up is the amplitude of the transformer's secondary output voltage (Figure 1-16-B normal half week); the UI is the input voltage of the N1 winding of the primary coil of the transformer; n is the ratio of the output voltage of the secondary coil of the switch transformer to the input voltage of the primary coil. n can also be regarded as the ratio of the turns between the N2 winding of the switch transformer and the N1 winding of the primary coil, that is, n = n2/N1.
It can be seen that, during the period when the control switch K is switched on, the amplitude of the secondary output voltage of the excited switch power supply transformer is only related to the input voltage and the secondary/Primary Voltage Ratio of the transformer.