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. It only needs to change the turns ratio of the transformer and the cross-sectional area of ​​the enameled wire. In addition, the transformer is initially The secondary is isolated from each other and does not need to share the same land. Therefore, the transformer switching power supply is also referred to as an off-line switching power supply. The offline here does not require input power, but there is no wire connection between the input power and the output power, and the energy is transmitted through the magnetic field coupling.
The biggest advantage of transformer switching power supply using transformer to isolate the input and output is to improve the insulation strength of the equipment, reduce the safety risk, and also reduce EMI interference, and it is easy to match the power.
Transformer switching power supply has single-excitation transformer switching power supply and double-excited transformer switching power supply. Single-excited transformer switching power supply is widely used in low-power electronic equipment. Therefore, single-excited transformer switching power supply is widely used. The double-excited transformer switching power supply is generally used in electronic equipment with high power, and the circuit is generally more complicated.
The disadvantage of the single-excited transformer switching power supply is that the volume of the transformer is larger than that of the excitation transformer of the double-excited transformer switching power supply, because the magnetic core of the transformer of the single-excited switching power supply only works at the single end of the magnetic circuit curve, the magnetic circuit The area of ​​the curve change is small.
Single-excited transformer switching power supply works
Figure 1-16-a is the simplest working principle diagram of a single-excited 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 resistor.
When the control switch K is turned on, the DC input voltage Ui first supplies power to the winding of the primary winding N1 of the transformer T, and the current generates a self-induced electromotive force e1 at both ends of the winding of the primary winding N1 of the transformer; meanwhile, by the action of the mutual inductance M, The induced electromotive force e2 is also generated at both ends of the secondary winding of the transformer N2; when the control switch K is suddenly turned from the on state to the off state, the energy (magnetic energy) stored in the winding of the primary winding N1 of the transformer is also generated. The counter electromotive force e1; at the same time, the induced electromotive force e2 is also generated in the winding of the secondary winding N2 of the transformer by the action of the mutual inductance M.
Therefore, the direction of the electromotive force induced in the primary and secondary coils of the transformer is different before and after the control switch K is turned on.
The so-called single-excited transformer switching power supply means that the switching power supply is within one working cycle, and the primary coil of the transformer is only excited by the DC voltage once. Generally, a single-excited transformer switching power supply provides power (or voltage) output to the load for only half a cycle within one duty cycle. When the primary coil of the transformer is just excited by the DC voltage, the secondary winding 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 winding of the transformer is just excited by the DC voltage, The secondary winding of the transformer does not provide a power output to the load, but only provides power output to the load after the excitation voltage of the primary winding of the transformer is turned off. This transformer switching power supply is called a flyback switching power supply.
Figure 1-16-b shows the waveform of the output voltage of a single-excited transformer switching power supply. Since the output voltage is the secondary output of the transformer, there is no DC component at 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, which is characteristic of the output voltage waveform of the single-excited 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; otherwise, when only the negative half-wave voltage is output, it is a flyback switching power supply.
Incidentally, the positive and negative polarity of the transformer output voltage waveform in Figure 1-16-b can be changed by adjusting the direction (phase) of the transformer coil. Strictly speaking, only when the duty ratio of the control switch is equal to 0.5, the output voltage of the switching power supply can be called the positive and negative half-cycle voltage, but since people are used to the positive and negative half-cycle, as long as there is a positive For power supplies with negative voltage output, we still used to call them positive and negative half cycles. However, in order to distinguish from the voltage waveform when the duty ratio is not equal to 0.5, we sometimes refer to the voltage waveform when the duty ratio is not equal to 0.5 as the positive and negative half waves. Therefore, in some cases, when we do not affect the understanding of the positive and negative half-wave voltages, or when the duty ratio is uncertain, we are accustomed to refer to the positive and negative half waves as positive and negative half cycles.
In Figure 1-16-a, during Ton, the control switch K is turned on, the input power source Ui starts to energize the winding of the primary winding N1 of the transformer, and the current passes from both ends of the winding of the primary winding N1 of the transformer, and the electromagnetic induction will be in the transformer. A magnetic field is generated in the iron core and magnetic lines of force are generated; at the same time, a self-induced electromotive force E1 is generated at both ends of the winding of the primary winding N1, and an induced electromotive force e2 is also generated at both ends of the winding of the secondary winding N2; the induced electromotive force e2 acts on the load R Both ends, thereby generating 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 is generated in the core of the transformer. The magnitude of this magnetic field can be obtained by magnetic flux (referred to as magnetic flux). The number of magnetic lines is represented by Ñ„.
== ф 1-ф2 —— K is turned on (1-60)
The magnetic flux ф1 generated by the primary coil current of the transformer can also be divided into two parts, one part is used to cancel the magnetic flux ф2 generated by the secondary winding current of the transformer, which is recorded as ф10, and the other part is the magnetic flux generated by the exciting current, which is denoted 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 primary coil of the transformer, and is independent of the current flowing through the secondary coil of the transformer; the magnetic flux generated by the current flowing through the secondary coil of the transformer is completely The magnetic flux generated by another portion of the current flowing through the primary winding of the transformer cancels.
According to the law of electromagnetic induction, the equation can be listed for the primary winding N1 winding circuit of the transformer:
E1 = N1*dф/dt = Ui —— K on period (1-61)
Similarly, the equation can be listed for the transformer secondary winding N2 winding loop:
E2 = N2 *dф/dt = Up - K is on (1-62)
According to (1-61) and (1-62), we can find:
Up = e2 =n*E1 = n*Ui - K is on (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 winding N1 winding of the forward switching power supply transformer; n is the variable voltage Ratio, that is, the ratio of the output voltage of the secondary winding of the switching transformer to the input voltage of the primary coil, n can also be regarded as the turns ratio of the winding of the secondary winding N2 of the switching transformer to the winding of the primary winding N1, ie: n = N2/N1.
It can be seen 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 voltage transformation ratio of the transformer.
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