Principle and Design of switching power supply)

By the way, it is pointed out that during toff of control switch K, the magnetic flux in the Transformer Core is mainly determined by the current in the circuit of the transformer secondary coil, this is equivalent to the magnetic field generated by the current flowing through the transformer secondary coil, which can demagnetize the Transformer Core and restore the magnetic field strength in the transformer core to the initial state.
The control switch is suddenly turned off, and the excitation current flowing through the primary coil of the transformer is suddenly 0. At this time, the current flowing through the secondary coil of the transformer takes over the excitation current in the original primary coil of the transformer, the magnetic induction intensity in the Transformer Core is returned from the maximum BM to the position of the magnetic induction intensity BR corresponding to the remaining magnetism, that is, the current flowing through the N3 winding is gradually changed from the maximum to 0. It can be seen that the Transformer Core is demagnetized when the switching power of the Reverse type transformer is output and the current flowing through the secondary coil circuit.

Figure 1-20 is the switching power supply of the anti-Exciting Transformer. When operating in the critical continuous current state, the input voltage of the rectification is UO, the load current IO, and the magnetic flux of the transformer core, and transformer initial and secondary current waveforms.
Figure 1-20-a), the output voltage of the transformer secondary coil uo is a pulse waveform with positive and negative polarity. Generally, the negative half week is a very regular rectangular wave, while the positive half week, because the output pulse is limited by the rectification diode, when the switching power supply is in the continuous current or critical continuous current state, the output waveform is also a Rectangular Wave. Therefore, the positive half-week amplitude of the input voltage of the rectification diode is basically the same as the voltage at both ends of the output voltage uo or the energy storage filtering capacitor. Therefore, the amplitude up of the input voltage of the rectification diode is basically the same as that of the half-wave average uPA and the output voltage of the rectification diode.
Figure 1-20 B) shows the process of changing the magnetic flux in the transformer core. The Transformer Core is magnetized when the control switch is on, and the Transformer Core is demagnetized when the control switch is off. Therefore, the magnetic flux in the Transformer Core changes from the residual magnetic s • Br to the maximum magnetic flux s • BM during the toff period, the magnetic flux in the Transformer Core changes from the maximum magnetic flux s • BM to the residual magnetic flux s • BR.
Figure 1-20-c) is the current waveform of the first and second coils of the transformer when the switching power supply of the anti-Exciting Transformer is in the critical current state. In the figure, I1 is the current flowing through the primary coil of the transformer, I2 is the current flowing through the secondary coil of the transformer (as shown in the dotted line), and Io is the current flowing through the load (as shown in the dotted line ). When the control switch is on, the Transformer Core is magnetized by the current of the primary coil. When the control switch is off, the Transformer Core is demagnetized by the current of the secondary coil and the current is output to the load. From Figure 1-20-c), we can also see that the current in the first and second coils of the transformer can jump suddenly. At the moment when the switch is switched off, the current flowing through the primary coil of the transformer changes from the maximum value to 0, while the current flowing through the secondary coil of the transformer changes from 0 to the maximum value at the same time. In addition, the maximum current of the primary coil of the transformer is equal to n times the maximum current of the secondary coil of the transformer (n is the ratio of the secondary voltage of the transformer to the Primary Voltage ).
By the way, although the result of (1-110) is obtained by the condition that the switching power supply works in the critical continuous current state, it is also true for the switch power supply to work in the continuous current state or cut-off state, because, when the capacity of the energy storage filter capacitor is large enough, the output voltage uo only depends on its peak voltage up, rather than its average value.
When the switching power supply is in the current discontinuous state, that is, the duty cycle of the control switch is reduced by an hour, I (0) and (1-100) in (1-108) the i2x type is 0, and the current flowing through the secondary coil of the transformer has been reduced to 0 before the switch-off period ends, this is equivalent to reducing both the output voltage and current of the switch power supply. In this case, the switch power supply will reduce the power output to the load.

When the switching power supply is in the continuous current state, that is, when the duty cycle of the control switch is increased, the I (0) in (1-100) type cannot be 0, (1-108) the i2x type cannot be 0, which is equivalent to an increase in the output voltage and current. In this case, the switching power supply will provide a larger power output to the load.
Figure 1-21 is the switching power supply of the anti-Exciting Transformer. When the current is not continuous, the input voltage of the rectifier diode is UO, the load current IO and the magnetic flux in the Transformer Core, and transformer initial and secondary current waveforms.

Figure 1-22 is the switching power supply of the anti-Exciting Transformer. When the current is in the continuous current state, the input voltage of the rectifier diode is UO, the load current IO, and the magnetic flux in the Transformer Core, and transformer initial and secondary current waveforms.
We can see that, the switching regulated power supply of the anti-excited transformer is to adjust the output voltage of the switching power supply by changing the duty cycle of the control switch and the charge and discharge current of the energy storage filter capacitor to achieve stable voltage output.

It should also be pointed out that the above analysis all assumes that the output voltage of the switch power supply is relatively unchanged. In fact, when the switch power supply is just starting to work, that is: when the energy storage filter capacitor is charging, the output voltage of the switching power supply is also changing, but the output voltage quickly transits from an initial value to a stable value, then it is transitioned from an initial value (the previous stable value) to the next stable value ....... Therefore, the process of switching the voltage or current from an initial value to a stable value is called the transition process of the switching power supply circuit.