Principle and Design of switching power supply)

1-6. Switch Power Supply of excited Transformer
The transient control characteristics of the output voltage of the switch power supply and the load characteristics of the output voltage are relatively good. Therefore, the operation is relatively stable, and the output voltage is not prone to jitter, it is often used in scenarios with high requirements on output voltage parameters.
1-6-1. Working Principle of switching power supply of the excited Transformer
The so-called excited transformer Switching Power Supply refers to when the transformer's primary coil is under DC voltage excitation, the transformer's secondary coil has power output.
 

Figure 1-17 shows a simple working principle of the switching power supply of the forward-facing transformer. In Figure 1-17, the UI is the input voltage of the switching power supply, T is the switching transformer, and K is the control switch, l is the energy storage filter inductor, C is the energy storage filter capacitor, D2 is the continued flow diode, D3 is the back-peak diode, r is the load resistance.
In Figure 1-17, note the same name of the first and second coils of the switch transformer. If you reverse the end of the initial or secondary coil of the switch transformer, Figure 1-17 will no longer be the active transformer switching power supply.
We can see from (1-76) and (1-77) that changing the duty cycle D of the control switch K can only change the average UA of the output voltage (1-16-B, the amplitude of the output voltage remains unchanged. Therefore, the switch power supply of the excited transformer is used for the regulated power supply, and only the average voltage output mode can be used.
In Figure 1-17, the energy storage filtering inductor L and the energy storage filtering capacitor C, as well as the continued stream diode D2, are the average voltage output filtering circuit. The working principle is exactly the same as that in Figure 1-2. For more information about the working principle of the average voltage output filter circuit, see "voltage filter output circuit of the tandem Switching Power Supply" in "1-2. tandem Switching Power Supply.
The main drawback of the switch power supply of the excited transformer is that the first and second winding of the switch power transformer will generate a high back potential when the switch K is switched off, this back-force is produced by the magnetic energy stored by the excitation current flowing through the first coil winding of the transformer. Therefore, in Figure 1-17, in order to prevent the generation of the Back-voltage force breakdown switch device when the control switch K is turned off, add a back-voltage energy absorption feedback coil N3 winding in the switch power supply transformer, and added a back-to-peak diode D3.
The feedback coil N3 winding and back-to-peak diode D3 are essential for the switch power supply of the forward-facing transformer. On the one hand, feed back the induced EMR generated by the N3 winding of the coil through the diode D3 to limit the back-EMR, return the limiting energy to the power supply, and charge the power supply, the magnetic field generated by the current in the N3 winding of the feedback coil can demagnetize the Transformer Core and restore the magnetic field strength in the transformer core to the initial state.
The control switch is suddenly shut down, and the excitation current flowing through the primary coil of the transformer is suddenly 0. At this time, the current flowing through the N3 winding of the feedback coil takes over the original excitation current, 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 in the N3 winding of the feedback coil is gradually changed from the maximum to 0. It can be seen that, when the induction voltage generated by the feedback coil N3 winding is charging the power supply, the current flowing through the feedback coil N3 winding is also demagnetizing the Transformer Core.

Figure 1-18 shows the voltage and current waveforms of several key points in the Exciting Transformer switching power supply from 1-17. Figure 1-18-a) shows the output voltage waveform of transformer second coil N2 winding rectification, figure 1-18-b) is the output voltage waveform of transformer second coil N3 winding rectification, figure 1-18-c) it is the current waveform flowing through the N1 winding and N3 winding of the primary coil of the transformer.
In Figure 1-17, during the ton period, the control switch K is switched on, the input power UI powers up the N1 winding of the primary coil of the transformer, And the N1 winding of the primary coil flows through i1, at the same time, both ends of N1 generate the self-inductive potential, and at the same time generate the induction potential at both ends of the N2 winding of the transformer secondary coil, and provide the output voltage to the load. The output voltage of the secondary coil of the switch transformer is determined by the equation (1-63), (1-69), (1-76), and (1-77). The output voltage waveform is 1-18-a ).
Figure 1-18-c) is the waveform flowing through the transformer's primary coil current i1. The current in the transformer is different from that in the inductor coil. The current in the transformer is abrupt, but the current in the transformer cannot be abrupt. Therefore, when the control switch K is connected, the current flowing through the transformer of the positive Switching Power Supply can immediately reach a stable value, which is related to the current size of the transformer secondary coil. If we record the current as I10 and the transformer's secondary coil current as I2, It is I10 = n I2, where N is the transformer's secondary voltage to the primary voltage ratio.
In addition to I10, i1 also has an excitation current. we record the excitation current as ∆ i1. From Figure 1-18-c), we can see that ∆ I1 is the linear growth of I1 over time. The excitation current ∆ I1 is given below:
∆ I1 = UI * t/L1 -- k During connection (1-80)
When the control switch K is switched from on to off, the current I1 flowing through the primary coil of the transformer is suddenly 0, because the magnetic flux in the Transformer Core cannot change, the current flowing through the secondary coil circuit of the transformer must also follow the abrupt changes to offset the effect of sudden changes in the current of the primary coil of the transformer, or a very high back-voltage will appear in the primary coil circuit of the transformer, the control switch or transformer is broken down.
If the magnetic flux in the Transformer Core changes, the initial and secondary Coils of the transformer will generate an infinite back-EMR, and the back-EMR will generate an Infinite Current, the current will resist the changes in the magnetic flux. Therefore, the magnetic flux changes in the Transformer Core will eventually be constrained by the current in the transformer's initial and secondary coils.
Therefore, when the switch K is suddenly switched from the on status to the off status and the current in the circuit of the primary coil of the transformer is suddenly 0, the current I2 in the circuit of the transformer's secondary coil must be exactly the same as the current I2 (ton +) during the connection of the control switch K ), the sum of the excitation current of the primary coil of the transformer, which is converted to the current of the secondary coil of the transformer. However, the excitation current of the primary coil of the transformer is converted to the current of the secondary coil of the transformer, and the current of the secondary coil of the transformer is I2 (ton +) in the opposite direction, the rectification diode D1 does not turn on the current ∆ I1/n. Therefore, the current ∆ I1/n can only be produced by the transformer secondary coil N3 winding, the rectified diode D3 recharges the input voltage UI.
During the ton period, because the I10 of the current of the switch power supply transformer is equal to 0, the current I2 in the N2 winding circuit of the transformer secondary coil is also equal to 0, so the current flowing through the N3 winding of the transformer secondary coil, only the excitation current in the primary coil of the transformer is converted to the current I3 (equal to ∆ I1/n) in the N3 winding circuit of the transformer secondary coil. The current size decreases over time.
Generally, the turns of the primary coil of the excited switch power supply transformer are equal to the turns of the N3 winding of the secondary back-voltage energy absorption feedback coil, that is, the ratio of turns of the Primary and Secondary Coils is: 1: 1. Therefore, I1 = I3. In Figure 1-18-c), I3 is represented by a dotted line.
Figure 1-18 B) the voltage waveform of the N3 winding of the positive-induced type Switching Power Supply transformer secondary back-to-Potential Energy Absorption feedback coil. Here, the ratio of turns in the first and second coils of the transformer is 1: 1. Therefore, when the back-voltage produced by the N3 winding of the second coil exceeds the input voltage UI, the rectification diode D3 is turned on, the back-voltage is limited by the input voltage UI and the rectification diode D3, and the current flowing through the rectification diode is sent back to the power supply circuit to charge the power supply or energy storage filter capacitor.
Accurately calculate the current I3 size, which can be obtained according to the (1-80) formula and the following equation. When the control switch K is off:
E3 =-L3 * di/dt =-UI -- k During connection (1-81)
I3 =-(ui * ton/nl1)-UI * t/L3 -- K during Shutdown (1-82)
The first item on the right of the above formula is the maximum excitation current flowing through the N1 winding of the transformer primary coil, which is converted to the current in the N3 winding of the secondary coil, and the second item is the time-varying component of I3. N is the ratio of the transformer secondary coil to the primary coil. It is worth noting that the inductance of the first and second coils of the transformer is not proportional to the number of turns N, but proportional to the number of turns N2. From the formula (1-82), we can see that the turns of the N3 winding of the transformer secondary coil increase, that is, the current I3 of the N3 winding of the transformer secondary coil decreases as the L3 inductance increases, it is also prone to interruption, which indicates that the energy of the Back-EMR is easy to release. Therefore, the ratio of the turns of the N3 winding of the transformer secondary coil to the turns of the N1 winding of the transformer primary coil is preferably greater than one or equal to one.
When N1 is equal to N3, that is, when L1 is equal to L3, the above formula can be changed:
I3 = UI (ton-T)/L3 -- k connection period (1-83)
(1-83) indicates that when the turns of the N1 winding of the primary coil of the transformer are equal to the turns of the N3 winding of the secondary coil, if the duty cycle of the control switch D is less than 0.5, the current I3 is discontinuous. If the duty cycle D is equal to 0.5, the current I3 is critical continuous. If the duty cycle D is greater than 0.5, the current I3 is continuous current.
By the way, in Figure 1-17, it is best to parallel a High-Frequency Capacitor at both ends of the rectification diode D1 (not shown in the figure ). On the one hand, it can absorb the high-voltage back-EMR energy generated by the transformer secondary coil when the control switch K is turned off to prevent the rectification diode D1 from breaking down. On the other hand, the energy absorbed by the capacitor is provided to the load by discharging (in connection with the output voltage) before the D1 of the rectification diode is turned on. This parallel capacitor not only improves the output voltage of the power supply (equivalent to double-pressure rectification), but also greatly reduces the loss of the rectification diode D1 and improves work efficiency. At the same time, it will also reduce the voltage rise rate of the Back-voltage, which is good for reducing electromagnetic radiation.