Switching Power Supply Principle and Design (Serial 13) Forward Transformer Switching Power Supply

1-6. Forward transformer switching power supply

The transient control characteristics and output voltage load characteristics of the forward transformer switching power supply are relatively good. Therefore, the operation is relatively stable and the output voltage is not prone to jitter. It is often used in some occasions where high requirements are placed on output voltage parameters.

1-6-1. Working principle of forward transformer switching power supply The
so-called forward transformer switching power supply means that when the primary coil of the transformer is being excited by the DC voltage, the secondary coil of the transformer just has power output.

Figure 1-17 is a simple working principle diagram of a forward transformer switching power supply. In Figure 1-17, Ui is the input voltage of the switching power supply, T is the switching transformer, K is the control switch, L is the energy storage filter inductor, C is the energy storage filter capacitor, D2 is the freewheeling diode, D3 is the reverse peak clipping diode, and R is the load resistor.

In Figure 1-17, special attention should be paid to the same-name terminals of the primary and secondary coils of the switching transformer. If the same-name terminals of the primary or secondary coils of the switching transformer are reversed, Figure 1-17 is no longer a forward transformer switching power supply.
From equations (1-76) and (1-77), we can see that changing the duty cycle D of the control switch K can only change the average value Ua of the output voltage (positive half cycle in Figure 1-16-b), while the amplitude Up of the output voltage remains unchanged. Therefore, when a forward transformer switching power supply is used as a voltage-stabilized power supply, it can only use the voltage average value output method.

In Figure 1-17, the energy storage filter inductor L, the energy storage filter capacitor C, and the freewheeling diode D2 are the voltage average output filter circuit. Its working principle is exactly the same as the series switching power supply voltage filter output circuit in Figure 1-2, and will not be repeated here. For the detailed working principle of the voltage average output filter circuit, please refer to the "Series switching power supply voltage filter output circuit" in the "1-2. Series switching power supply" section.

The biggest disadvantage of the forward transformer switching power supply is that at the moment when the control switch K is turned off, the primary and secondary coil windings of the switching power supply transformer will generate very high back electromotive force, which is generated by the magnetic energy stored in the excitation current flowing through the primary coil winding of the transformer. Therefore, in Figure 1-17, in order to prevent the back electromotive force from breaking down the switching device at the moment when the control switch K is turned off, a back electromotive force energy absorbing feedback coil N3 winding is added to the switching power supply transformer, and a reverse peak clipping diode D3 is added.

The feedback coil N3 winding and the reverse peak clipping diode D3 are very necessary for the forward transformer switching power supply. On the one hand, the induced electromotive force generated by the feedback coil N3 winding can limit the reverse electromotive force through the diode D3, and return the limited energy to the power supply to charge the power supply; on the other hand, the magnetic field generated by the current flowing through the feedback coil N3 winding can demagnetize the transformer core and restore the magnetic field strength in the transformer core to its initial state.

Since the control switch is suddenly turned off, the excitation current flowing through the primary coil of the transformer suddenly becomes 0. At this time, the current flowing through the winding of the feedback coil N3 just takes over the role of the original excitation current, so that the magnetic induction intensity in the iron core of the transformer returns from the maximum value Bm to the position of the magnetic induction intensity Br corresponding to the residual magnetism, that is, the current flowing through the winding of the feedback coil N3 gradually changes from the maximum value to 0. It can be seen that the induced electromotive force generated by the winding of the feedback coil N3 is charging the power supply, and the current flowing through the winding of the feedback coil N3 is also demagnetizing the iron core of the transformer.

Figure 1-18 is a voltage and current waveform diagram of several key points in the forward transformer switching power supply in Figure 1-17. Figure 1-18-a) is the rectified output voltage waveform of the transformer secondary coil N2 winding, Figure 1-18-b) is the rectified output voltage waveform of the transformer secondary coil N3 winding, and Figure 1-18-c) is the current waveform flowing through the transformer primary coil N1 winding and secondary coil N3 winding.

In Figure 1-17, during the Ton period, the control switch K is turned on, and the input power supply Ui energizes the primary coil N1 winding of the transformer. The current i1 flows through the primary coil N1 winding, and while the self-induced electromotive force is generated at both ends of N1, the induced electromotive force is also generated at both ends of the secondary coil N2 winding of the transformer, and the output voltage is provided to the load. The output voltage of the secondary coil of the switching transformer is given by equations (1-63), (1-69), (1-76), and (1-77), and the voltage output waveform is shown in Figure 1-18-a).

Figure 1-18-c) is the waveform of the current i1 flowing through the primary coil of the transformer. The current flowing through the forward switching power supply transformer is different from the current flowing through the inductor coil. The current flowing through the forward switching power supply transformer has a sudden change, while the current flowing through the inductor coil cannot change suddenly. Therefore, the current flowing through the forward switching power supply transformer can immediately reach a certain stable value when the control switch K is turned on. This stable current value is related to the current size of the transformer secondary coil. If we record this current as i10 and the transformer secondary coil current as i2, then it is: i10 = n i2, where n is the ratio of the transformer secondary voltage to the primary voltage.

In addition, the current i1 flowing through the forward switching power supply transformer has an excitation current in addition to i10, which we record as ∆i1. As can be seen from Figure 1-18-c), ∆i1 is the part of i1 that increases linearly with time. The excitation current ∆i1 is given by the following formula:
∆i1 = Ui*t/L1 —— During the K on period (1-80)
When the control switch K suddenly turns from on to off, the current i1 flowing through the primary coil of the transformer suddenly becomes 0. Since the magnetic flux ф in the transformer core cannot change suddenly, the current flowing through the secondary coil loop of the transformer must also change suddenly to offset the influence of the sudden change of the current in the primary coil of the transformer. Otherwise, a very high back electromotive force voltage will appear in the primary coil loop of the transformer, breaking down the control switch or the transformer.

If the magnetic flux in the transformer core changes suddenly, the primary and secondary coils of the transformer will generate infinite back electromotive force, which in turn will generate infinite current, and the current will resist the change of magnetic flux. Therefore, the change of magnetic flux in the transformer core will ultimately be constrained by the current in the primary and secondary coils of the transformer.

Therefore, when the control switch K suddenly turns from the on state to the off state, and the current in the transformer primary coil loop suddenly becomes 0, the current i2 in the transformer secondary coil loop must be exactly equal to the sum of the current i2 (Ton+) during the on period of the control switch K and the current converted from the transformer primary coil excitation current ∆i1 to the transformer secondary coil. However, since the direction of the current ∆i1/n converted from the transformer primary coil excitation current ∆i1 to the transformer secondary coil is opposite to the direction of the original transformer secondary coil current i2 (Ton+), the rectifier diode D1 does not conduct the current ∆i1/n. Therefore, the current ∆i1/n can only be reversely charged to the input voltage Ui through the rectifier diode D3 through the back electromotive force generated by the transformer secondary coil N3 winding.

During the Ton period, since the current i10 of the switching power supply transformer is equal to 0, the current i2 in the transformer secondary coil N2 winding loop is naturally equal to 0. Therefore, of the current flowing through the transformer secondary coil N3 winding, only the excitation current ∆i1 in the transformer primary coil is converted to the current i3 in the transformer secondary coil N3 winding loop (equal to ∆i1/n), and the magnitude of this current decreases with time.

Generally, the number of turns of the primary coil of the forward switching power supply transformer is equal to the number of turns of the secondary back electromotive force energy absorption feedback coil N3 winding, that is, the ratio of the number of turns of the primary and secondary coils is 1:1, so ∆i1 = i3. In Figure 1-18-c), i3 is represented by a dotted line.

Figure 1-18-b) Voltage waveform of the secondary back-EMF energy absorption feedback coil N3 winding of the forward switching power supply transformer. Here, the transformer primary and secondary coil turns ratio is 1:1. Therefore, when the back-EMF voltage generated by the secondary coil N3 winding exceeds the input voltage Ui, the rectifier diode D3 is turned on, and the back-EMF voltage is limited by the input voltage Ui and the rectifier diode D3, and the current flowing through the rectifier diode during the limiting is sent back to the power supply circuit to charge the power supply or energy storage filter capacitor.

The current i3 can be accurately calculated based on (1-80) and the following equation, when the control switch K is closed:

e3 = -L3*di/dt = -Ui - K on-time (1-81)
i3 = -(Ui*Ton/nL1)- Ui*t/L3 - K off-time (1-82)

The first term on the right side of the above formula is the maximum excitation current flowing through the transformer primary coil N1 winding converted to the current in the secondary coil N3 winding, and the second term is the component in i3 that changes with time. Where n is the transformer secondary coil to primary coil transformation ratio. It is worth noting that the inductance of the transformer primary and secondary coils is not proportional to the number of coil turns N, but proportional to the number of coil turns N2. It can be seen from formula (1-82) that as the number of turns of the transformer secondary coil N3 winding increases, that is, the inductance of L3 increases, the current i3 of the transformer secondary coil N3 winding becomes smaller, and it is easy to break, indicating that the energy of the back electromotive force is easy to be released. Therefore, the ratio n of the number of turns of the transformer secondary coil N3 winding to the number of turns of the transformer primary coil N1 winding is preferably greater than or equal to one.

When N1 is equal to N3, that is, L1 is equal to L3, the above formula can be changed to:
i3 =Ui(Ton-t)/L3 —— K on-time (1-83)
Formula (1-83) shows that when the number of turns of the transformer primary coil N1 winding is equal to the number of turns of the secondary coil N3 winding, if the duty cycle D of the control switch is less than 0.5, the current i3 is discontinuous; if the duty cycle D is equal to 0.5, the current i3 is critically continuous; if the duty cycle D is greater than 0.5, the current i3 is a continuous current.

By the way, in Figure 1-17, it is best to connect a high-frequency capacitor (not shown) in parallel at both ends of the rectifier diode D1. The advantage is that on the one hand, it can absorb the high-voltage back electromotive force energy generated by the secondary coil of the transformer when the control switch K is turned off, preventing the rectifier diode D1 from breaking down; on the other hand, the energy absorbed by the capacitor will be provided to the load in the form of discharge (in series with the output voltage) before the rectifier diode D1 is turned on in the second half of the cycle. This parallel capacitor can not only increase the output voltage of the power supply (equivalent to the role of voltage doubling rectification), but also greatly reduce the loss of the rectifier diode D1 and improve work efficiency. At the same time, it will also reduce the voltage rise rate of the back electromotive force, which is beneficial to reducing electromagnetic radiation.