Forward transformer switching power supply works

The transient control characteristics and output voltage load characteristics of the output voltage of the forward-type transformer switching power supply are relatively good. Therefore, the operation is relatively stable, and the output voltage is not easy to generate jitter. In some occasions where the output voltage parameters are relatively high, frequently used.

1-6-1. Forward Transformer Switching Power Supply Working Principle

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 winding of the transformer has just the power output.

Figure 1-17 shows the simple working principle of the switching power supply of the forward transformer. 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, and C is the energy storage. Filter capacitor, D2 is a freewheeling diode, D3 is a reversed peak diode, and R is the load resistor.

In Figure 1-17, special attention should be paid to the same name of the primary and secondary coils of the switching transformer. If the same name end of the primary or secondary coil of the switching transformer is reversed, Figure 1-17 is no longer a forward-mode transformer switching power supply.

We can see from the two equations (1-76) and (1-77) 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), and the output voltage. The magnitude of Up is unchanged. Therefore, the forward-type transformer switching power supply is used for a regulated power supply, and only the voltage average output mode can be used.

In Figure 1-17, the energy storage filter inductor L and the energy storage filter capacitor C, as well as the freewheeling diode D2, are the voltage average output filter circuit. The working principle is exactly the same as the series switching power supply voltage filter output circuit of Figure 1-2, and will not be described here. For details on the working principle of the voltage average output filter circuit, please refer to the contents of "Series Switching Power Supply Voltage Filter Output Circuit" in section "1-2. Series Switching Power Supply".

The biggest disadvantage of the forward-type transformer switching power supply is that the switching of the primary and secondary windings of the switching power supply transformer generates a high back electromotive force at the moment when the control switch K is turned off. This back electromotive force is generated by the initial winding winding of the transformer. The excitation current is stored by the magnetic energy generated. Therefore, in Figure 1-17, in order to prevent the back electromotive force breakdown switching device from being generated 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 diode is added. D3.

The feedback coil N3 winding and the reverse peak diode D3 are necessary for the forward-type transformer switching power supply. On the one hand, the induced electromotive force generated by the feedback coil N3 winding can limit the back electromotive force through the diode D3 and return the limiting energy. To the power supply, the power supply is charged; on the other hand, the magnetic field generated by the current flowing through the winding of the feedback coil N3 can demagnetize the core of the transformer, and restore the magnetic field strength in the transformer core to the initial state.

Since the control switch is suddenly turned off, the excitation current flowing through the primary coil of the transformer is suddenly zero. At this time, the current flowing through the winding of the feedback coil N3 takes over the original excitation current, so that the magnetic induction intensity in the transformer core is from the maximum value Bm. Return to the position of the magnetic induction Br of the residual magnetism, that is, the current flowing through the feedback coil N3 is gradually changed from the maximum value to zero. It can be seen that the induced electromotive force generated by the winding of the feedback coil N3 charges the power supply, and the current flowing through the winding of the feedback coil N3 also demagnetizes the transformer core.

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

In Figure 1-17, during Ton, the control switch K is turned on, the input power source Ui powers up the transformer primary winding N1 winding, and the primary winding N1 winding has a current i1 flowing through it, generating a self-induced electromotive force at both ends of N1. Both ends of the N2 winding of the secondary winding of the transformer also generate an induced electromotive force and provide an output voltage to the load. The output voltage of the secondary winding of the switching transformer is given by (1-63), (1-69), (1-76), (1-77), and the voltage output waveform is shown in Figure 1-18-a).

Figure 1-18-c) is the waveform of the current flowing through the transformer primary coil i1. The current flowing through the forward switching power supply transformer is different from the current flowing through the inductor. The current flowing through the forward switching power supply transformer is abrupt, and the current flowing through the inductor cannot be abrupt. Therefore, the current flowing through the forward switching power supply transformer immediately after the control switch K is turned on can immediately reach a certain stable value, which is related to the magnitude of the secondary winding current of the transformer. If we record this current as i10 and the transformer secondary coil current is i2, then: i10 = n i2 , where n is the transformer secondary voltage to primary voltage ratio.

In addition, the current i1 flowing through the forward switching power supply transformer has an exciting current in addition to i10, and we record the exciting current as ∆i1. It can be seen from Fig. 1-18-c) that ∆i1 is the part of i1 that grows linearly with time, and the excitation current ∆i1 is given by:

∆i1 = Ui*t/L1 —— K during the 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 is suddenly zero. Since the magnetic flux Ñ„ in the transformer core cannot be abruptly changed, the current flowing through the secondary coil of the transformer must also be changed. To counteract the effects of sudden changes in the primary winding current of the transformer, either a very high back EMF voltage will appear in the primary coil circuit of the transformer, and the control switch or transformer will be broken down.

If the magnetic flux in the transformer core is abrupt, the primary and secondary coils of the transformer will produce an infinitely high back electromotive force, and the counter electromotive force will generate an infinite current, which in turn will resist the change of the magnetic flux. Therefore, the transformer iron The flux change in the heart is ultimately constrained by the current in the primary and secondary coils of the transformer.

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

During Ton, since i10 of the current of the switching power supply transformer is equal to 0, the current i2 in the winding loop of the transformer secondary winding N2 is naturally equal to 0, so the current flowing through the winding of the secondary winding N3 of the transformer is only in the primary coil of the transformer. The field current ∆i1 is converted to the current i3 (equal to ∆i1/n) in the winding loop of the transformer secondary winding N3. 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 absorbing feedback coil N3 winding, that is, the turns ratio of the primary and secondary coils is 1:1:1, therefore, ∆i1 = i3 . In Figure 1-18-c), i3 is indicated by a dotted line.

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

Accurate calculation of the current i3 can be obtained according to the equation (1-80) and the following equation, when the control switch K

When closed:

E3 = -L3*di/dt = -Ui - K During the on period (1-81)

I3 = -(Ui*Ton/nL1)- Ui*t/L3 ——K Shutdown period (1-82)

The first term on the right side of the above equation is that the maximum excitation current flowing through the primary winding of the transformer N1 is converted to the current in the winding of the secondary winding N3, and the second term is the component of time in i3. Where n is the transformation ratio of the secondary winding of the transformer to the primary coil. It is worth noting that the inductance of the primary and secondary coils of the transformer is not proportional to the number of turns of the coil N, but is proportional to the number of turns of the coil N2. It can be seen from (1-82) that the number of turns of the N3 winding of the secondary winding of the transformer increases, that is, the inductance of L3 increases, the current i3 of the winding of the secondary winding of the transformer becomes smaller, and the current is prone to breakage. The energy of the counter electromotive force is easily released.

Therefore, the ratio n of the transformer secondary winding N3 winding turns to the transformer primary winding N1 winding turns is preferably greater than one or equal to one.

When N1 is equal to N3, ie: L1 is equal to L3, the above equation can be changed to:

I3 = Ui(Ton-t)/L3 - K During the on period (1-83)

Equation (1-83) shows that when the number of turns of the primary winding N1 winding of the transformer is equal to the number of turns of the secondary winding N3 winding, if the duty ratio D of the control switch is less than 0.5, the current i3 is discontinuous; The ratio i3 is critically continuous with a ratio D equal to 0.5, and the current i3 is a continuous current if the duty ratio D is greater than 0.5.

By the way, in Fig. 1-17, it is preferable to connect a high frequency capacitor (not shown) in parallel across the rectifier diode D1. On the one hand, it can absorb the high-voltage back electromotive force energy generated by the secondary coil of the transformer when the switch K is turned off to prevent the breakdown diode D1 from breakdown; on the other hand, the energy absorbed by the capacitor is not turned on before the rectifier diode D1 is turned on in the second half. It supplies energy to the load in the form of a discharge (in series with the output voltage). This shunt capacitor can not only improve the output voltage of the power supply (equivalent to the function of voltage doubler rectification), but also greatly reduce the loss of the rectifier diode D1 and improve the working efficiency. At the same time, it also reduces the voltage rise rate of the back EMF, which is good for reducing electromagnetic radiation.