The forward transformer switching power supply is a widely used design in power electronics due to its excellent transient response and stable output voltage characteristics under load. This makes it particularly suitable for applications where high output voltage parameters are required, as the output remains relatively jitter-free and consistent.
In a forward-type transformer switching power supply, power is delivered through the secondary winding of the transformer when the primary coil is energized by a DC voltage. The basic working principle involves a control switch (K) that turns on and off periodically, allowing energy to be transferred from the input to the output through the transformer. A filter inductor (L), a filter capacitor (C), and a freewheeling diode (D2) help smooth out the output voltage, ensuring a stable DC output.
Figure 1-17 illustrates the circuit configuration of a forward transformer switching power supply. Key components include the input voltage (Ui), the switching transformer (T), the control switch (K), the filter inductor (L), the filter capacitor (C), the freewheeling diode (D2), the reverse peak diode (D3), and the load resistor (R). It's crucial to note the polarity of the primary and secondary windings of the transformer. If the same-polarity ends are reversed, the circuit will no longer operate in forward mode.
The output voltage of the forward transformer switching power supply is primarily controlled by adjusting the duty cycle (D) of the control switch (K). However, this only affects the average output voltage (Ua), not the peak voltage (Up). As a result, the forward transformer switching power supply is typically used in applications that require regulated DC output at a constant average level.
One of the main challenges with this type of power supply is the high back electromotive force (EMF) generated when the control switch (K) turns off. This EMF is caused by the stored magnetic energy in the transformer’s primary winding. To prevent damage to the switching device, a feedback coil (N3) and a reverse peak diode (D3) are added. These components help absorb and return the excess energy back to the power source while also demagnetizing the transformer core.
Figure 1-18 shows the voltage and current waveforms for key points in the circuit. During the on-time (Ton), the control switch is closed, allowing current to flow through the primary winding (N1) and inducing a voltage in the secondary winding (N2), which supplies power to the load. When the switch opens, the current in the primary winding drops abruptly, causing a sudden change in the magnetic flux. This results in a large back EMF unless properly managed.
To address this, the feedback coil (N3) and reverse peak diode (D3) work together to limit the back EMF and return the energy to the power supply. The current through the feedback coil (i3) gradually decreases over time, ensuring that the transformer core is reset to its initial magnetic state.
The relationship between the primary and secondary currents is important in determining the behavior of the system. During the on-time, the primary current (i1) consists of both the reflected load current (i10) and the excitation current (∆i1). The excitation current increases linearly with time according to ∆i1 = Ui*t/L1. When the switch turns off, the primary current drops to zero, and the secondary current must adjust accordingly to maintain the magnetic flux.
In summary, the forward transformer switching power supply offers a stable and efficient method of converting AC to DC, especially in high-voltage applications. With proper design and component selection, it can deliver reliable performance while minimizing losses and electromagnetic interference.
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