The two main states of a switching transistor are on and off. When the switching transistor is on, the electrical energy inside the power supply is transferred to the output capacitor for storage through the switching transistor. When the switching transistor is off, the electrical energy stored in the output capacitor is released to the load. This allows for precise control of the output voltage and current.
Furthermore, switching power supplies are highly efficient because they convert AC power into high-frequency AC power in the transformer circuit, which significantly improves efficiency, reduces transformer size, and lowers production costs. Switching power supplies also include various protection circuits, such as input over/under voltage protection, output over/under voltage protection, output overcurrent protection, and output short-circuit protection, to enhance the reliability and safety of the power supply.
Different types of switching circuits, such as relay switches, field-effect transistor switches, bipolar transistor switches, and MOSFET switches, have different working principles, but they all share the common feature of using high-speed switching characteristics to control the on and off of the circuit, thereby achieving control of current and voltage.
A switching power supply is a power device that converts AC to DC power. It uses the switching action of a transistor to switch the input voltage into a pulse signal at a high frequency, and then converts it into a DC voltage output through a rectifier and filter circuit. Switching power supplies are characterized by high efficiency, small size, light weight, and good stability.
Working principle of switching power supply
The basic working principle of a switching power supply is to switch the input voltage into a high-frequency pulse signal by the switching action of a transistor. This high-frequency pulse signal is then transformed by a transformer or inductor and processed by a filter circuit to finally obtain a stable DC output voltage. The output voltage of a switching power supply can be adjusted and stabilized as needed to meet the power requirements of different devices. Switching power supplies have many advantages, including high efficiency, good stability, small size, light weight, and high reliability. They are widely used in electronic equipment, communication equipment, computer equipment, industrial control systems, and other fields, becoming one of the most common power supply types in modern electronic technology.
Switching power supply classification
Switching power supplies can be classified according to different criteria. Here are some common classification methods:
1. Classified by input power type:
AC-DC switching power supply: converts alternating current (AC) to direct current (DC).
DC-DC switching power supply: Converts direct current to another type of direct current voltage.
2. Classified by work method:
Single-ended switching power supply: has only one switching transistor and is suitable for low-power applications.
Dual-ended switching power supply: It has two switching transistors and is suitable for high-power applications.
3. Classification by topology:
Based on topology, switching power supplies can be broadly classified into Buck, Boost, Buck-Boost, Flyback, Forward, Two-Transistor Forward, Push-Pull, Half Bridge, and Full Bridge, among others. These classifications are only a part of the overall classification; switching power supplies can be further categorized according to other specific requirements and applications.
Next, we will introduce the commonly used Flyback and Forward technologies. Forward and Flyback are two different switching power supply technologies. Forward switching power supplies use a forward high-frequency transformer to isolate and couple energy, while the corresponding technology is the flyback switching power supply.
Forward switching power supply
Forward converter switching power supplies have a relatively complex structure but very high output power. They are suitable for switching power supplies ranging from 100W to 300W and are generally used in low-voltage, high-current switching power supplies, making them widely applicable.
As shown in the diagram below, a forward converter switching power supply specifically refers to a power supply where, when the switching transistor is turned on, the output transformer acts as a dielectric to directly couple magnetic field energy, converting electrical energy into magnetic energy and enabling simultaneous input and output. However, it also has drawbacks in practical applications: for example, it requires an additional back EMF winding (to prevent the back EMF generated by the primary coil of the transformer from damaging the switching transistor), and an additional inductor is needed on the secondary side for energy storage and filtering. Therefore, its cost is higher than that of a flyback converter switching power supply, and the transformer in a forward converter switching power supply is also larger.
flyback switching power supply
As shown in the diagram, a flyback switching power supply refers to a switching power supply that uses a flyback high-frequency transformer to isolate the input and output circuits. Its transformer not only transforms the voltage and transmits energy but also acts as an energy storage inductor; therefore, the flyback transformer is similar in design to an inductor. The circuit is relatively simple and easy to control, and flyback power supplies are widely used in low-power applications from 5W to 100W. In a flyback switching power supply, when the switching transistor is turned on, the current in the transformer's primary inductor rises. Because the output coil's polarity is reversed, the output diode is cut off, and the transformer stores energy. The load is powered by the output capacitor. When the switching transistor is turned off, the voltage induced in the transformer's primary inductor reverses, and the output diode turns on. The transformer's energy is supplied to the load through the diode, while simultaneously charging the capacitor.
As can be seen from the comparison, the transformer in a forward converter only has a transformer function, and the whole circuit can be regarded as a buck circuit with a transformer. The transformer in a flyback converter can be regarded as an inductor with a transformer function, which is a buck-boost circuit. In general, the working principles of forward and flyback converters are different. In a forward converter, both the primary and secondary circuits are active. If the secondary circuit is not active, a freewheeling inductor will provide current. It is generally in CCM mode. The power factor is generally not high, and the input/output ratio is proportional to the turns ratio and duty cycle. In a flyback converter, the primary circuit is active, and the secondary circuit is not active. The two circuits are independent. It is generally in DCM mode, but the transformer inductance is smaller, and an air gap is required. Therefore, it is usually suitable for small to medium power applications.
Forward transformers are ideal, as they do not store energy. However, because the magnetizing inductance is finite, the magnetizing current results in a large core current. To avoid flux saturation, the transformer requires an auxiliary winding for flux reset. Flyback transformers can be viewed as coupled inductors, where the inductor first stores energy and then releases it. Because the input and output voltages of a flyback transformer have opposite polarities, the secondary winding can provide a reset voltage to the core after the switching transistor is turned off. Therefore, flyback transformers do not require an additional flux reset winding.
