A switch-mode power supply (SMPS), also known as a switching power supply or switching converter, is a high-frequency power conversion device and a type of power supply. Its function is to convert a voltage at a certain level into the voltage or current required by the user through different structural forms. The input of a switching power supply is mostly AC power (such as mains power) or DC power, while the output is mostly devices that require DC power, such as personal computers. The switching power supply performs the voltage and current conversion between these two sources.
Unlike linear power supplies, switching power supplies utilize transistors that primarily switch between fully on (saturation) and fully off (cutoff) modes. Both modes feature low dissipation. While the transition between modes involves higher dissipation, the time is very short, resulting in energy savings and less heat generation. Ideally, the switching power supply itself does not consume electrical energy. Voltage regulation is achieved by adjusting the on and off times of the transistors. Conversely, linear power supplies consume electrical energy because the transistors operate in the amplification region during output voltage generation. The high conversion efficiency of switching power supplies is a major advantage. Furthermore, due to their high operating frequency, smaller and lighter transformers can be used, making switching power supplies smaller and lighter than linear power supplies. If efficiency, size, and weight are primary considerations, switching power supplies are superior to linear power supplies. However, switching power supplies are more complex, with internal transistors switching frequently. If the switching current is not properly managed, it can generate noise and electromagnetic interference that could affect other devices. Additionally, without specific design features, the power factor of a switching power supply may be low.
Switching power supplies are widely used in industrial automation control, military equipment, scientific research equipment, LED lighting, industrial control equipment, communication equipment, power equipment, instruments and meters, medical equipment, semiconductor refrigeration and heating, air purifiers, electronic refrigerators, LCD displays, LED lamps, communication equipment, audio-visual products, security monitoring, LED light strips, computer cases, digital products and instruments, and other fields.
A switching power supply is a power supply device used to convert alternating current (AC) into stable direct current (DC). It receives power from the mains through an AC input port.
It processes voltage and current through internal circuit components such as transformers, rectifiers, filters, and voltage regulators to output stable DC power for the load.
Switching power supplies are characterized by high efficiency, stability, and reliability, and are widely used in modern electronic equipment, communication equipment, industrial automation, and other fields.
A switching power supply converts alternating current (AC) into stable direct current (DC) for use by various electronic devices. Its main functions include:
1. Stable voltage output: Switching power supplies can output stable DC power, effectively avoiding damage to electronic equipment caused by instantaneous voltage fluctuations.
2. Increased circuit efficiency: The switching power supply uses high-frequency switching technology, which makes its conversion efficiency, stability and reliability higher, thereby improving the efficiency and quality of the entire circuit.
3. Reduced heat loss: Compared with traditional linear power supplies, switching power supplies have lower heat loss, which not only reduces the power consumption of equipment, but also extends the life of the power supply.
4. Supports a wide range of input voltages: Switching power supplies have a wide input range, typically supporting AC 90-240V input voltage, making electronic devices more adaptable.
5. Smaller size: Due to the use of high-frequency switching technology, the overall size of the switching power supply is smaller, which can be used more flexibly in the design and save space.
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I. The function of switching power supplies
The main function of a switching power supply is to break down high energy into numerous low-energy components, which are then transmitted to the output terminal via transmission lines. The output terminal provides feedback to the input terminal based on the output voltage level. This feedback mechanism effectively stabilizes the output voltage, ensuring its stability under various conditions.
Furthermore, switching power supplies generate relatively little heat because excess energy can be converted into heat and dissipated, which to some extent helps reduce voltage. Since no other losses occur during this energy conversion process, switching power supplies are highly efficient.
Meanwhile, switching power supplies also have advantages such as small size, light weight, energy saving and environmental protection, and are therefore widely used in various electronic devices.
I. What is a power switch?
A power switch uses a circuit to control a switching transistor to quickly turn the circuit on and off. It converts direct current (DC) into high-frequency alternating current (AC) to power a transformer, which then transforms the voltage to produce one or more desired voltages.
II. Function of the power switch
1. The AC power input is rectified and filtered into DC;
2. The switching transistor is controlled by a high-frequency PWM (Pulse Width Modulation) signal to apply DC power to the primary winding of the switching transformer;
3. A high-frequency voltage is induced in the secondary winding of the switching transformer, which is then rectified and filtered to supply the load.
4. The output section feeds back to the control circuit through a certain circuit to control the PWM duty cycle in order to achieve stable output.
III. Working principle of power switch
A power switch has a door inside. When the door is open, power flows through; when the door is closed, power stops flowing. So what is the door? Some switching power supplies use thyristors, while others use switching transistors. These two components have similar performance. They both rely on pulse signals applied to the base (of the switching transistor) and the control electrode (of the thyristor) to achieve conduction and cutoff. When the positive half-cycle of the pulse signal arrives, the voltage on the control electrode rises, and the switching transistor or thyristor conducts. The 300V voltage output after rectification and filtering of 220V is then conducted. It is transmitted to the secondary side through the switching transformer, and then the voltage is increased or decreased through the transformation ratio to power various circuits.
When the negative half-cycle of the oscillation pulse arrives, the voltage at the base of the power supply regulator or the control electrode of the thyristor falls below the original set voltage. The power supply regulator is cut off, the 300V power supply is shut off, and there is no voltage on the secondary side of the switching transformer. At this time, the operating voltage required by each circuit is maintained by the discharge of the filter capacitor after rectification in the secondary circuit. The process repeats when the positive half-cycle of the next pulse arrives.
This switching transformer is called a high-frequency transformer because its operating frequency is higher than the low-frequency 50Hz. So how is the pulse to drive the switching transistor or thyristor obtained? This requires an oscillation circuit. We know that a transistor has a characteristic: its base-emitter voltage is 0.65-0.7V for amplification, above 0.7V for saturation conduction, and -0.1V to -0.3V for oscillation. After adjusting its operating point, a deep negative feedback is used to generate a negative voltage, causing the oscillator to oscillate. The frequency of the oscillator is determined by the charging and discharging time of the capacitor at the base; a higher oscillation frequency results in a larger output pulse amplitude, and vice versa. This determines the output voltage of the power supply regulator.
So how is the operating voltage of the transformer secondary output regulated? Generally, a single coil is wound on the switching transformer. The voltage obtained at its upper end is rectified and filtered to serve as a reference voltage. This reference voltage is then fed back to the base of the oscillator tube through an optocoupler to adjust the oscillation frequency. If the transformer secondary voltage increases, the voltage output of this sampling coil also increases, and the positive feedback voltage obtained through the optocoupler also increases. This voltage is applied to the base of the oscillator tube, thus reducing the oscillation frequency and stabilizing the secondary output voltage.
