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Switched-mode Power Supply (SMPS), also known as switching converter, is a high-frequency electric energy conversion device and a type of power supply. Its function is to convert a level of voltage into the voltage or current required by the user through different forms of architecture.
The Switched-mode power supply (SMPS) is different from linear power supplies. Most of the switching transistors used in switched-mode power supplies are switched between fully open mode (saturation zone) and fully closed mode (cut-off zone). Both modes have low dissipation. Switch between the conversion will have higher dissipation, but the time is very short, so it saves energy and generates less waste heat. Ideally, the switched-mode power supply itself does not consume power. Voltage regulation is achieved by adjusting the time when the transistor is turned on and off. On the contrary, in the process of generating the output voltage of the linear power supply, the transistor works in the amplifying area, and it consumes power. The high conversion efficiency of the switched-mode power supply is one of its major advantages, and because it has a high operating frequency, small size and lightweight transformer can be used, so the switched-mode power supply will be smaller in size and lighter than the linear power supply.
Switched-mode Power Supply
If the high efficiency, volume, and weight of the power supply are the key considerations, the switched-mode power supply is better than the linear power supply. However, the switched-mode power supply is more complicated, and the internal transistors will switch frequently. If the switching current has not been processed, noise and electromagnetic interference may be generated to affect other equipment, and if the switch mode power supply is not specially designed, its power factor may not be high.
The switched-mode power supply is roughly composed of four parts: main circuit, control circuit, detection circuit, and auxiliary power supply.
The internal structure of the switched-mode power supply
Inrush current limit: limit the inrush current on the input side when the power is turned on.
Input filter: Its function is to filter the clutter that exists in the grid and prevent the clutter generated by the machine from being fed back to the grid.
Rectification and filtering: Directly rectify the AC power of the grid into a smoother DC.
Inverter: Convert the rectified DC power into high-frequency AC power, which is the core part of the high-frequency switched-mode power supply.
Output rectification and filtering: Provide a stable and reliable DC power supply according to load needs.
On the one hand, samples are taken from the output terminal and compared with the set value, and then the inverter is controlled to change its pulse width or pulse frequency to stabilize the output. On the other hand, according to the data provided by the test circuit, the control circuit carries out various protection measures for the power supply.
Provide various operating parameters and various instrument data in the protection circuit.
Realize the software (remote) start of the power supply, and supply power for the protection circuit and control circuit (chips such as PWM).
In the field of switched-mode power supply technology, people are developing related power electronic devices and switching frequency conversion technology. The two promote each other to promote the switched-mode power supply to the direction of anti-interference, light, small, thin, low noise, high reliability, with a growth rate of more than two digits every year. Switched-mode power supplies can be divided into two categories: AC/DC and DC/DC.
Switched-mode power supplies are becoming popular and miniaturized. They will gradually replace all applications of transformers in life. The application of low-power micro-switch mode power supplies must first be reflected in digital display meters, smart meters, mobile phone chargers, etc. At this stage, the country is vigorously promoting the construction of smart grids, and the requirements for electric energy meters are greatly increased. Switched-mode power supplies will gradually replace the application of transformers in electric energy meters.
The difference between the reversing series switched-mode power supply and the general series switched-mode power supply is that the output voltage of this reversing series switched-mode power supply is a negative voltage, which is exactly the opposite of the positive voltage output by the general series switched-mode power supply; and because of energy storage inductor L only outputs current to the load when the switch K is turned off. Therefore, under the same conditions, the output current of the reversing series switched-mode power supply is twice as small as the output current of the series switched-mode power supply.
The working process of the switched-mode power supply is quite easy to understand. In the linear power supply, the power transistor is operated in linear mode. The difference from the linear power supply is that the PWM switched-mode power supply allows the power transistor to work in the on and off state. In the state, the volt-ampere product added to the power transistor is very small (when it is turned on, the voltage is low and the current is large; when it is off, the voltage is high, and the current is small) / the volt-ampere product on the power device is the loss generated on the power semiconductor device.
Bert diagram of switch mode power supply
Compared with the linear power supply, the more effective working process of the PWM switched-mode power supply is achieved by "chopping", that is, cutting the input DC voltage into a pulse voltage whose amplitude is equal to the amplitude of the input voltage. The duty cycle of the pulse is adjusted by the controller of the switched-mode power supply. Once the input voltage is chopped into an AC square wave, its amplitude can be increased or decreased by the transformer. The output voltage value can be increased by increasing the number of secondary windings of the transformer. Finally, these AC waveforms are rectified and filtered to obtain a DC output voltage.
The main purpose of the controller is to keep the output voltage stable, and its working process is very similar to the linear controller. That is to say, the functional blocks, voltage reference, and error amplifier of the controller can be designed to be the same as the linear regulator. The difference between them is that the output of the error amplifier (error voltage) passes through a voltage/pulse width conversion unit before driving the power tube.
Switched-mode power supplies have two main operating modes: forward conversion and boost conversion. Although the layout of their parts is very different, the working process is very different, and each has its own advantages in specific applications.
A high-frequency switched-mode power supply is the direction of development. High-frequency makes switched-mode power supply miniaturized and enables it to enter a wider range of applications, especially in high-tech fields, which promotes the development of switched-mode power supply. The growth rate of more than two digits is developing towards the direction of lightness, smallness, thinness, low noise, high reliability, and anti-interference. Switched-mode power supplies can be divided into two major categories: AC/DC and DC/DC. DC/DC converters have been modularized, and the design technology and production process have been mature and standardized, and have been recognized by users. The modularity of AC/DC, due to its own characteristics, makes it encounter more complicated technical and technological manufacturing problems in the process of modularization. In addition, the development and application of switched-mode power supplies are of great significance in saving energy, saving resources, and protecting the environment.
The power electronic devices used in switched-mode power supplies are mainly diodes, IGBTs and MOSFETs, and transformers. SCR has a small number of applications in switched-mode power supply input rectifier circuits and soft-start circuits. GTR is difficult to drive and has a low switching frequency. It is gradually replaced by IGBTs and MOSFETs.
The development direction of switched-mode power supply is high frequency, high reliability, low consumption, low noise, anti-interference, and modularization. Since the key technology of light, small and thin switched-mode power supply is high frequency, the major switched-mode power supply manufacturers are committed to the simultaneous development of new and highly intelligent components, especially to improve the loss of secondary rectifier devices, and to improve the Increasing technological innovation in ferrite materials to improve the high magnetic properties obtained at high frequencies and large magnetic flux density (Bs). The application of SMT technology has made great progress in switched-mode power supplies. Components are arranged on both sides of the circuit board to ensure that the switched-mode power supply is light, small, and thin. The high frequency of the switched-mode power supply will inevitably innovate the traditional PWM switching technology. The realization of the soft switching technology of ZVS and ZCS has become the mainstream technology of the switched-mode power supply and has greatly improved the working efficiency. For high-reliability indicators, American switched-mode power supply manufacturers reduce the stress of the device by reducing the operating current and lowering the junction temperature, which greatly improves the reliability of the product.
The switched-mode power supply module
Modularity is the overall trend of the development of switched-mode power supplies. Modular power supplies can be used to form a distributed power supply system, which can be designed as an N+1 redundant power supply system and realize capacity expansion in parallel mode. In view of the shortcoming of high operating noise of switched-mode power supply, if the high frequency is pursued alone, the noise will inevitably increase. However, the use of partial resonant conversion circuit technology can theoretically achieve high frequency and reduce noise. There are still technical problems in the practical application of resonant conversion technology, so a lot of work is still needed in this field to make this technology practical.
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