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The Principle of High Frequency Switching Power Supply

With the application of power system, high-frequency switching power supply has been more innovative and developed. On the premise of understanding the development trend of high-frequency switching power supply, let us first familiarize ourselves with the principle of high-frequency switching power supply.
High frequency switching power supply circuit principle

The high frequency switching power supply consists of the following parts:


1. Main circuit

The whole process of inputting from AC grid and outputting DC, including:
1). Input filter: its function is to filter the clutter existing in the grid, and also prevent the clutter generated by the machine from being fed back to the public grid.
2). Rectification and filtering: The AC power of the grid is directly rectified into a smoother DC power for the next-level transformation.
3). Inverter: Convert the rectified direct current into high-frequency alternating current, which is the core part of the high-frequency switching power supply. The higher the frequency, the smaller the ratio of volume, weight and output power.
4). Output rectification and filtering: Provide stable and reliable DC power according to the needs of the load.
2. Control circuit
On the one hand, take samples from the output end, compare it with the set standard, and then control the inverter to change its frequency or pulse width to achieve stable output. The control circuit carries out various protection measures for the whole machine.
3. Detection circuit
In addition to providing various parameters in operation in the protection circuit, various display instrument data are also provided.
4. Auxiliary power supply
Provides power for different requirements of all single circuits.
The second section of switch control voltage regulation principle
The switch K is repeatedly turned on and off at time intervals, and when the switch K is turned on, the input power E is supplied to the load RL through the switch K and the filter circuit. During the entire switch-on period, the power supply E provides energy to the load. When the switch K is turned off, the input power supply E interrupts the supply of energy. It can be seen that the input power supply provides energy to the load intermittently. In order to enable the load to obtain continuous energy supply, the switching regulated power supply must have a set of energy storage devices. A part of the energy is stored when the switch is turned on and released to the load when the switch is turned off.
The average voltage EAB between AB can be expressed as:
In the formula, TON is the time when the switch is turned on each time, and T is the duty cycle of the switch on and off (ie, the sum of the switch on time TON and the off time TOFF).
It can be seen from the formula that the average value of the voltage between AB also changes by changing the ratio of the switch on time and the duty cycle. Therefore, with the change of the load and the input power supply voltage, the ratio of TON and T can be adjusted automatically to make the output voltage V0 stay the same. Changing the on-time TON and the duty cycle ratio is to change the duty cycle of the pulse. This method is called "time ratio control" (TimeRatioControl, abbreviated as TRC).
According to the TRC control principle, there are three ways:
1). Pulse Width Modulation (Pulse Width Modulation, abbreviated as PWM)
The switching period is constant, and the duty cycle is changed by changing the pulse width.
2). Pulse Frequency Modulation (Pulse Frequency Modulation, abbreviated as PFM)
The turn-on pulse width is constant, and the duty cycle is changed by changing the switching frequency. Information from: Transmission and Distribution Equipment Network
3). Hybrid modulation
The on-pulse width and the switching frequency are not fixed and can be changed by each other. It is a mixture of the above two methods.
Section III Development and Trend of Switching Power Supply
In 1955, the self-excited oscillating push-pull transistor single-transformer DC converter invented by American Roger (GH. Roger) was the beginning of the realization of high-frequency conversion control circuits. Transformer, in 1964, American scientists proposed the idea of canceling the series switching power supply of the power frequency transformer, which obtained a fundamental way to reduce the size and weight of the power supply. In 1969, due to the improvement of the withstand voltage of high-power silicon transistors and the shortening of diode reverse recovery time, a 25 kHz switching power supply was finally made.


At present, switching power supplies are widely used in almost all electronic equipment such as various terminal equipment and communication equipment dominated by electronic computers due to their small size, light weight and high efficiency. power mode. Among the switching power supplies currently on the market, the 100kHz power supply made of bipolar transistors and the 500kHz power supply made of MOS-FET have been put into practical use, but their frequency needs to be further improved. To increase the switching frequency, it is necessary to reduce switching losses, and to reduce switching losses, high-speed switching components are required. However, as the switching speed increases, surges or noise can be generated due to the distributed inductance and capacitors in the circuit or the stored charge in the diodes. In this way, it will not only affect the surrounding electronic equipment, but also greatly reduce the reliability of the power supply itself. Among them, in order to prevent the voltage surge that occurs with the opening and closing of the switch, R-C or L-C buffers can be used, and for the current surge caused by the stored charge of the diode, a magnetic buffer made of an amorphous magnetic core can be used . However, for high frequencies above 1MHz, a resonant circuit should be used, so that the voltage on the switch or the current through the switch is a sine wave, which can not only reduce switching losses, but also control the occurrence of surges. This switching method is called resonant switching. At present, the research on this kind of switching power supply is very active, because this method can theoretically reduce the switching loss to zero without greatly increasing the switching speed, and the noise is also small, which is expected to become one of the high frequencies of the switching power supply. main way. At present, many countries in the world are working on the practical research of multi-terahertz converters.