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The principle and development trend of switching power supply

Section 1 Principle of High Frequency Switching Power Supply Circuit

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

1. Main circuit

The whole process of input from AC grid and output of DC, including:

1. Input filter: Its function is to filter the clutter existing in the power grid, and also prevent the clutter generated by the machine from being fed back to the public power 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: according to the needs of the load, provide a stable and reliable DC power supply.

2. Control circuit

On the one hand, take samples from the output end, compare 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, it also provides various display instrument data.

4. Auxiliary power supply

Provides power for different requirements of all single circuits.

Section 2 The principle of switching control voltage regulation

The switch K is repeatedly turned on and off at a certain time interval. When the switch K is turned on, the input power supply E is provided to the load RL through the switch K and the filter circuit, and the power supply E provides energy to the load during the entire switch-on period; 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. When the switch is turned on, a part of the energy is stored. When disconnected, release to load. In the figure, the circuit composed of inductor L, capacitor C2 and diode D has this function. The inductor L is used to store energy. When the switch is turned off, the energy stored in the inductor L is released to the load through the diode D, so that the load can obtain continuous and stable energy. Because the diode D makes the load current continuous, it is called freewheeling. diode. 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 (that is, 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" (Time Ratio Control, 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.

3. Mixed modulation

The on-pulse width and the switching frequency are not fixed, and can change each other. It is a mixture of the above two methods.

Section 3 Development and Trend of Switching Power Supplies

The self-excited oscillating push-pull transistor single-transformer DC converter invented by GH. Roger in 1955 was the beginning of the realization of high-frequency conversion control circuits. In 1957, Jen Sen of the United States invented the self-excited push-pull Double 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 improved 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 practical, 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.

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