The principle of switching power supply

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The principle of switching power supply [Copy link]

1.
The whole process of the main circuit from AC grid input to DC output includes:
1. Input filter: its function is to filter the noise in the grid, and also prevent the noise generated by the machine from being fed back to the public grid.
2. Rectification and filtering: directly rectify the AC power of the grid into smoother DC power for the next level of conversion.
3. Inversion: convert the rectified DC power into high-frequency AC power, 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 supply according to the load requirements.
2.
On the one hand, the control circuit samples from the output end, compares it with the set standard, and then controls the inverter to change its frequency or pulse width to achieve output stability. On the other hand, according to the data provided by the test circuit, after identification by the protection circuit, the control circuit provides various protection measures for the whole machine.
3.
In addition to providing various parameters in the protection circuit, the detection circuit also provides various display instrument data.
4. The auxiliary power supply
provides different power requirements for all single circuits.
Switch control voltage regulation principle

Switch K is repeatedly turned on and off at a certain time interval. When switch K is turned on, the input power supply E is provided to the load RL through switch K and the filter circuit. During the entire switch-on period, the power supply E provides energy to the load; when 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 voltage stabilization power supply must have a set of energy storage devices to store part of the energy when the switch is turned on and release it to the load when the switch is turned off. In the figure, the circuit composed of inductor L, capacitor C2 and diode D has this function. Inductor L is used to store energy. When the switch is turned off, the energy stored in inductor L is released to the load through diode D, so that the load obtains continuous and stable energy. Because diode D makes the load current continuous, it is called a freewheeling diode. The average voltage EAB between AB can be expressed by the following formula:
EAB=TON/T*E
, where TON is the time when the switch is turned on each time, and T is the duty cycle of the switch (i.e., the sum of the switch on time TON and the switch off time TOFF).
It can be seen from the formula that by changing the ratio of the switch on time and the duty cycle, the average voltage between AB will also change accordingly. Therefore, the output voltage V0 can be maintained unchanged by automatically adjusting the ratio of TON and T as the load and input power supply voltage change. Changing the ratio of the on time TON and the duty cycle is to change the duty cycle of the pulse. This method is called "time ratio control" (abbreviated as TRC).
According to the TRC control principle, there are three methods:
1. Pulse Width Modulation (abbreviated as PWM)
The switching cycle is constant and the duty cycle is changed by changing the pulse width.
2. Pulse Frequency Modulation (abbreviated as PFM)
The conduction pulse width is constant and the duty cycle is changed by changing the switch operating frequency.
3. Hybrid
modulation The conduction pulse width and the switching operating frequency are not fixed, and both can be changed. It is a mixture of the above two methods.
Section 3 Development and Trend of Switching Power Supply
In 1955, the self-excited oscillation push-pull transistor single transformer DC converter invented by GH. Roger of the United States was the beginning of realizing high-frequency conversion control circuit. In 1957, Jen Sen of the United States invented the self-excited push-pull double transformer. In 1964, American scientists proposed the idea of eliminating the series switching power supply of the power frequency transformer, which provided a fundamental way for the power supply to reduce the volume and weight. In 1969, due to the improvement of components such as the increase in the withstand voltage of high-power silicon transistors and the shortening of the reverse recovery time of diodes, a 25 kHz switching power supply was finally made.
　　At present, the switching power supply is widely used in various terminal equipment, communication equipment and almost all electronic equipment dominated by electronic computers with its small size, light weight and high efficiency. It is an indispensable power supply method for the rapid development of today's electronic information industry. The 100kHz power supplies made of bipolar transistors and 500kHz power supplies made of MOS-FETs sold on the market are already practical, but their frequencies need to be further improved. To increase the switching frequency, it is necessary to reduce the switching loss, and to reduce the switching loss, high-speed switching components are needed. However, when the switching speed is increased, surges or noise will be generated due to the influence of the distributed inductance and capacitance in the circuit or the stored charge in the diode. This will not only affect the surrounding electronic equipment, but also greatly reduce the reliability of the power supply itself. Among them, RC or LC buffers can be used to prevent voltage surges caused by the opening and closing of the switch, and magnetic buffers made of amorphous magnetic cores can be used for current surges caused by the stored charge of the diode. However, for high frequencies above 1MHz, a resonant circuit should be used to make the voltage on the switch or the current passing through the switch sinusoidal, which can reduce switching losses and control the occurrence of surges. This switching method is called resonant switching. At present, the research on this switching power supply is very active, because this method can theoretically reduce the switching loss to zero without significantly increasing the switching speed, and the noise is also small, which is expected to become a major way to increase the high frequency of switching power supplies. At present, many countries in the world are committed to the practical research of multi-megahertz converters.