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We often use switching power supply in our system, mainly used to obtain a certain power of DC power supply (most is 24V), we often see switching power supply looks like a small main box, through the surface development of many heat holes can be seen inside the circuit board, but its working principle may not be clear to us. The following is an introduction to its common sense, so that we can have a better understanding of electrical and electronic knowledge.
The high frequency switching power supply consists of the following parts:
First, the main circuit from the AC grid input, DC output of the whole process, including: 1, input filter: its role is to filter the clutter existing in the grid, but also hinder the clutter generated by the local feedback to the public grid. 2. Rectification and filtering: the grid AC power supply is directly rectified to a smoother DC power for the next level of conversion. 3. Inverter: converting the rectified DC into high-frequency AC, which is the core part of high-frequency switching power supply. The higher the frequency, the higher the volume, weight and output power ratio is. 4. Output rectification and filtering: according to the load needs, provide a stable and reliable DC power supply.
On the one hand, the control circuit sampled from the output terminal and compared with the set standard, then controlled the inverter to change its frequency or pulse width, so as to achieve output stability. On the other hand, according to the data provided by the test circuit, through the protection circuit identification, the control circuit provides a variety of protection measures for the whole machine.
3. The detection circuit provides not only the parameters in the protection circuit, but also the data of various display instruments.
Four, auxiliary power supply provides different power requirements for all single circuits. Switch control and voltage stabilization principle Switch K is repeatedly switched on and off at a certain time interval. When switch K is switched on, input power E is supplied to the load RL through switching K and filtering circuit. During the whole switching on period, power E provides energy to the load; when switch K is disconnected, input power E interrupts the energy supply. It can be seen that the input power supply provides energy to the load is intermittent, in order to make the load be able to provide continuous energy, the switching regulated power supply must have a set of energy storage device, when switching on, part of the energy stored up, when switching off, to the load release. In this diagram, the circuit composed of inductance L, capacitance C2 and diode D has such functions. Inductance L is used to store energy. When the switch is off, the energy stored in the inductance L is released to the load through diode D, so that the load gets continuous and stable energy. Because diode D makes the load current continuous, it is called a reflow diode.
The average voltage EAB between AB can be expressed as follows:
The TON is the time of switching on and T is the working period of switching on and off (that is, the sum of the switching on time TON and the switching off time TOFF). It can be seen from the formula that the average value of AB voltage will change with the change of switching time and the ratio of working period. Therefore, the output voltage V0 can be kept unchanged by automatically adjusting the ratio of TON and T with the change of load and input power supply voltage. Changing the TON of the turn-on time and the ratio of the working period, that is, changing the duty cycle of the pulse, is called "time ratio control"
(TimeRatioControl, abbreviated as TRC). According to the TRC control principle, there are three ways:
1. Pulse Width Modulation (PWM) has a constant switching period and changes the duty cycle by changing the pulse width.
2. Pulse Frequency Modulation (PFM) turns on the pulse width constant and changes the duty cycle by changing the switching frequency.
3. The pulse width and switching frequency of the hybrid modulation turn-on are not fixed and can be changed with each other. It is a mixture of the above two modes.
