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Composition and working principle of switching power supply

The switching power supply is quickly applied to all walks of life due to its small size and high efficiency. The internal components of the switching power supply are 20%-30% more efficient than the linear power supply under high frequency operation. The switching power supply has many advantages, but it is inevitable that occasional failures will occur. The switching power supply circuit is composed of four parts: the main circuit, the control circuit, the detection circuit and the auxiliary power supply. The maintenance and inspection should follow the rules of easy first and then difficult. The following is the working principle of the switching power supply and the introduction of each component.

1. Composition of switching power supply: It consists of four parts: main circuit, control circuit, detection circuit and auxiliary power supply.


1. Main circuit of switching power supply

Current rush limit part: limit the rush current on the input side when the power is turned on.

Input filter: Repair surges existing in the grid, filter clutter, etc.

Rectification and filtering: Directly rectify the AC power of the grid into a smoother DC.

Inverter: Convert the rectified direct current into high-frequency alternating current, which is the core part of the high-frequency switching power supply.

Output rectification and filtering: According to the needs of the load, provide a stable and regulated and reliable DC power supply.

2. Control circuit

①After receiving the output terminal information and comparing the rated value, control the inverter to adjust and change the pulse width or pulse frequency to make the output stable

②Receive circuit value transmission protection circuit identification result adjust circuit to achieve circuit protection purpose

3. Detection circuit: Provide various parameters and various instrument data in operation in the protection circuit.

4. Auxiliary power supply: realize the software (remote) startup of the power supply, and supply power for the protection circuit and control circuit (chips such as PWM).

three. The working principle of switching power supply

A switching power supply uses a power semiconductor device as a switching element, and controls the duty cycle of the switching element to adjust the output voltage by periodically switching on and off. The switching element is repeatedly turned on and off at a certain time interval. When the switch is not turned on, the input power supply Vi provides energy to the load RL through the switch S and the filter circuit. When the switch S is turned off, the energy storage device in the circuit (The circuit composed of L1, C2, and diode D) releases the energy stored when the switch is turned on to the load RL, so that the load can obtain continuous and stable energy.


VO is the average voltage across the load

TON is the time that the switch is turned on each time

T is the duty cycle of the switch on and off


It can be seen from the formula that the average value of the voltage between VO also changes when the ratio of the switch on time and the duty cycle is changed. Therefore, the ratio of TON and T can be automatically adjusted with the change of the load and the input power supply voltage, so that the output voltage VO can remain unchanged. Change. 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" (TimeRationControl, abbreviated as TRC).

According to the TRC control principle, there are three ways:

1. Pulse width modulation (PulseWithModulation, 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 conduction pulse width is constant, and the duty cycle is changed by changing the switching frequency.

3. Hybrid modulation: The on-pulse width and the switching operating frequency are not fixed and can be changed with each other. It is a mixture of the above two methods.

Four. For details of common faults and troubleshooting knowledge of switching power supply, click "Switching Power Supply Fault" to understand

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