Switching power supply composition and working principle

　　Power supply is an indispensable component of various electronic devices, and its performance is directly related to the technical indicators of electronic devices and whether they can work safely and reliably. Since the key components inside the switching power supply work in a high-frequency switching state, the power consumption is small, the conversion rate is high, and the volume and weight are only 20% to 30% of the linear power supply, it has become the mainstream product of voltage-stabilized power supply .

　　1. Composition of switching power supply

　　The switching power supply is generally composed of four parts: main circuit, control circuit, detection circuit, and auxiliary power supply (see figure)

　　1. Main circuit

　　Inrush current limiter: Limits the instantaneous inrush current on the input side when the power is turned on.

　　Input filter: Its function is to filter out the noise in the power grid and prevent the noise generated by the machine from being fed back to the power grid.

　　Rectification and filtering: directly rectify the grid AC power into smoother DC power.

　　Inversion: Converts rectified DC power into high-frequency AC power, which is the core part of high-frequency switching power supply.

　　Output rectification and filtering: Provide stable and reliable DC power supply according to load requirements.

　　2. Control circuit

　　On the one hand, samples are taken from the output end and compared with the set value, and then the inverter is controlled to change its pulse width or pulse frequency to stabilize the output. On the other hand, based on the data provided by the test circuit and identified by the protection circuit, the control circuit is provided with various protection measures for the power supply.

　　3. Detection circuit

　　Provide various parameters and instrument data in operation of the protection circuit.

　　4. Auxiliary power supply

　　Realize software (remote) startup of the power supply to provide power for the protection circuit and control circuit (PWM and other chips).

　　2. Working principle of switching power supply

　　The switching power supply uses power semiconductor devices as switching elements, and adjusts the output voltage by periodically switching on and off and controlling the duty cycle of the switching elements. The switching element is repeatedly turned on and off at a certain time interval. When the switch is 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 obtains continuous and stable energy.

　　VO=TON/T*Vi

　　VO is the average voltage across the load

　　TON is the time the switch is turned on each time

　　T is the on-off duty cycle of the switch

　　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 VO will also change. Therefore, as the load and input power voltage change, the ratio of TON and T is automatically adjusted to keep the output voltage VO unchanged. Changing the ratio of the on time TON and the duty cycle is also called changing the duty cycle of the pulse. This method is called "Time Ration Control" (abbreviated as TRC).

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

　　1. Pulse Width Modulation (PWM): The switching period is constant and the duty cycle is changed by changing the pulse width.

　　2. Pulse Frequency Modulation (PFM): The on-pulse width is constant and the duty cycle is changed by changing the switching operating frequency.

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