Explanation of the composition principle of switching power supply
（1） Input circuit:
Linear filtering circuit, surge current suppression circuit, rectification circuit.
Function: Converts the AC power supply input to the power grid into a DC input power supply that meets the requirements of a switching power supply.
1. Linear filtering circuit:
Suppress harmonics and noise.
2. Surge filtering circuit:
Suppress surge currents from the power grid.
3. Rectifier circuit:
Turn AC into DC.
There are two types of input: capacitor input type and choke input type, with most switching power supplies using the former.
（2） Transform circuit:
Including switch circuits, output isolation (transformer) circuits, etc., it is the main channel for switching power supply transformation, completing chopping modulation and output of power supply waveforms with power.
The switching power transistor at this level is its core device.
1. Switch circuit
Drive mode: self excited or self excited.
Conversion circuit: isolated, non isolated, resonant.
Power devices: The most commonly used ones include GTR, MOSFET, and IGBT.
Modulation methods: PWM, PFM, and hybrid. PWM is the most commonly used.
2. Transformer output
Divided into no tap and with tap. During half wave rectification and current doubling rectification, there is no need for a tap, and a tap must be present during full wave rectification.
（3） Control circuit:
Provide modulated rectangular pulses to the driving circuit to adjust the output voltage.
Reference circuit: Provide voltage reference. For example, parallel reference LM358, AD589, series reference AD581, REF192, etc.
Sampling circuit: Taking all or part of the output voltage.
Comparison amplification: Compare the sampled signal with the reference signal to generate an error signal, which is used to control the power supply PM circuit.
V/F conversion: Converts the error voltage signal into a frequency signal.
Oscillator: Produces high-frequency oscillation waves.
Base drive circuit: Converts the modulated oscillation signal into a suitable control signal to drive the base of the switch tube.
（4） Output circuit:
Rectification and filtering.
Rectify the output voltage into pulsating DC and smooth it into low ripple DC voltage. Output rectification technology now includes half wave, full wave, constant power, current doubling, synchronous and other rectification methods.
