Basic control principle of switching power supply

1. Control structure of switching power supply:
Generally speaking, the switching power supply is roughly composed of four main parts: input circuit, converter, control circuit, and output circuit.
If divided in detail, it includes: input filtering, input rectification, switching circuit, sampling, reference power supply, comparison amplifier, oscillator, V/F conversion, base drive, output rectification, output filtering circuit, etc. The
actual switching power supply also needs protection circuit, power factor correction circuit, synchronous rectification drive circuit and some other auxiliary circuits.
The following is a typical switching power supply principle block diagram, which is of great significance to our understanding of switching power supply.

Figure 2-1: Basic structure diagram of switching power supply
Depending on the control type, PM (pulse modulation) circuits may have many forms. Here is a typical PFM structure.

2. The composition principle of the switching power supply:
(I) Input circuit:
linear filter circuit, surge current suppression circuit, rectifier circuit.
Function: Convert the input grid AC power supply into the DC input power supply of the switching power supply that meets the requirements.
1. Linear filter circuit:
suppress harmonics and noise.
2. Surge filter circuit:
suppress surge current from the grid.
3. Rectifier circuit:
convert AC to DC.
There are two types: capacitor input type and choke input type. Most switching power supplies are the former.
(II) Conversion circuit:
Contains switching circuit, output isolation (transformer) circuit, etc. It is the main channel for switching power supply power conversion, and completes the chopping modulation and output of the power waveform with power.
The switching power tube at this level is its core device.
1. Switching circuit
Driving mode: self-excited type, externally excited type.
Conversion circuit: isolated type, non-isolated type, resonant type.
Power devices: The most commonly used are GTR, MOSFET, IGBT.
Modulation mode: PWM, PFM, and hybrid type. PWM is the most commonly used.
2. Transformer output
There are two types: tapless and tapped. For half-wave rectification and current-doubler rectification, no tap is required, but for full-wave, there must be a tap.
(III) Control circuit:
Provides modulated rectangular pulses to the drive circuit to adjust the output voltage.
Reference circuit: Provides voltage reference. Such as parallel reference LM358, AD589, series reference AD581, REF192, etc.
Sampling circuit: Takes all or part of the output voltage.
Comparison amplification: Compares the sampling signal with the reference signal to generate an error signal for controlling the power supply PM circuit.
V/F conversion: Converts the error voltage signal into a frequency signal.
Oscillator: Generates a high-frequency oscillation wave.
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.