The basic control principle of switching power supply
one 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 it is divided into details, it includes: input filter, input rectification, switching circuit, sampling, reference power supply, comparison amplification, shaker, V/F conversion, base drive, output rectification, output filtering circuit, etc.
The actual switching power supply also has protection circuit, power factor correction circuit, synchronous rectifier driver circuit and some other auxiliary circuits.
The following is a typical block diagram of switching power supply, to master it is of great significance to our understanding of switching power supply.
Figure 2-1: Basic structure diagram of the switching power supply
Depending on the type of control, PM (pulse modulation) circuits may take many forms. Here is a typical PFM structure.
two The principle of switching power supply:
(1) Input circuit:
Linear filter circuit, surge current suppression circuit, rectifier circuit.
Function: Convert the input AC power supply into the DC input power supply that meets the requirements of the switching power supply.
1. Linear filter circuit:
Suppress harmonics and noise.
2. Surge filter circuit:
Suppress the surge current from the grid.
3. Rectifier circuit:
Change AC to DC.
There are two kinds of capacitor input type and choke input type, and most switching power supplies are the former.
(2). Conversion circuit:
Including switching circuit, output isolation (transformer) circuit, etc., is the main channel of switching power supply power conversion, to complete the power waveform with power chopper modulation and output.
This level of switching power tube is its core device.
1. Switching circuit
Driving mode: self-excited, other excited.
Conversion circuit: isolated type, non-isolated type, resonant type.
Power devices: The most commonly used are GTR, MOSFET, IGBT.
Modulation mode: PWM, PFM, hybrid three. PWM is most commonly used.
2. Transformer output
Divided without tap, with tap. No tapping is required for half wave rectification and double current rectification, and tapping is required for full wave rectification.
(3). Control circuit:
The modulated rectangular pulse is provided to the drive circuit to adjust the output voltage.
Reference circuit: Provides a voltage reference. Such as parallel type reference LM358, AD589, series type reference AD581, REF192 and so on.
Sampling circuit: takes all or part of the output voltage.
Comparison amplification: the sampling signal is compared with the reference signal to generate an error signal, which is used to control the power supply PM circuit.
V/F conversion: The error voltage signal is converted into a frequency signal.
Oscillator: Produces high frequency oscillating waves.
Base drive circuit: converts the modulated oscillation signal into a suitable control signal to drive the base of the switching tube.
(4). Output circuit:
Rectification and filtering.
The output voltage is rectified to pulsating DC and smoothed to low ripple DC voltage. The output rectifier technology now has half wave, full wave, constant power, double current, synchronous and other rectification modes.