A switching power supply is a voltage conversion circuit. Its main work content is to lift and lower. It is widely used in modern electronic products. Because the switching transistor always works in the "on" and "off" states, it is called a switching power supply. A switching power supply is essentially an oscillator circuit. This way of converting electrical energy is not only used in power supply circuits, but also widely used in other circuits, such as LCD backlight circuits, fluorescent lamps, etc. Compared with transformers, switching power supplies have high efficiency, good stability and small size. The disadvantage of switching power supply is that the power is relatively small, which will cause high-frequency interference to the circuit, and the circuit is complex and difficult to maintain.
There are three control principles of switching power supply:
The first is that the switching period is constant at the end of the pulse width adjustment, and the duty ratio is changed by changing the pulse width;
The second is pulse frequency adjustment, the conduction pulse width is constant, and the duty cycle is changed by changing the switching frequency;
The third method is to mix and adjust the on-pulse width and the switching operating frequency, both of which are not fixed and can be changed. It is a mixture of the above two methods.
Switching power supply classification:
1. According to the technology, the connection method of the switching tube, and the power supply technology, the switching power supply can be divided into a series switching power supply and a parallel switching power supply. The switching tube of the series switching power supply is connected in series between the input voltage and the output load, which is a step-down voltage regulator circuit; while the switching tube of the parallel switching power supply is connected in parallel between the input voltage and the output load, similar to redundancy. The power supply belongs to the step-up voltage regulator circuit.
2. According to the excitation mode, the switching power supply can be divided into self-excited and other-excited. In the self-excited switching power supply, a positive feedback loop is formed by the switching tube and the transformer technology' and the high-frequency transformer to complete the self-excited oscillation, which is similar to the intermittent oscillator; while the other-excited switching power supply must be attached with an oscillator to oscillate The switching pulse generated by the device is added to the switch tube to control the on and off of the switch tube, so that the switch circuit works and has a DC voltage output.
3. According to the modulation method, the switching power supply such as the server power supply can be divided into the pulse width modulation (PWM) method and the pulse frequency modulation (PFM) method. PWM is a way to control the output voltage stability by changing the switching pulse width, while PFM is when the output voltage changes, through sampling and comparison, the error value is amplified to control the switching pulse period (ie frequency), so that the output voltage is stable.
4. According to the size of the output DC value, the switching power supply can be divided into a boost switching power supply and a step-down switching power supply, and can also be divided into high-voltage switching technology' "high-voltage switching power supply and low-voltage switching power supply.
5. According to the output waveform, the switching power supply can be divided into rectangular wave and sine wave circuits.
6. According to the output performance, the switching power supply can be divided into constant voltage and constant frequency circuits and variable voltage and frequency conversion circuits.
7. According to the number of switching tubes and the connection method, the switching power supply can be divided into single-ended type, push-pull type, half-bridge type and full-bridge type. The single-ended type uses only one switch, the push-pull and half-bridge type uses two switches, and the full-bridge type uses four switches.
8. The switching power supply can be divided into forward excitation and flyback type according to the energy transfer method.
9. According to the soft switching mode, the switching power supply has current resonance type, voltage resonance type, E-type and quasi-E-type resonance type and partial resonance type.
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