ADD：The basic principle of switching power supply
Switching power supply is a voltage conversion circuit, the main tasks are ascending and descending pressure, are widely used in modern electronic products. Transistor always work because the switch "on" and "off" state, so called switching power supply. Switching power supply is a real oscillator circuit, this change the way energy is not only used in power circuit, the circuit in other applications are also common, such as the LCD backlight circuits, fluorescent lamps and so on. Switch the source and the transformer compared to high efficiency, good stability, small size, the advantages and disadvantages are relatively small power, and the circuit will produce high-frequency interference, the circuit complexity of easy maintenance.
Before talking about switching power supply, first familiarize yourself with the transformer feedback oscillation circuit, can produce a regular pulse current or voltage of the circuit called the oscillator circuit, transformer feedback oscillator circuit is able to meet this condition the circuit; it in basic amplifier circuit and a feedback loop composed of C2, L1 election to form a parallel resonant frequency circuit, instantaneous power in the VT circuit conduction, this time in the C2, L1 parallel resonant circuit formed have a very rich harmonic, when the applied frequency and parallel resonant circuit is equal to the natural frequency of the circuit into oscillation state, and by L3 feedback to further enlarge the base of VT, the final formation of a regular pulse current or voltage output to the load RL on. Switching power supply is the feedback oscillator circuit around the transformer designed only to increase the original basis of a number of protection and control circuit, we can use analysis methods to analyze the oscillator circuit switching power supply.
Vibration mode by switching power swing points, can be divided into self-excited oscillation and its two, self-excited without external signal source is able to self-oscillation, self-excited can see it as a transformer feedback oscillation circuit, excited and it is entirely dependent on external sustained oscillations, in the practical application of self-excited type used widely. Under the excitation signal of structure; can be divided into pulse width and pulse amplitude are two kinds of pulse width is the width of the control signal, that is, frequency, pulse amplitude modulation control signal amplitude, the role of the two the same are to maintain oscillation frequency within a certain range, to the effect of voltage, the transformer windings can generally be divided into three types, a group involved in the primary winding oscillation, a group is to maintain the oscillation of the feedback winding, there is a load coil is used in household appliances switching power supply, the 220V AC through the bridge rectifier, converted into about 300V DC, filtered into the transformer switch added to the collector after the high frequency oscillation, the feedback winding back to the base to maintain oscillation circuit, the load winding induction the signal, the rectifier, filter, get the DC voltage regulator to provide power to the load. Load winding to provide power, but also the ability task of stabilizing the voltage, the principle is the voltage output circuit connected in a voltage sampling device to monitor the output voltage changes, and timely feedback to adjust the oscillation frequency of oscillator circuit to achieve the stable voltage purpose, in order to avoid the interference of the circuit, the voltage fed back to oscillator circuit will be used optocoupler isolated. Most switching power supply with standby circuitry, switching power supply in standby mode still oscillating, but the frequency is lower than the normal working hours.
Some switching power supply is complex, dense components, many protection and control circuit, in the case of no technical support, maintenance up is a very troublesome thing. I faced this situation is, first of all find the switch and I will participate in the external oscillator circuit, to separate it from the circuit to see whether it satisfies the conditions of oscillation, such as detection bias are normal, whether positive feedback failure, there is switch itself, switching power supply with extremely powerful protection, control and protection of prosecution removed, and the load circuit.
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