Switching power supply circuit diagram analysis

Photocoupler

Switching power supply circuit diagram

Switching power supply circuit diagram analysis

The so-called switching power supply, hence the name, is that there is a door here, a door power supply will pass, a power supply will stop passing, then what is the door, some of the switching power supply uses thyristor, and some use the switch tube, this The performance of the two components is similar. It is connected to the base (switching tube) control electrode (thyristor) with a pulse signal to complete the turn-on and turn-off. The positive half-cycle of the pulse signal comes, and the voltage on the control electrode rises. The switch tube or the thyristor is turned on, and the 300V voltage outputted by the 220V rectification and filtering is turned on, passed to the secondary through the switching transformer, and then the voltage is raised or lowered by the transformation ratio for each circuit to work. When the oscillation pulse is negative for half a week, the base voltage of the power supply regulating tube or the control electrode voltage of the thyristor is lower than the original set voltage, the power supply regulating tube is cut off, the 300V power supply is turned off, and the switching transformer secondary has no voltage. At this time, each circuit The required operating voltage is maintained by the secondary capacitor rectified filter capacitor discharge.

The previous process is repeated until the signal of the positive half cycle of the next pulse arrives. This switching transformer is called a high frequency transformer because its operating frequency is higher than the 50Hz low frequency. Then how to get the pulse of the switch tube or thyristor, this requires an oscillation circuit. We know that the transistor has a characteristic that the base-to-emitter voltage is 0.65-0.7V, which is an amplification state, 0.7V. The above is the saturation conduction state, -0.1V- -0.3V works in the oscillation state, then after the working point is adjusted, the deep negative feedback is used to generate the negative pressure, so that the oscillation tube starts to oscillate, the frequency of the oscillation tube It is determined by the length of time that the capacitor on the base is charged and discharged. The oscillation frequency is high and the output pulse amplitude is large, and vice versa. This determines the output voltage of the power supply regulating tube.

Then how to regulate the working voltage of the secondary output of the transformer, generally on the switching transformer, winding a group of coils, the voltage obtained at the upper end is rectified and filtered, as a reference voltage, and then the reference is passed through the optocoupler. The voltage returns to the base of the oscillating tube to adjust the oscillation frequency. If the secondary voltage of the transformer rises, the voltage output from the sampling coil also rises, and the positive feedback voltage obtained by the photocoupler also rises. This voltage is added. On the base of the oscillating tube, the oscillation frequency is reduced, which stabilizes the stability of the secondary output voltage. The too thin working condition does not need to be elaborated, and it is not necessary to understand the fineness, so that the high-power voltage is controlled by the switching transformer. Passed and separated from the latter stage, the returned sampling voltage is transmitted by the optocoupler and also separated from the latter stage, so the mains voltage of the former stage is separated from the latter stage. This is called cold plate, it is safe, the transformer The front power supply is independent, which is called a switching power supply.