A.Introduction The power switch is used to control the […]
The power switch is used to control the switch tube through the circuit for high-speed pass and cut-off. The direct current is converted into high-frequency alternating current and supplied to the transformer for transformation, thereby generating the required one or more sets of voltages!
B. The role of the switch
1. The AC power input is rectified and filtered into DC
2. Control the switching tube through the high frequency PWM (pulse width modulation) signal, and add that DC to the primary of the switching transformer
3. High-frequency voltage is induced in the secondary of the switching transformer, which is rectified and filtered to supply the load
4. The output part is fed back to the control circuit through a certain circuit to control the PWM duty cycle to achieve the purpose of stable output.
C. Working principle
There is a door in the power switch. When the door is opened, the power will pass through, and when the door is closed, the power will stop. So what is a door? Some switching power supplies use thyristors and some use switching tubes. The performance of these two components is similar. They are all turned on and off by adding a pulse signal to the base and (switch tube) control electrode (SCR). When the pulse signal arrives in the positive half cycle, the voltage on the control electrode rises, and the switch tube or SCR Turn on, the 300v voltage output after 220v rectification and filtering is turned on, and is transmitted to the secondary through the switching transformer, and then the voltage is raised or lowered through the transformation ratio for each circuit to work. The negative half cycle of the oscillating pulse arrives, the voltage of the base electrode of the power regulator tube or the control electrode of the thyristor is lower than the original setting voltage, the power regulator tube is cut off, the 300v power supply is turned off, and the secondary of the switching transformer has no voltage. The required working voltage is maintained by the discharge of the filter capacitor after the rectification of the secondary circuit. When the signal arrives at the positive half cycle of the next pulse period, repeat the previous process. This switching transformer is called a high-frequency transformer because its operating frequency is higher than 50hz low frequency. So how to obtain the pulse that drives the switch tube or the thyristor? This requires an oscillating circuit to generate it. We know that the transistor has a characteristic, that is, the base-to-emitter voltage is 0.65-0.7v, which is the amplified state, 0.7v The above is the saturated conduction state, -0.1v- -0.3v is working in the oscillating state, then after the working point is adjusted, the negative pressure is generated by the deeper negative feedback to make the oscillation tube vibrate and the frequency of the oscillation tube Determined by the length of time the capacitor on the base is charged and discharged, the output pulse amplitude is large when the oscillation frequency is high, and vice versa, which determines the output voltage of the power regulator tube. So how to stabilize the working voltage of the secondary output of the transformer? Generally, a set of coils are wound on a switching transformer. The voltage obtained at the upper end is rectified and filtered as the reference voltage, and then the reference voltage is passed through a photocoupler. 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 by the sampling coil also rises, and the positive feedback voltage obtained through the photocoupler also rises. This voltage is added to On the base of the oscillating tube, the oscillation frequency is reduced, which stabilizes the secondary output voltage.