Working Principle of Switching Tubes in Switching Power Supplies
The switching of a switching tube from conduction to cut-off is strictly speaking a very complex process, but when we carry out an analysis of the principle of operation, we generally start by simplifying some of the non-major issues. For example, when a power switching tube conducts or cuts off, we think of it as an ideal switch that operates in only two states, on or off. But in fact, the switching tube on and off is a very complex process, it is in addition to on or off, there is a high-frequency can not be ignored, is the switching tube conduction, from the cut-off region to the amplification region, and then from the amplification region to the saturation region of the working process. This process needs to be solved using differential equations, where I do not want to introduce you to too complex.
Simply put, the power supply switching tube conduction and shutdown are time-consuming. Generally are simply the switching tube conduction time ton divided into conduction delay time td and conduction rise time tr, while the switching tube off time toff is divided into off delay time tstg (or off storage time) and off fall time tf.
Switching power supply in the work cycle, due to the output voltage to the filter storage capacitor charging, because the charging current is very large, the load will be very heavy (or the equivalent of a load short-circuit), so the general switching power supply have to take soft-start measures, the beginning of the duty cycle is very small, and then slowly tends to normal, that is, the beginning of the time when the output power is very small, and then slowly become larger. Or at the beginning, the operating voltage is relatively low, and then slowly rise to normal values.
Strictly speaking, the switching power supply will always work in a non-stable state, the so-called stable is only relative. For example, the switching power supply voltage regulation process is this: when the output voltage increases, after sampling and comparison, the sampling circuit will output an error signal to the pulse width modulation circuit, so that the duty cycle decreases, thus reducing the output voltage; when the output voltage decreases, after sampling and comparison, the sampling circuit will output an error signal to the pulse width modulation circuit, so that the duty cycle increases, thus making the output voltage increases, so that Repeated cycles, the output voltage of the switching power supply will always be a certain frequency in the average voltage swing up and down, the so-called voltage regulator is only the average output voltage is more stable.
For the current flowing through the primary coil of the switching transformer is not a stable value, generally a sawtooth wave, rectifier output current is the same. Constant-current drive for LEDs, generally also means that after filtering, the filter output current is more stable, this stability also refers to the average value, and the input current of the filter is generally a sawtooth wave.
Switching power supply to start the cycle is generally considered to be from the switching tube conduction counted as the beginning, which mainly depends on where you want to analyse the circuit is cut from, if it refers to all the circuits of the switching power supply when the start of the work, this can be considered to be from the power switch on the start of the work, if you need to analyse the waveform of each point, it is necessary to take the waveform of the circuit in the work of a device as a reference point (or synchronous).
