Working principle of switching tube in switching power supply
Analysis of the operation of switching tube in switching power supply by heating ring
Strictly speaking, it is a very complicated process for the switch tube to turn on and off, but when we analyze the working principle, we usually simplify some non-main problems first. For example, when the power switch tube is turned on or off, we regard it as an ideal switch, and there are only two states when it works, on or off. But in fact, the switching on and off of the switch tube is a very complicated process. Besides switching on or off, there is also a problem that cannot be ignored at high frequency, that is, when the switch tube is turned on, it is the working process from the cut-off area to the amplification area, and then from the amplification area to the saturation area. This working process needs to be solved by differential equations, and I don't want to introduce it to you too complicated here.
Simply put, it takes time to turn on and off the power switch tube. Generally, the on-time ton of the switch tube is simply divided into the on-time delay td and the on-time rise tr, while the off-time toff of the switch tube is divided into the off-time delay tstg (or the off-time storage time) and the off-time fall tf.
In the first working cycle of switching power supply, the output voltage will charge the filter energy storage capacitor, and the load will be heavy (or equivalent to load short circuit) because of the large charging current. Therefore, the general switching power supply should take soft-start measures, with a small duty ratio at the beginning, and then it will gradually become normal, that is, the output power will be small at the beginning, and then it will gradually increase. Or at the beginning, the working voltage is relatively low, and then it slowly rises to the normal value.
Strictly speaking, switching power supply always works in an unstable state, and the so-called stability is only relative. For example, the voltage stabilization process of switching power supply is like this: when the output voltage rises, after sampling and comparison, the sampling circuit will output an error signal to the pulse width modulation circuit, which will reduce the duty cycle and thus reduce the output voltage; When the output voltage is reduced, after sampling and comparison, the sampling circuit will output an error signal to the pulse width modulation circuit, which will increase the duty ratio and thus increase the output voltage. In this way, the output voltage of the switching power supply will always swing up and down at a certain frequency, and the so-called voltage stabilization is just that the average value of the output voltage is relatively stable.
The current flowing through the primary winding of switching transformer is not a stable value, and it is generally a sawtooth wave, as is the rectified output current. Driving LED with constant current generally means that the output current of the filter is relatively stable after filtering, and this stability also refers to the average value, while the input current of the filter is generally a sawtooth wave.
Generally speaking, the first cycle of switching power supply starts from the conduction of the switch tube, which mainly depends on where the circuit you want to analyze is cut in. If it refers to when all the circuits of switching power supply start to work, it can be considered as starting to work as soon as the power switch is turned on. If it is necessary to analyze the waveforms of various points, it is necessary to take the waveform of a device in the circuit as a reference point (or synchronization).
In the first cycle of switching power supply, the general sampling circuit basically does not work, because the output voltage charges the filter capacitor, and it takes several cycles to charge it to the normal value, and only after the output voltage reaches the normal value can the sampling circuit work normally. However, before the sampling circuit works normally, its output voltage is equal to 0, which is also a special case of error signal output (negative maximum). In this case, if the switching power supply has no soft start circuit, the duty cycle of the switching tube will be very large when it works, which will easily saturate the transformer and damage the switching tube.
