Talking about the conduction, cutoff and start of switching power supply

Strictly speaking, the process from the switch tube in the switching power supply from on to off is a very complicated one, but when we analyze the working principle, we usually simplify some non-main issues first. For example, when the power switch tube is on or off, we regard it as an ideal switch, which has only two states when working, on or off. But in fact, the on and off of the switch tube is a very complicated process. In addition to on or off, there is another problem that cannot be ignored at high frequencies, that is, when the switch tube is on, it is a working process from the cutoff 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 complicatedly here.

Simply put, it takes time for the power switch to turn on and off. Generally, the switch on time ton is simply divided into the on delay time td and the on rise time tr, and the switch off time toff is divided into the off delay time tstg (or off storage time) and the off fall time tf.

Figure 1 is the test waveform of the power switch tube's on and off process. In fact, the test waveform in Figure 1 is also simplified. In the actual waveform, the rising edge of the waveform is generally an exponential curve with a positive change rate, while the falling edge of the waveform is an exponential curve with a negative change rate. For simplicity, straight lines are used in Figure 1 instead.

In the first working cycle of the switching power supply, the output voltage needs to charge the filter energy storage capacitor. Because the charging current is large, the load will be heavy (or equivalent to a short circuit), so the general switching power supply must take soft start measures. At the beginning, the duty cycle is very small, and then it slowly tends to normal, that is, the output power is very small at the beginning, and then it slowly increases. Or at the beginning, the working voltage is relatively low, and then it slowly rises to the normal value.

Strictly speaking, the switching power supply always works in an unstable state, and the so-called stability is only relative. For example, the voltage stabilization process of the switching power supply is as follows: 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 is reduced, thereby 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 again, so that the duty cycle is increased, thereby increasing the output voltage. This cycle is repeated, and the output voltage of the switching power supply will always fluctuate around the average voltage at a certain frequency. The so-called voltage stabilization is nothing more than a relatively stable average output voltage.

The current flowing through the primary coil of the switching transformer is not a stable value, but generally a sawtooth wave, and the same is true for the rectifier output current. Constant current driving of LEDs generally means that after filtering, the output current of the filter is relatively stable, and this stability also refers to the average value, while the input current of the filter is generally a sawtooth wave.

The first cycle of a switching power supply is generally considered to start from the time the switch is turned on. This mainly depends on where the circuit you want to analyze starts. If it refers to when all the circuits of the switching power supply start working, this can be considered to start working as soon as the power switch is turned on. If you need to analyze the waveforms at each point, you must take the waveform of a device in the circuit as a reference point (or synchronization).

In the first cycle of the switching power supply, the general sampling circuit is basically not working, because the output voltage charges the filter capacitor, and it takes several cycles to charge to the normal value. Only after the output voltage reaches the normal value can the sampling circuit work normally. However, before the sampling circuit works normally, the absolute value of its output voltage is equal to 0, which is also a special case of error signal output (negative maximum value). In this case, if the switching power supply does not have a soft start circuit, the duty cycle of the switch tube will be very large when it works, which can easily saturate the transformer and damage the switch tube.