On, off, and start of a switch tube in a switching power supply - News - Global IC Trade Starts Here.

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Switching from the conduction to the cut-off in the switching power supply is strictly a very complicated process, but when doing the analysis of the working principle, we generally first simplify some non-primary problems. For example, when the power switch is turned on or off, we see it as an ideal switch that operates in only two states, on or off. But in fact, the turn-on and turn-off of the switch tube is a very complicated process. In addition to on or off, there is a problem that can not be ignored at high frequencies, that is, when the switch tube is turned on, it is from the cut-off area. Go to the enlargement area and then work from the enlargement area to the saturation area. This work process needs to be solved by using differential equations. I don't want to introduce you too complicated here.

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

Figure 1 is the test waveform of the switching power supply switch tube 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 rate of change. The falling edge of the waveform is an exponential curve with a negative rate of change. For simplicity, Figure 1 is replaced by a straight line.

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

Strictly speaking, the switching power supply always works in an unstable state, so the stability is only relative. For example, the voltage regulation process of the switching power supply is such that when the output voltage rises, after sampling and comparison, the sampling circuit outputs an error signal to the pulse width modulation circuit to reduce the duty ratio, thereby reducing the output voltage; After the output voltage is reduced, after sampling and comparison, the sampling circuit will output an error signal to the pulse width modulation circuit to increase the duty ratio, thereby increasing the output voltage. In this way, the output voltage of the switching power supply will always be constant. The frequency swings up and down the average value of the voltage. The so-called voltage regulation is only that the average value of the output voltage is relatively stable.

The current flowing through the primary winding of the switching transformer is also not a stable value, generally a sawtooth wave, and the rectified output current is also the same. Constant current driving of the LED generally means that the filtered output current is relatively stable after filtering, and the stability is also the average value, and the input current of the filter is generally a sawtooth wave.

The first cycle of switching power supply is generally considered to start from the conduction of the switch. This depends mainly on where the circuit you are analyzing is cut from. If it refers to when all the circuits of the switching power supply start working, this can be considered as Start working when the power switch is turned on. If you need to analyze the waveform of 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 inoperative. Because the output voltage charges the filter capacitor, it takes several cycles to charge to the normal value. Only after the output voltage reaches the normal value, the sampling circuit In order to work properly. However, before the sampling circuit is not working properly, the absolute value of its output voltage is equal to 0, which is also a special case of the error signal output (negative maximum). In this case, if the switching power supply does not have a soft-start circuit, the duty cycle of the switching tube will be large, and it is easy to saturate the transformer and damage the switching tube.