As one of the most important devices in the switching power supply, the switching tube is also the focus of attention in power supply debugging. The turn-off and turn-on action of the switch is actually a more complicated process, but we can first simplify it. 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.
In the past, to test the characteristics of the power supply, we just stopped measuring the quiescent current and voltage with a multimeter and used the calculator to perform complex calculations. Today, oscilloscopes have become the power measurement platform of choice for many engineers. Some cost-effective oscilloscopes even feature power analysis software that simplifies setup and makes dynamic measurements easier.
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, as shown in Figure 1.
Figure 1 Test simplified waveform of the power switch tube conduction and shutdown process
In the first working cycle of the switching power supply, since the output voltage is to charge the filter storage capacitor, the charging current is very large and the load will be heavy. Therefore, the general switching power supply must adopt soft start measures, and the duty ratio is small at the beginning. Then slowly become normal. That is, at the beginning, the working voltage is relatively low, and then slowly rises to the normal value. This time is generally long and can be as long as several hundred milliseconds. This time it will test the storage capacity of the oscilloscope. The ZDS2024 Plus comes standard with a 250M memory depth, which allows you to observe a one-second waveform while still maintaining a sampling rate of 500Msa/s, which is very redundant in many cases.
The power-up waveform of the output of a switching power supply can be seen that the power supply is in 1.64 seconds and there are two buffer rise processes. as shown in picture 2.
Figure 2 Power-on output waveform of a switching power supply
Figure 3 switch tube test
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