Analysis of soft start process of switching power supply
As one of the most important devices in the switching power supply, the switching tube is also the most important in the power supply debugging. The turn-off and turn-on action of the switching tube is actually a more complicated process, but we can simplify it first. For example, when the power switch tube is on or off, we regard it as an ideal switch, which operates in only two states, on or off.
In the past, to test the characteristics of a power supply, we simply stopped at measuring static current and voltage with a multimeter and performing complex calculations with a calculator. Today, oscilloscopes have become the preferred power measurement platform for many engineers. Some cost-effective oscilloscopes even come with power analysis software that simplifies setup and makes dynamic measurements easier.
Simply put, it takes time for the power switch tube to open and close. Generally, the on-time ton of the switching tube is simply divided into on-time delay time td and on-time rise time tr, while the on-time shutdown time toff is divided into on-time delay time tstg (or closed storage time) and on-time fall time tf, as shown in Figure 1.
In the first working cycle of the switching power supply, due to the output voltage to charge the filter energy storage capacitor, the charging current is large, and the load will be very heavy, so the general switching power supply must take soft starting measures, the duty cycle is small at the beginning, and then slowly tends to normal. That is, at the beginning, the operating voltage is relatively low, and then it slowly rises to a normal value. This time is generally relatively long, may be up to a few hundred milliseconds, this time will be very testing the oscilloscope storage capacity. The 250M storage depth of the ZDS2024 Plus is standard, which can observe and record the waveform of one second while guaranteeing a sample rate of 500Msa/s, which is very abundant in many cases.
On the output waveform of a switching power supply, you can see that the power supply has two buffer rising processes within 1.64 seconds.
