The Working Principle of Switch Tubes in Switching Power Supplies
Strictly speaking, the process of switching from conduction to cutoff is a very complex one, but when analyzing the working principle, we usually simplify some non main problems first. For example, when the power switch is on or off, we consider it an ideal switch that operates in only two states: on or off. But in fact, the conduction and turning off of a switch tube are both very complex processes. In addition to making or breaking, there is also a problem that cannot be ignored at high frequencies. When the switch tube is turned on, it operates from the cutoff region to the amplification region, and then from the amplification region to the saturation region. This work process requires the use of differential equations to solve, and I don't want to introduce you too complex here.
Simply put, it takes time for the power switch to turn on and off. Generally, the on time ton of the switch tube is simply divided into the on delay time td and the on rise time tr, while the off time toff of the switch tube is divided into the off delay time tstg (also known as the off storage time) and the off fall time tf.
In the first working cycle of a switching power supply, the output voltage needs to charge the filter energy storage capacitor. Due to the large charging current, the load will be heavy (or equivalent to a load short circuit). Therefore, general switching power supplies need to adopt soft start measures. At the beginning, the duty cycle is very small, and then it gradually becomes normal, that is, the output power is very small at the beginning, and then it gradually increases. At the beginning, the working voltage is relatively low, and then it slowly rises to the normal value.
Strictly speaking, switching power supplies always operate in an unstable state, and stability is only relative. For example, the voltage stabilization process of a switching power supply is as follows: when the output voltage rises, after sampling and comparison, the sampling circuit will output an error signal to the pulse width modulation circuit, reducing the duty cycle and thus reducing the output voltage; After the output voltage decreases, after sampling and comparison, the sampling circuit will output an error signal to the pulse width modulation circuit, increasing the duty cycle and increasing the output voltage. In this repeated cycle, the output voltage of the switching power supply will always oscillate up and down at a certain frequency at the average voltage value, and the so-called voltage stabilization is just that the average output voltage value is relatively stable.
The current flowing through the primary coil of a switching transformer is not a stable value, usually a sawtooth wave, and the rectifier output current is also the same. Constant current driving of LED generally refers to the stable output current of the filter after filtering, which 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 conduction of the switch transistor, which mainly depends on where the circuit you want to analyze starts. If it refers to when all circuits of the switching power supply start working, it can be considered as starting from the moment the power switch is turned on. If you need to analyze the waveform of each point, you must take the waveform of a certain device in the circuit as a reference point (or synchronization).
In the first cycle of the switching power supply, the sampling circuit is generally not working because the output voltage charges the filter capacitor, which takes several cycles to charge to the normal value. Only after the output voltage reaches the normal value can the sampling circuit operate normally. However, before the sampling circuit works properly, its output voltage is equal to 0, which is also considered 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 large during operation, which can easily saturate the transformer and damage the switch tube.
