Working Principle of Switching Transistors for Switching Power Supplies
Strictly speaking, the process of switching a transistor from conduction to cutoff is a very complex one, but when analyzing its working principle, we usually simplify some non major issues first. For example, when a power switch tube is turned on or off, we consider it an ideal switch that operates in only two states, on or off. But in reality, the conduction and turn off of the switching transistor are both very complex processes. In addition to conduction or turn off, there is another issue that cannot be ignored at high frequencies, which is the working process of the switching transistor from the cutoff region to the amplification region, and then from the amplification region to the saturation region when it conducts. This work process requires the use of differential equations to solve, and I don't want to introduce it too complicated to you here.
Simply put, it takes time for the power switch tube to turn on and off. Generally, the conduction time ton of the switch tube is simply divided into a conduction delay time td and a conduction rise time tr, while the shutdown time toff of the switch tube is divided into a shutdown delay time tstg (or shutdown storage time) and a shutdown fall time tf.
Switching power supplies have working cycles, and due to the output voltage, the filtering energy storage capacitor needs to be charged. As the charging current is large, the load will be heavy (or equivalent to a load short circuit), so general switching power supplies need to adopt soft start measures. At the beginning, the duty cycle is small, and then it gradually tends to be normal, that is, the output power is small at the beginning and then gradually increases. At the beginning, the working voltage is relatively low, and then it gradually rises to the normal value.
Strictly speaking, switch mode 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 increases, 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 to increase the duty cycle, thereby increasing the output voltage. This cycle repeats, and the output voltage of the switching power supply will always oscillate around the average voltage at a certain frequency. The so-called voltage stabilization is just that the average output voltage is relatively stable.
The current flowing through the primary coil of the switching transformer is not a stable value, usually a sawtooth wave, and the rectified output current is 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. The input current of the filter is usually a sawtooth wave.
