Overview of the working principle of high-frequency switching power supply
The working principle of high-frequency switching power supply is power conversion.
When switch S is closed, current flows through inductor L, generating output voltage across load RL. Due to the polarity relationship of the input voltage, the diode VD1 is in reverse configuration, and L stores energy at this time. When switch S is turned on, the magnetic field polarity of inductor L changes, and the energy stored in L is released through load RL. Diode VD1 conducts forward, and the voltage polarity at both ends of the load remains unchanged. The diode VD1 is called a freewheeling diode due to its role in the circuit.
When switch S is closed, there is current input to the input circuit, but when the switch is opened, the current suddenly terminates. However, due to the action of inductor L and freewheeling diode VD1, the output current is continuous. Inductance L and capacitor C also serve as filters, making the voltage on RL smoother.
In practical applications, switching transistors are used as switches. At the same time, in the circuit of Figure 1, there is a lack of safety isolation measures between the input and output circuits, so high-frequency transformers are generally used as isolation devices.
VT1 is a switching transistor whose base is controlled by a square wave S1. When S1 is at high level, VT1 conducts, generating power at the primary of transformer T and storing energy. Due to the fact that the secondary and primary of the transformer are in phase, all quantities are also transmitted to the secondary of the transformer. The current flows through the forward biased diode VD2 and inductor L, transferring energy to the load RL, while the inductor L stores capacity. At this time, diode VD1 is reverse biased.
When S1 is at a low level, VT1 is turned off, the voltage in the transformer T winding is reversed, diode VD2 is turned off, freewheeling diode VD1 is turned on, and the energy stored in inductor L continues to be transferred to load RL.
Obviously, the output voltage VRL=V2 × Ton/T=V2 × X, where X=Ton/T is the duty cycle; Ton is the conduction time of VT1, and changing the pulse duty cycle δ can change the output voltage (or current).
