A switching power supply uses power semiconductor devices as switching elements. By periodically switching the power supply on and off, the duty cycle of the switching elements is controlled to adjust the output voltage. Simply put, the working principle of a switching power supply is to first rectify alternating current (AC) into direct current (DC), then invert the DC back into AC, and finally rectify the output to produce the required DC voltage.
① AC power is rectified and filtered into DC;
② The switching transistor is controlled by a high-frequency PWM (Pulse Width Modulation) signal to switch on and off at high speed, converting DC power into high-frequency AC power to supply the switching transformer for voltage transformation;
③ The secondary winding of the switching transformer induces a high-frequency AC voltage, which is then rectified and filtered into DC power to supply the load;
④ The output section is fed back to the control circuit through a certain circuit to control the PWM duty cycle in order to achieve stable output.
The main advantages of switching power supplies are: small size, light weight (only 20-30% of the size and weight of linear power supplies), high efficiency (generally 60-70%, while linear power supplies are only 30-40%), strong anti-interference ability, wide output voltage range, and modularity.
Switching power supplies are a relatively new type of power supply. They have advantages such as high efficiency, light weight, step-up and step-down voltage conversion, and high output power. However, because the circuit operates in a switching state, the noise is relatively high. Let's briefly explain the working principle of a step-down switching power supply using the diagram below. As shown in the diagram, the circuit consists of a switch K (actually a transistor or MOSFET), a freewheeling diode D, an energy storage inductor L, and a filter capacitor C. When the switch is closed, the power supply provides power to the load through switch K and inductor L, storing some electrical energy in inductor L and capacitor C. Due to the self-inductance of inductor L, the current increases relatively slowly after the switch is turned on, meaning the output cannot immediately reach the power supply voltage. After a certain time, the switch is opened. Due to the self-inductance of inductor L (which can be visualized as the current in the inductor having inertia), the current in the circuit remains constant, continuing to flow from left to right. This current flows through the load, returns from the ground wire, flows to the positive terminal of the freewheeling diode D, passes through diode D, and returns to the left end of inductor L, thus forming a loop. The output voltage can be controlled by controlling the closing and opening times of the switch (i.e., PWM - Pulse Width Modulation).
If the switching time is controlled by detecting the output voltage to maintain a constant output voltage, voltage regulation is achieved. During the switch's closed state, the inductor stores energy; during the switch's open state, the inductor releases energy, hence the inductor L is called an energy storage inductor. During the switch's open state, the diode D provides a current path to the inductor L, therefore the diode D is called a freewheeling diode.
Switching power supply function:
1. Main circuit
Alternating current enters the power supply and is first rectified and filtered to become high-voltage direct current. Then, it passes through a switching circuit and a high-frequency switching transformer to become high-frequency low-voltage pulses. After that, it passes through a rectification and filtering circuit again and finally outputs low-voltage direct current.
2. Control and feedback loops
A given switching signal is input to the switching device, causing it to open and close, thereby achieving energy transfer. Furthermore, the output is collected through sampling and feedback loops to form a negative feedback closed loop, ensuring stable output from the switching power supply.
3. Protection circuit
When a switching power supply malfunctions, current can be limited or the power supply can be shut off to protect the power load. Protection types mainly include undervoltage protection, overvoltage protection, overcurrent protection, and overheat protection.
