Switching power supply chips are becoming increasingly essential in our daily lives, finding their way into almost every electronic device we use. From charging the digital gadgets we rely on to powering essential home appliances, these chips play a crucial role behind the scenes. Today, I'd like to introduce you to this reliable and familiar component: the switching power supply chip. First, let's take a look at the circuit composition of a switching power supply. The main circuit consists of several key components, including an input electromagnetic interference (EMI) filter, a rectification and filtering circuit, a power conversion circuit, a PWM control circuit, and an output rectification and filtering circuit. In addition, there are auxiliary circuits that provide various forms of protection, such as input over-voltage protection, output over-voltage protection, output over-current protection, and output short-circuit protection. Next, let's explore the working principles of the input circuit and some common circuits. One important part is the lightning protection circuit, which includes components like MOV1, MOV2, MOV3, F1, F2, F3, and FDG1. When a surge or lightning strike occurs, the varistors reduce their resistance when the voltage exceeds their threshold, allowing the high-energy surge to be dissipated safely. If the current becomes too large, the fuses F1, F2, and F3 will blow, protecting the rest of the circuit from damage. Another key part is the input filter circuit, which typically uses a double π-type network made up of capacitors C1, L1, C2, and C3. This circuit helps suppress electromagnetic noise and unwanted signals from the power source, preventing them from interfering with the power supply or the grid. When the power is first turned on, capacitor C5 needs to be charged. To prevent a large inrush current, a thermistor (RT1) is often used. Initially, RT1 has a high resistance, limiting the current. As it heats up, its resistance decreases, allowing the circuit to operate normally with minimal energy loss. The rectifier and filter circuit follows, where the AC voltage is converted to DC by BRG1 and then smoothed out by C5. If the capacitance of C5 decreases over time, the output will experience more AC ripple, affecting the overall performance of the power supply. Now, moving on to the power conversion circuit. The most commonly used component in this stage is the MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). It works based on the electroacoustic effect of the semiconductor surface, also known as a surface field-effect device. Due to its high input impedance, it can reach up to 10^5 ohms. The gate-source voltage controls the amount of induced charge on the semiconductor surface, which in turn regulates the drain current. For example, the 60W series 12V5A energy-efficient six-level solution offers excellent performance. It features ±5% constant voltage and constant current accuracy, fast dynamic response control, and standby power consumption of less than 75mW. Additionally, it automatically compensates for input voltage fluctuations and inductive inductance changes, ensuring stable operation. It also has an ultra-low starting current, making it ideal for a wide range of applications.

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