**First, the Introduction of Switching Power Supply**
A switching power supply is a type of power supply that uses modern power electronics to regulate the on and off time of a switch, ensuring a stable output voltage. It typically consists of a Pulse Width Modulation (PWM) control IC and a MOSFET. With continuous advancements in power electronics, switching power supply technology has evolved significantly. Today, these power supplies are widely used in almost all electronic devices due to their compact size, lightweight design, and high efficiency. As a result, they have become an essential component in the fast-paced development of the electronics and information industries.
**Second, the Basic Composition of the Switching Power Supply**
A switching power supply is generally made up of four main components: the main circuit, the control circuit, the detection circuit, and the auxiliary power supply.
**1. Main Circuit**
The main circuit includes several key parts:
- **Inrush Current Limiting:** This part limits the initial surge of current when the power is first turned on.
- **Input Filter:** It filters out noise from the grid and prevents internal noise from being fed back into the grid.
- **Rectification and Filtering:** Converts the AC input from the grid into a smoother DC voltage.
- **Inverter:** Converts the DC voltage into high-frequency AC, which is the core of the switching power supply.
- **Output Rectification and Filtering:** Provides a stable and reliable DC output based on the load requirements.
**2. Control Circuit**
The control circuit samples the output voltage, compares it with a reference value, and adjusts the pulse width or frequency of the inverter to maintain a stable output. Additionally, it works with the detection circuit to implement various protection functions such as overvoltage, overcurrent, and thermal protection.
**3. Detection Circuit**
This circuit monitors and provides real-time data on various operational parameters, helping the system respond to changes and ensure safe and efficient operation.
**4. Auxiliary Power Supply**
The auxiliary power supply supports the startup of the system and powers the control and protection circuits, including components like the PWM controller.
**Third, the Working Principle of the Switching Power Supply**
The voltage conversion process in a switching power supply involves a pulse oscillator made up of a switching transistor and a pulse transformer. This oscillator generates pulsed electrical energy, converting the 300V DC input into the desired output voltage through the secondary winding of the transformer. The working principle is illustrated in Figure 2.
**1. Working Principle of the Pulse Oscillator**
- **Start of the Pulse Oscillator:** The power supply applies a forward bias to the base and emitter of Q3 via resistors R10, R10A, and R15, forcing Q3 into conduction.
- **Oscillation Process:** When Q3 turns on, current flows through the primary coil of the pulse transformer, generating a magnetic field. The secondary coil induces a voltage, which is applied back to the base of Q3 through R12 and C8, increasing the base current and driving Q3 into saturation. Once saturated, the polarity of the induced voltage reverses, causing Q3 to turn off rapidly. An absorption circuit consisting of D8, R17, and C7 then dissipates any residual energy in the primary coil. This cycle repeats continuously, determining the oscillation frequency based on the capacitance of C8 and the inductance of the secondary coil.
The oscillation mechanism ensures efficient and stable power conversion, making switching power supplies ideal for a wide range of applications.
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