Switching power supplies are advanced power systems that use modern power electronics to control the on and off time of a switch, ensuring a stable output voltage. These power supplies typically consist of a Pulse Width Modulation (PWM) control IC and a MOSFET. As power electronics technology continues to evolve, switching power supply designs have become more efficient and compact. Today, they are widely used in nearly all electronic devices due to their small size, lightweight, and high efficiency. They play a crucial role in supporting the rapid growth of the electronics industry.
**First, the components of the switching circuit**
A switching power supply is generally composed of four main parts: the main circuit, the control circuit, the detection circuit, and the auxiliary power supply.
1. **Main Circuit**
- Inrush current limiting: Prevents excessive current from entering the system during startup.
- Input filter: Reduces noise from the grid and prevents internal noise from returning to the grid.
- Rectification and filtering: Converts AC input into a smooth DC voltage.
- Inverter: Converts the DC voltage into high-frequency AC, which is the core of high-frequency switching.
- Output rectification and filtering: Provides a stable 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 inverter’s pulse width or frequency to maintain stability. It also monitors the system through the detection circuit and implements various protection mechanisms.
3. **Detection Circuit**
This circuit provides real-time data about the operation of the power supply, such as voltage, current, and temperature, to support the control and protection functions.
4. **Auxiliary Power Supply**
Supplies power to the control circuit and other components, including PWM chips, allowing for remote or software-based control.
**Second, analysis of the output waveform of the switching circuit**
1. **Basic structure of a single-tube flyback circuit**
A flyback converter is a common type of switching power supply, where energy is stored in the transformer’s primary winding and then transferred to the secondary side when the switch is off.
2. **DCM vs. CCM modes**
- **CCM (Continuous Conduction Mode)**: Primary and secondary currents remain continuous throughout the cycle.
- **DCM (Discontinuous Conduction Mode)**: At some point, both primary and secondary currents drop to zero.
- **BCM (Boundary Conduction Mode)**: A hybrid between CCM and DCM.
The waveforms differ significantly between these modes. For example, in CCM, the primary current is trapezoidal, while in DCM, it is triangular. Similarly, the secondary rectifier current follows the same pattern.
3. **Effect of parasitic parameters on MOSFET waveforms**
When the MOSFET turns off, the Vds voltage can spike significantly due to leakage inductance. This energy must be absorbed by an RCD snubber circuit to prevent damage to the MOSFET. During turn-on, resonant oscillations may occur between the MOSFET's junction capacitance and the primary inductance, causing damped oscillations on the Vds waveform.
4. **Design considerations**
- Keep the maximum duty cycle below 50% to avoid subharmonic oscillation. If higher duty cycles are needed, slope compensation should be added.
- Ensure proper grounding, especially between the power ground and control ground, to minimize noise interference.
- Design the inner loop bandwidth to be at least ten times faster than the outer loop to ensure stability.
These design tips help improve the performance and reliability of switching power supplies, making them essential in modern electronics.
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