With the continuous advancement and innovation in power electronics technology, switching power supply systems have undergone significant improvements. These power supplies are now widely used across nearly all electronic devices due to their compact size, lightweight design, and high efficiency. As a result, they have become an essential component in the rapid development of the electronics and information industry. There are two main types of modern switching power supplies: DC switching power supplies and AC switching power supplies.
This article primarily focuses on DC switching power supplies, which are designed to convert low-quality input power—such as from the mains or battery sources—into high-quality DC voltage that meets the requirements of connected equipment. The core component of a DC switching power supply is the DC/DC converter, which determines its classification and functionality.
A typical switching power supply consists of four main parts: the main circuit, control circuit, detection circuit, and auxiliary power supply. The main circuit includes components like inrush current limiting, input filtering, rectification and filtering, inverter, and output rectification and filtering. The control circuit monitors the output, adjusts the pulse width or frequency of the inverter, and provides protection based on feedback. The detection circuit gathers operational data for monitoring and protection, while the auxiliary power supply ensures stable operation of the control and protection circuits.
Here are some common questions and expert answers related to switching power supplies:
1. **Switching Power Supply Transformer**: If copper strips replace enameled wires, how does current flow? A copper strip with a thickness of 0.1mm can handle up to 4.5 A/mm² current density. This is calculated by dividing the current by the cross-sectional area (thickness × width).
2. **EMI Sources**: The most significant source of electromagnetic interference (EMI) in a switching power supply is the primary and secondary coils of the transformer. The power line is particularly susceptible to EMI, acting as a half-wave antenna, so isolation at the input is crucial.
3. **Transformer Temperature Rise**: Reducing the maximum flux density (Bm), lowering the operating frequency, and minimizing coil losses can help reduce temperature rise. Current density in enameled wire should not exceed 4.5 A/mm².
4. **Flyback Duty Cycle**: The duty cycle depends on the input voltage and the switch's voltage rating. For example, at 260V AC, the duty cycle is around 0.306, while at 170V AC, it increases to about 0.5.
5. **Forward vs. Flyback**: In forward mode, the power is delivered when the switch is on, while in flyback, energy is stored during the on-time and released during off-time. Output voltages differ in waveform and phase.
6. **Feedback Loop Design**: The loop gain must be carefully balanced. Too high leads to oscillation, too low causes instability. Typically, three loops are used to adjust differential, integral, and DC gains for stability.
7. **Flyback MOSFET Minimum Voltage**: Reducing the duty cycle lowers the voltage, but this can also reduce efficiency and voltage regulation range.
8. **Copper Foil Loss**: Copper foil loss is minimal. If the temperature rises above 80°C, the coating may discolor, but the loss is comparable to a 1–3W resistor.
9. **Drive Waveform Distortion**: Capacitive or transformer-coupled drive circuits can distort waveforms if they contain DC components. Proper coupling and duty cycle adjustment are necessary.
10. **Rectifier Bridge Selection**: The choice depends on current, voltage, and frequency. A 3A 700V bridge in a 30W supply may overheat if the current exceeds its capacity or if the switching speed is too slow.
11. **Feedback Compensation**: Proper loop design prevents oscillation. Gain settings must be adjusted carefully to maintain stability.
12. **Efficiency Improvement**: Lowering the working frequency or using faster switches can improve efficiency. Increasing the transformer size reduces magnetic flux density and associated losses.
13. **Minimum DC Voltage Calculation**: The minimum DC voltage is typically derived from the lowest AC input after rectification and filtering. For example, 100V AC becomes approximately 120V DC average.
14. **Output Glitch Elimination**: Adding an inductor with a large air gap between the secondary rectifier and filter capacitor can help reduce noise.
15. **Optimizing Flyback Frequency**: Operating frequency affects efficiency and component size. Higher frequencies increase losses, while lower frequencies improve efficiency but may require larger components.
16. **Flyback Voltage Optimization**: The flyback voltage is determined by the duty cycle and input voltage. It must not exceed 70% of the switch's breakdown voltage.
17. **Initial Peak Current and VOR**: The peak current and flyback voltage depend on the duty cycle and input voltage. They must be chosen within safe limits to avoid damaging the switch.
By understanding these principles and applying them effectively, engineers can design more efficient, reliable, and stable switching power supplies tailored to specific applications.
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