The power supply circuit is a crucial component in any electronic device, directly influencing its overall performance. Our products utilize two main types of power supplies: linear power supplies and high-frequency switching power supplies. In theory, a linear power supply draws as much current at the input as the user requires at the output. On the other hand, a switching power supply takes as much power from the input as the user needs at the output. This fundamental difference affects both efficiency and heat generation.
**Linear Power Supply**
A linear power supply operates in a linear mode, typically using voltage regulator ICs like the LM7805, LM317, or SPX1117. These devices regulate voltage by dissipating excess energy as heat. The schematic of an LM7805-based power supply includes rectification, filtering, regulation, and energy storage components. The input current (I1) equals the output current (I2) plus the reference current (I3), which is usually very small. Therefore, I1 ≈ I3. Understanding current flow is essential during PCB design because trace widths are determined based on the current that flows through each node. Proper planning ensures that the board can handle the required current without overheating or causing signal integrity issues.
When laying out a linear power supply PCB, components should be placed closely to minimize trace lengths. The layout should follow the functional relationships in the schematic—starting with rectification, followed by filtering, regulation, and finally, energy storage via capacitors. A well-designed layout not only improves performance but also reduces noise and interference. For example, in Figure 2, the right-hand PCB layout demonstrates better placement of the rectifier, filter capacitor, and regulator chip, ensuring efficient power delivery and improved thermal management.
Heat dissipation is another critical factor in linear power supplies. If the input voltage is significantly higher than the output, the regulator chip can generate substantial heat. For instance, a 10V input with a 5V output and 500mA load results in 2.5W of heat, while a 15V input would produce 5W. To prevent overheating, sufficient space or a heat sink must be provided. Linear power supplies are best suited for low-voltage and low-current applications. For higher power demands, a switching power supply is more appropriate.
**High-Frequency Switching Power Supply**
Switching power supplies use high-speed switching transistors to generate PWM signals, allowing efficient voltage regulation. Common ICs include the LM2575, MC34063, and SP6659. Unlike linear power supplies, switching power supplies maintain power balance between input and output, meaning that voltage and current are inversely proportional. This makes them highly efficient and suitable for high-power applications.
In a typical switching power supply, when the switch (U1) turns on, current flows through the inductor (L1). Due to the inductor’s property of opposing sudden changes in current, the current gradually increases. When the switch turns off, the inductor resists the sudden drop in current, and the freewheeling diode (D1) allows the current to continue flowing through the circuit, maintaining energy transfer to the output capacitor (C3).
Proper PCB layout is essential for switching power supplies. The feedback line must be connected after filtering to avoid excessive output ripple. Additionally, the placement of the freewheeling diode and the feedback point significantly impacts performance. Many designers overlook these details, assuming that similar network connections behave the same way. However, even small differences in layout can lead to significant performance variations.
Figure 4 illustrates a common mistake in the layout of an LM2575 switching power supply. Poor placement of the feedback line and the inductor can result in instability and increased noise. By following the correct layout guidelines, such as keeping the feedback path short and placing the inductor close to the switch, the performance of the power supply can be greatly improved.
Understanding the schematic is key to successful PCB design. It provides critical information about component pin connections, current paths, and signal flow. The LM7805 and LM2575 circuits represent standard layouts for linear and switching power supplies, respectively. While each product may have unique requirements, the general principles of layout and routing remain consistent.
In conclusion, mastering the principles of power supply circuits and their PCB layout techniques is essential for any electronics designer. Whether working with linear or switching power supplies, attention to detail in both the schematic and the physical layout ensures reliable and efficient performance. With practice and proper knowledge, designing power circuits becomes a straightforward task.
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