The power supply circuit is a critical component in any electronic product, directly influencing its overall performance. Our electronic devices typically use 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 as the user needs at the input, while a switching power supply delivers as much power as required by the user, regardless of the input voltage.
**Linear Power Supply**
A linear power supply operates in a linear mode, commonly using voltage regulator ICs such as LM7805, LM317, or SPX1117. The schematic diagram of an LM7805-based power supply is shown below. This type of power supply usually consists of rectification, filtering, voltage regulation, and energy storage components. A typical linear power supply is a series-regulated design, where the output current is nearly equal to the input current (I1 ≈ I2 + I3, with I3 being a small reference current). Understanding the current flow between components is essential for PCB layout, as trace widths are determined based on current requirements. Proper current management ensures that the board performs efficiently without overheating or voltage drops.
When designing the PCB for a linear power supply, it's important to place components compactly and keep all connections as short as possible. The layout should follow the functional relationships seen in the schematic—starting with rectification, followed by filtering, voltage regulation, and finally the output capacitor. Figure 2 shows the PCB layout for this design. While both layouts are similar, the right-hand version demonstrates better placement, ensuring that the power line and ground return are close together for improved signal integrity and reduced noise.
Heat dissipation is also a major concern when working with linear power supplies. For example, if a regulator chip has an input voltage of 10V and an output of 5V with a 500mA load, it will generate 2.5W of heat. If the input increases to 15V, the heat rises to 5W. Therefore, sufficient space or a heat sink must be allocated to ensure proper cooling. Linear power supplies are best suited for low-voltage and low-current applications. For higher power or larger voltage differences, a switching power supply is more appropriate.
**High-Frequency Switching Power Supply**
Switching power supplies operate by rapidly turning a switch on and off, generating PWM signals. These signals are used to control the charging and discharging of an inductor, which helps regulate the output voltage. Common ICs include 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.
In the LM2575 switching power supply schematic (Figure 3), when the switch (U1) turns on, current flows through the inductor (L1). Inductors resist sudden changes in current, so the current gradually increases. When the switch turns off, the inductor tries to maintain the current, causing the freewheeling diode (D1) to conduct and allow the current to continue flowing. This process helps charge the output capacitor (C3), effectively boosting the voltage.
Proper PCB layout is crucial for switching power supplies. Key considerations include the placement of feedback lines and the correct routing of the freewheeling diode. The feedback network should be placed after filtering to minimize output ripple. Many designers overlook these details, thinking that similar net connections are interchangeable, but this can lead to significant performance issues.
Figure 4 shows a poorly designed PCB layout for the LM2575. A well-designed layout ensures that high-current paths are kept short and that sensitive feedback traces are isolated from noisy areas. Understanding the schematic thoroughly helps in identifying critical connections and current paths, making PCB design more straightforward.
Both linear and switching power supplies have their own advantages and are used depending on the application’s requirements. Learning the principles behind these circuits not only improves your design skills but also gives you a deeper understanding of how electronic systems function. Every electronic product relies on a stable power supply, so mastering these concepts is essential for any engineer or designer.
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