A well-designed PCB layout plays a crucial role in optimizing performance, reducing thermal stress, and minimizing noise between components and traces. These benefits stem from the designer's deep understanding of the current conduction path and signal flow within the power supply. When a prototype power board is first tested, ideally it should function smoothly, operate quietly, and generate minimal heat—though this is rare in practice.
One common issue with switching power supplies is an unstable switching waveform, which can lead to audible noise from magnetic components. If the problem arises from PCB layout, it can be challenging to identify the root cause. Therefore, getting the PCB layout right from the start is essential for reliable and efficient power design.
A good layout enhances power efficiency, reduces thermal stress, and most importantly, minimizes noise and interference between components. To achieve these goals, designers must have a clear understanding of the internal current paths and signal flow in the power supply. This article outlines key considerations for proper PCB layout in non-isolated switching power supplies.
**Layout Planning**
When placing an embedded DC/DC power supply on a large board, it’s important to position the output close to the load device to minimize interconnect impedance and voltage drop. Ensure good airflow and manage thermal stress. If forced air cooling is used, place the power supply near the fan. Also, avoid blocking airflow around sensitive components like MOSFETs or PWM controllers by positioning larger passive components (e.g., inductors and electrolytic capacitors) away from them.
To prevent switching noise from interfering with analog signals, avoid placing sensitive signal lines under the power supply. If unavoidable, include a ground plane between the power layer and the small signal layer for shielding.
Early planning is key—designers often overlook power layout, focusing instead on other parts of the system. This can lead to inefficient and unreliable designs. On multilayer boards, consider placing a DC voltage layer between high-current power components and sensitive signal traces. This layer acts as an AC ground, helping to shield small signals from noise.
Avoid separating the ground or DC voltage layer if possible. If separation is necessary, keep the number and length of traces minimal and route them in the same direction as the main current to reduce interference.
Poor layer structures, such as those shown in Figures A and C, sandwich the small signal layer between high-current layers and the ground, increasing capacitive coupling. Good designs, like B and D, use a ground plane to shield the small signal layer effectively. Always place a ground plane next to the outer power layer and use thick copper for high-current areas to reduce losses and thermal resistance.
**Power Stage Layout**
Switching power supplies typically consist of a power stage and a control stage. The power stage handles high currents, so its components should be placed first, followed by the control circuit at specific points.
High-current traces should be short and wide to minimize inductance, resistance, and voltage drop—especially for high di/dt pulses. In a synchronous buck converter, the continuous current path is represented by a solid line, while the pulse current path (thermal loop) includes components like input capacitors, FETs, and Schottky diodes.
Parasitic inductance in these loops can cause voltage ringing and spikes. To minimize inductance, keep the loop area as small as possible and ensure the traces are wide and direct. High-frequency decoupling capacitors (0.1μF–10μF, X5R/X7R ceramic) should be placed close to the power components to reduce noise.
In boost converters, the high-frequency capacitor should be near the MOSFET and rectifier diode. Proper placement of these components helps reduce loop size and improve stability.
**High dv/dt Switch Area**
The switch node (SW) has a high dv/dt, making it a major source of EMI. Minimize the copper area around the SW junction to reduce coupling capacitance with other traces. However, the area must also allow for heat dissipation. A grounded copper foil beneath the switch junction can provide additional shielding.
For surface-mount power components without heatsinks, the copper area must be sufficient for cooling. For DC voltage junctions, maximize the copper area where possible. Use multiple vias to help dissipate heat and reduce thermal stress.
**Control Circuit Layout**
Keep the control circuit away from high-noise areas. In a buck converter, place the control circuit near the VOUT+ terminal; in a boost converter, place it near VIN+. Maintain a small distance (0.5" to 1") between the control IC and high-noise components if space allows. If not, ensure proper grounding to isolate the control circuit.
Use separate signal and power grounds, especially if the controller has SGND and PGND pins. Connect them at a single point to minimize noise. Solder the ground pad to the PCB and use multiple vias to reduce impedance and thermal resistance.
**Loop Area and Crosstalk**
Adjacent conductors can create capacitive coupling, leading to noise. Keep high-noise traces away from sensitive signal lines. If possible, place them on different layers and use internal ground planes for shielding.
**Trace Width Selection**
Different signals require different trace widths. Small signal traces can be narrower (10–15 mil), while high-current traces (gate drive, VCC, PGND) should be wider based on current levels. Choose widths that match the specific requirements of each pin and signal.
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