Switching regulators, which are commonly used for voltage conversion, rely on inductors to temporarily store energy. These inductors are typically large components that need to be integrated into the printed circuit board (PCB) layout. This task, while straightforward, requires careful attention, as the current flowing through the inductor can vary, though not instantaneously. The current changes in a continuous, generally slow manner.
A switching regulator alternates the current flow between two distinct paths. This switching occurs at high speeds, with the exact switching frequency depending on the characteristics of the switching edges. The path through which the switching current flows is known as the *thermal loop* or *AC current path*. In one state, this loop conducts current, while in the other state, it does not.
In PCB design, minimizing the thermal loop’s area and keeping its path as short as possible are critical considerations. This helps reduce parasitic inductance, which can lead to unwanted voltage fluctuations and increase the risk of electromagnetic interference (EMI). Keeping these traces short and tightly coupled minimizes both parasitic inductance and EMI, contributing to the overall efficiency and performance of the switching regulator.
**Best Practices for Routing Sensitive Control Traces Near Inductors in PCB Design**
When designing printed circuit boards (PCBs) for switching regulators, careful consideration must be given to the placement and routing of sensitive control traces, especially in relation to inductors. Sensitive signal paths should never be routed underneath an inductor—whether on the surface, within inner layers, or on the back of the PCB. This precaution is due to the magnetic field generated by the inductor, which can negatively impact weak signals in nearby paths. One of the most critical signal paths in switching regulator designs is the feedback path, which connects the output voltage to the switching regulator’s IC or resistor divider.
In addition to inductive effects, it’s important to understand that inductors also exhibit a capacitive behavior. The first coil winding in a step-down switching regulator is directly connected to the switching node, as shown in Figure 1. As the voltage at this switching node rapidly changes, the voltage across the coil also fluctuates significantly. Since switching events occur at very high speeds and input voltages can be substantial, significant coupling effects can be introduced onto adjacent PCB traces. Consequently, it is essential to keep sensitive traces as far away from the inductor as possible to prevent interference.
Some designers go even further by eliminating any copper under the inductor, even in the ground plane layer. In these designs, they may incorporate cuts under the inductor to prevent the formation of eddy currents in the ground plane caused by the magnetic field of the coil. While this approach can be effective, there are counterarguments in favor of maintaining an uninterrupted ground plane:
– **Shielding Efficiency**: An unbroken ground plane serves as an effective shield, protecting sensitive signals from external noise.
– **Improved Heat Dissipation**: A continuous copper plane enhances heat dissipation across the PCB, benefiting thermal management.
– **Eddy Current Considerations**: While eddy currents may form under the inductor, their impact is generally localized. These currents cause minimal losses and have negligible effects on the overall performance of the ground plane.
Thus, it is widely accepted that the ground plane should remain intact, even beneath the inductor, as long as there are no major issues with thermal management or electrical interference.
In summary, while the inductor in a switching regulator is not part of the critical thermal path, it is prudent to avoid routing sensitive control traces underneath or near the inductor. PCB layers, such as the ground plane or VDD (supply voltage) plane, should be designed to be continuous and uninterrupted, ensuring both signal integrity and efficient heat dissipation.
A switching regulator alternates the current flow between two distinct paths. This switching occurs at high speeds, with the exact switching frequency depending on the characteristics of the switching edges. The path through which the switching current flows is known as the *thermal loop* or *AC current path*. In one state, this loop conducts current, while in the other state, it does not.
In PCB design, minimizing the thermal loop’s area and keeping its path as short as possible are critical considerations. This helps reduce parasitic inductance, which can lead to unwanted voltage fluctuations and increase the risk of electromagnetic interference (EMI). Keeping these traces short and tightly coupled minimizes both parasitic inductance and EMI, contributing to the overall efficiency and performance of the switching regulator.
**Best Practices for Routing Sensitive Control Traces Near Inductors in PCB Design**
When designing printed circuit boards (PCBs) for switching regulators, careful consideration must be given to the placement and routing of sensitive control traces, especially in relation to inductors. Sensitive signal paths should never be routed underneath an inductor—whether on the surface, within inner layers, or on the back of the PCB. This precaution is due to the magnetic field generated by the inductor, which can negatively impact weak signals in nearby paths. One of the most critical signal paths in switching regulator designs is the feedback path, which connects the output voltage to the switching regulator’s IC or resistor divider.
In addition to inductive effects, it’s important to understand that inductors also exhibit a capacitive behavior. The first coil winding in a step-down switching regulator is directly connected to the switching node, as shown in Figure 1. As the voltage at this switching node rapidly changes, the voltage across the coil also fluctuates significantly. Since switching events occur at very high speeds and input voltages can be substantial, significant coupling effects can be introduced onto adjacent PCB traces. Consequently, it is essential to keep sensitive traces as far away from the inductor as possible to prevent interference.
Some designers go even further by eliminating any copper under the inductor, even in the ground plane layer. In these designs, they may incorporate cuts under the inductor to prevent the formation of eddy currents in the ground plane caused by the magnetic field of the coil. While this approach can be effective, there are counterarguments in favor of maintaining an uninterrupted ground plane:
– **Shielding Efficiency**: An unbroken ground plane serves as an effective shield, protecting sensitive signals from external noise.
– **Improved Heat Dissipation**: A continuous copper plane enhances heat dissipation across the PCB, benefiting thermal management.
– **Eddy Current Considerations**: While eddy currents may form under the inductor, their impact is generally localized. These currents cause minimal losses and have negligible effects on the overall performance of the ground plane.
Thus, it is widely accepted that the ground plane should remain intact, even beneath the inductor, as long as there are no major issues with thermal management or electrical interference.
In summary, while the inductor in a switching regulator is not part of the critical thermal path, it is prudent to avoid routing sensitive control traces underneath or near the inductor. PCB layers, such as the ground plane or VDD (supply voltage) plane, should be designed to be continuous and uninterrupted, ensuring both signal integrity and efficient heat dissipation.
