Should You Put an Inductor Above Ground? | PCB Layout

Summary

In this video from Altium Academy, host Zach Peterson addresses the common question of whether it's advisable to place a ground cutout beneath an inductor in a switching regulator. He argues in favor of placing ground under the inductor, citing reasons like reducing noise, shielding, and preventing EMI issues. He explains how a cutout could expose circuits to noise and offers insights into inductor orientation, Eddy currents, and the effects of capacitance. Zach concludes by encouraging feedback and interaction from viewers on this debated design practice.

Takeaways

• 🛠️ The topic revolves around whether it's okay to place a ground cutout beneath an inductor in a switching regulator.
• 🔧 Zach Peterson advocates for placing ground beneath the inductor, arguing it helps reduce noise and EMI issues.
• 💡 The inductor type and orientation matter when considering noise impact, with shielded and toroidal inductors reducing magnetic field leakage.
• 📉 The magnetic field generated by the inductor can cause Eddy currents, which generate opposing magnetic fields, affecting inductance.
• 🛡️ A solid ground plane below the inductor helps shield other circuits from switching noise, reducing EMI risk.
• ⚡ A cutout could allow switching magnetic fields to propagate into interior PCB layers, potentially creating noise in other circuits.
• 🔌 Concerns about capacitance between the switching node and ground are minimal, as modern MOSFETs can handle the small capacitance created.
• 🚫 Peterson advises against using cutouts, emphasizing that they expose nearby circuits to noise and offer no real benefit.
• 📊 Ground beneath the inductor provides shielding, reducing parasitic capacitance to nearby traces, and improving overall design.
• 📞 The video encourages viewers to share their experiences and reminds them to consult fabricators for best practices.

Q & A

What is the main topic of the video?

The video discusses whether it is acceptable to place a ground cutout beneath an inductor in a switching regulator, focusing on noise generation and its effects on the PCB.

Why is the placement of ground or a cutout below an inductor important?

The placement affects noise generation and potential interference with other circuits. Ground placement can help shield components from electromagnetic interference (EMI), while a cutout could expose circuits to noise.

What factors influence whether or not to put ground below an inductor?

Key factors include the current in the switching regulator, the type of inductor (shielded or unshielded), the magnetic field intensity, and the inductor’s orientation on the PCB.

What role does a shielded inductor play in this context?

A shielded inductor helps reduce the leakage of the magnetic field, confining it within the inductor package, which decreases the impact of noise on nearby components.

What are Eddy currents, and how do they relate to the inductor's magnetic field?

Eddy currents are induced by changing magnetic fields, which create opposing magnetic fields (according to Lenz’s law) to counteract the original field. This interaction can reduce the effective inductance of the inductor.

What is the reasoning behind placing a cutout below the inductor?

The cutout is meant to eliminate Eddy currents that could be induced in the ground plane below the inductor, reducing noise from those currents. However, it may also allow noise to propagate into other layers of the PCB.

What is the main disadvantage of placing a ground cutout below an inductor?

The main disadvantage is that it exposes internal layers of the PCB to noise from the switching magnetic field, which can create EMI problems and affect other circuits on the board.

What benefit does placing ground beneath an inductor offer?

Placing ground beneath the inductor helps shield other PCB layers and components from the switching magnetic field, reducing EMI and potential noise interference.

How does the capacitance between the switching node and ground affect the regulator's performance?

Although there is some capacitance between the switching node and ground, it is usually small and negligible compared to the capacitance of the switching elements (MOSFETs). Therefore, it has little impact on power conversion efficiency.

Why does the speaker recommend placing ground beneath the inductor?

The speaker recommends placing ground beneath the inductor to provide shielding and prevent EMI, rather than creating a cutout which could allow noise to propagate into other circuits.