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1. The three critical current loops

In a synchronous buck converter there are many nets, but only a few loops really define EMI, switching stress, and hot spots. For layout, think in terms of current loops, not traces:

1. Input hot loop (VIN → high-side FET → switch node → input cap → back to VIN)
2. Switching loop (high-side FET / low-side FET / inductor / output caps)
3. Output load loop (inductor → output caps → load → return)

The input hot loop is the most critical for EMI and ringing. It must be as small and tight as possible.

2. Place input capacitors first

The input capacitors are not a cosmetic add-on, they are part of the high-frequency power loop. For a 20–30 A buck:

• place MLCC input caps directly between VIN and PGND pins of the power stage
• add bulk electrolytic / polymer caps slightly further away (to cover lower frequencies)
• avoid long “wires” between VIN pin and input cap pad

A good mental model: the MOSFETs and high-frequency input caps should almost look like a single compact power block.

3. Switch node (SW) routing

The SW node is the noisiest point in a buck converter. It experiences fast transitions with dv/dt in the range of 20–50 kV/µs. Bad habits here almost guarantee failed EMC tests.

• Keep the SW copper region as small as functionally possible
• Do NOT route sensitive signals under or over SW
• Do NOT flood layers with a huge SW polygon unless it is intentional (e.g. as a heatsink) and shielded
• If a copper pour is needed for thermal reasons, keep it mostly on one layer and shield it with a solid ground plane below

4. Inductor placement and output caps

The inductor and the output capacitors form the core of the output loop. For 10–50 A designs:

• place the inductor immediately next to the SW node pad / FETs
• place the output caps directly at the inductor's VOUT side, forming a short loop
• keep the route from inductor to output caps wide and short
• place the load connection point close to the output caps, not “somewhere at the end of the board”

This minimizes output ripple, improves transient response, and reduces EMI from long current paths.

5. Ground planes and power grounds

For high currents, ground must be treated as a low-impedance distributed conductor, not a “wire”. A few rules:

• Use a solid ground plane on an internal layer whenever possible
• Keep power ground (PGND) and signal ground (AGND) conceptually separate near the controller
• Connect PGND and AGND at a single, well-defined “star point” near the controller or sense point
• Avoid breaking the ground plane with many cuts under sensitive or fast-switching regions

6. Sense lines and feedback routing

The feedback network is the “nervous system” of the regulator. Noise here leads to oscillations, jitter, or incorrect output voltage.

• Use Kelvin sense from the output cap node (not from the inductor or some remote pad)
• Route FB and COMP traces away from SW, gate drive and inductor
• Use a small ground pour around feedback components tied to AGND
• Do not share the AGND trace with high-current PGND return

7. Thermal paths and copper shapes

High-current buck converters often run near thermal limits. Layout can make or break thermal performance:

• Use wide copper pours for high-current paths (VIN, SW, VOUT, PGND)
• Add thermal vias under MOSFETs, power stages and inductors to spread heat into inner/other layers
• Consider forced airflow direction and keep hot parts in the airflow path
• Avoid placing temperature-sensitive components (references, precision op-amps) next to inductors or FETs

8. Example layout flow for a 30 A buck

Practical “build order” when starting a PCB:

1. Place the controller or power-stage IC
2. Place high-side and low-side FETs (if external) next to the IC
3. Place high-frequency input MLCCs directly between VIN and PGND pins
4. Place inductor at the SW node
5. Place output caps at inductor VOUT side
6. Draw the high-current loops with wide copper and minimal loops
7. Add feedback network close to the controller, route FB from output cap node
8. Fill ground plane, add thermal vias and check return currents

9. Common layout mistakes

• Long “wire-like” traces between input caps and FETs
• Feedback taken from the wrong point (e.g. inductor or remote load without proper sense routing)
• SW node routed as a big polygon under the whole board
• Sensitive analog ground sharing return with high-current PGND under the IC
• Inductor too far from the FETs, creating a big current loop
• High dI/dt loops running over split ground planes or gaps

10. EMI and measurement tips

To verify layout quality, measurements are essential:

• Observe SW node ringing and overshoot with a low-inductance probe loop
• Check inductor current ripple with a current probe (or sense resistor + differential probe)
• Use near-field probes around input caps and inductors to locate noisy regions
• Compare layout revisions under the same load/line/transient conditions

A clean layout will show controlled ringing, limited overshoot, and stable regulation under fast load transients.

11. Conclusion

High-current buck converters live or die by their PCB layout. Thinking in terms of current loops, minimizing the input hot loop, keeping the switch node compact, routing feedback cleanly and providing solid ground and thermal paths are far more important than minor schematic tweaks.

If you treat the power stage as a single “RF object” with defined loops and impedances, EMC testing, thermal margins and long-term reliability all improve at once.