Lesson · Problem 10
Lay Out a Buck Converter
The hot loop of a buck
You should be able to
- Identify the switch node and the hot loop.
- Keep feedback off the switch node even when that route is longer.
Power · Routing · PCB Fundamentals
What it is
That hot loop should be small. The switch node itself should also be small, because it is a fast voltage antenna. The feedback trace senses the output and should stay away from the switch node so it measures the output rather than the switching edge.
Engineers sketch the chopped-current loop before they chase a shorter geometric path. In a buck, that loop is the input capacitor, the switch, and the diode or low-side device, closed through ground. The inductor current is comparatively smooth. A short feedback route that shares copper with the switch node is electrically long: it measures the edge. This challenge grades that geometry. It does not simulate ripple or efficiency.
Why this matters
A buck switch node moves quickly. The loop that contains the switch, the diode or synchronous device, and the input capacitor carries the chopped current.
What you are building
A fixed buck schematic. You place and route the hot loop and keep feedback off the switch node.
Prerequisites
This lesson uses input capacitor, output capacitor, routed length, return path, trace, loop area. It introduces buck converter, switch node, hot loop, loop inductance, feedback, inductor current.
Component guide
What the parts are. The requirements panel is still the list that is graded.
Buck converter
- What it does
- A switching regulator that chops the input and filters it to a lower output.
- Why it is here
- The schematic can be correct while the layout still makes a large, noisy loop.
- Symbol
- Switch, diode or low-side device, inductor, input capacitor, output capacitor.
- Beginner mistake
- Routing the feedback along the switch node because that path is shorter.
- What an engineer checks
- Which pin is the switch node and which pin is feedback.
Input capacitor
- What it does
- Supplies the chopped current each cycle.
- Why it is here
- It is inside the hot loop. The inductor is not.
- Symbol
- A capacitor from the input node to ground, next to the switch.
- Beginner mistake
- Placing it on the far side of the board and calling the schematic “done”.
- What an engineer checks
- The value, and that it must sit at the switch input.
Catch diode
- What it does
- Gives the inductor current a path when the switch is open.
- Why it is here
- Its cathode is at the switch node and its anode is at ground in this buck.
- Symbol
- Diode bar at the switch node.
- Polarity
- Cathode at SW, anode at GND.
- Beginner mistake
- Reversing the diode so it shorts the switch node to ground.
- What an engineer checks
- Which pin is the cathode.
Interview lens
Sketch Cin, the switch, and the diode before you talk about the inductor. Say the feedback measures the output.
What PCBGrade measures
Loop geometry and how close feedback runs to the switch node. It does not simulate ripple or efficiency.
Where this shows up
A buck that rings or whose output wanders is often a large input-capacitor loop or a sense trace on the switch node.
Terms
- Switch node
- The node that snaps between the input and ground each cycle. It is noisy.
- Hot loop
- The smallest loop that carries the chopped current.
- Feedback
- The sense connection that tells the regulator what the output is doing.
- Loop inductance
- The inductance of the hot-loop path. A larger loop rings harder when the switch opens.
- Input capacitor
- The capacitor that supplies the chopped current. It is part of the hot loop, unlike the inductor.
What is happening electrically
A buck moves energy with a switch. While the switch is on, current ramps in the inductor from the input. While it is off, the inductor current keeps flowing, and the diode or the low-side device carries it. The input capacitor supplies the chopped current the inductor does not want to change instantly. That chopped path — input capacitor, switch, and diode or low-side device, closed through ground — is the hot loop.
Loop inductance is the inductance of that path. A larger loop stores more energy in the magnetic field and rings when the switch opens. The switch node is the copper that snaps between the input and ground every cycle. It is a fast voltage. The inductor current itself is comparatively smooth. The output capacitor takes what the inductor does not deliver at that instant and holds the output.
Feedback is a measurement of the output. If that trace shares copper or runs beside the switch node, it measures the edge as well as the output, and the control loop chases noise.
Why the geometry matters
Place the input capacitor against the switch and the catch device so the hot loop is a few small pieces of copper, not a tour of the board. Keep the switch-node copper small: every extra millimetre is antenna and coupling. Route the inductor from that node to the output capacitor, and put the output capacitor’s ground near the loop’s ground so the output current has a short return. Route feedback on a quiet path back to the output sense point, away from the switch node, even when that path is longer.
This challenge turns those sentences into lengths and areas. A pour does not replace the short routed leads the rules name. The input capacitor, the switch, and the diode share one tight loop. The inductor leaves that loop. Feedback does not re-enter it.
How an engineer reasons
Sketch the chopped current before you shorten a trace. Ask which edges actually switch. The inductor is not the hot loop. Then ask what the feedback pin will see. A geometrically short sense line beside the switch node is electrically a bad measurement. Only after the loop and the sense line are placed should you tidy the rest.
Worked example
Keep the input-capacitor / switch loop tight, limit switch-node copper, and route feedback so it is not hugging the switch node. The exact lengths are in the requirements.
Good and bad
Cin tight against the switch and diode, a small switch-node island, Cout at the inductor’s output, and feedback arriving from the quiet side: the geometry matches the currents. The same schematic with Cin across the board, a large switch-node pour, and feedback glued to that pour can look “routed” and still be a noisy converter. The inductor path can be neat in both pictures. Neat is not the hot loop.
Common mistake
Routing feedback alongside the switch node because that path is geometrically shorter.
Where this rule stops
PCBGrade grades that geometry. It does not simulate ripple, efficiency, or compensation. A real design also chooses frequency, inductor saturation, capacitor voltage rating, and a snubber when the loop still rings. The geometric limits are the challenge. The principle that survives is: the chopped loop wants to be small, and the sense line wants to be quiet.
Before the challenge
The schematic is already the converter. The challenge grades the hot loop, the switch-node copper, and where feedback runs. A shorter path is not automatically the right path.
Ready for the challenge
You can name the hot loop and refuse a short feedback route that hugs the switch node.
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