DC-DC Converter Layout Rules

The Layout Sets the Performance

A switching converter’s efficiency, its output noise and its electromagnetic behaviour are decided largely by the layout. The components are usually chosen from a reference design, and the difference between a good and a poor result is usually the placement and the routing rather than the part numbers. The two loops that matter are the hot loop, which carries the switched current, and the feedback path, which sets the regulation. A layout that respects those two will usually work; one that ignores them will usually produce ringing, noise and a regulation error.

The Hot Loop

The hot loop is the path that the switched current takes during each cycle: from the input capacitor, through the high side switch, through the inductor, and back through the low side switch or the diode to the capacitor. Its area should be minimised, because the loop is an antenna and its parasitic inductance produces the ringing that stresses the switches and radiates noise. The input capacitor should be placed as close as possible to the switches, with the shortest possible connection on both sides, and the return should be directly under the loop on an adjacent layer rather than routed around. A loop that is a few millimetres larger can double the ringing.

The Feedback Path

The feedback signal is a small voltage that sets the output, and it has to reach the controller without picking up the switching noise. The divider should be placed close to the controller, the sense point should be taken at the output capacitor rather than at the switch node, and the trace should be kept away from the inductor and the switch node. A feedback trace that runs near the inductor couples the switching field into the control loop, which produces jitter, an unstable regulation or an audible noise at a light load. Where the divider is remote, a shielded or a differential sense is sometimes used, but the simplest answer is to move the divider.

Grounding

The converter’s ground carries the switched current, and the control’s ground must not share the same path. The usual method is a single connection point between the power ground and the analogue or control ground, placed so that the switched current does not flow through the control’s reference. A ground plane that is continuous under the loop is desirable for the return path, but the control’s components should be placed so that their return currents do not pass through the loop’s return. The distinction is a matter of where the currents flow, and it should be considered for each component rather than only for the plane as a whole.

Component Placement and Thermal

The components should be placed in the order of the current path, and the small, sensitive components should be away from the switch node. The inductor should be positioned so that its field does not couple into the feedback or the output sense, which usually means turning it so that its axis is not aligned with the sensitive trace. The thermal design follows the dissipation: the switch and the inductor are the components that heat, and their copper area and their connection to the plane determine the temperature rise. A converter that is efficient but thermally marginal will derate itself and lose its advantage.

Noise and Filtering

The converter is a noise source, and its output filter and its input filter are part of the design. The input capacitor’s placement is the first line of defence, and an additional ferrite and capacitor at the input can prevent the switching current from flowing back into the supply. The output filter reduces the ripple, and its components should be placed close to the load rather than at the converter, because the trace between them carries the ripple. Where the converter supplies a sensitive load, an additional filter or a linear regulator after it is sometimes the practical answer, with the linear stage placed close to the load.

Verification

The converter should be measured rather than assumed. The switching waveform and the ringing are the first checks, followed by the output ripple, the efficiency across the load range and the regulation at the extremes of the line and the load. A thermal image shows the temperature of the switch and the inductor, and a near field probe or a spectrum analyser shows the radiated and the conducted noise. The measurements should be made at the worst case, and the layout should be changed if the ringing or the noise is higher than expected, since a component change seldom fixes a layout problem.

Multiphase and Multi-Rail Designs

Where the current is high or the load has several rails, the converter may have more than one phase or several converters on one board. The phases have to be arranged so that their loops do not share, and their inductors should be oriented so that their fields do not couple. The output capacitors are shared, so their placement has to serve all the phases, and the sense lines have to be routed without picking up the neighbouring phase’s switching. On a board with several rails the layout becomes a matter of keeping each converter’s loop local and presenting a single, quiet reference to the loads.

FAQ

What is the hot loop? The path the switched current takes each cycle, whose area should be minimised to reduce inductance and ringing.

Where should the feedback be sensed? At the output capacitor, with the divider close to the controller and away from the switch node.

How should the grounds be arranged? With a single connection point so that the switched current does not flow through the control’s reference.

Why does the inductor’s orientation matter? Its field can couple into the feedback or the sense trace if it is aligned with them.

What does a thermal image show? The temperature of the switch and the inductor, which decides whether the design derates itself.

Conclusion

A DC-DC converter’s behaviour is decided by the hot loop, the feedback path and the grounding, so place the components by the current path and keep the sense clean. Measure the ringing and the ripple. Power layout belongs to PCB design and layout, the assembly sits in PCB assembly, and the verification is part of PCBA testing. Converter layouts are first built during prototype PCB assembly in 2026.