Magnetic Coil PCB versus Inductor PCB: Key Differences

Once a coil or an inductor is built into a board rather than wound as a separate component, two different things are being described. A magnetic coil PCB forms an electromagnetic structure that couples to another coil or to a magnetic field, while an inductor PCB integrates a wound component into the copper layers so that it replaces a discrete part. Their purposes differ, and so do their design rules.

What Each One Is

A magnetic coil PCB is a planar winding whose job is coupling: transferring power across a gap, sensing a field or generating one. The winding is usually a spiral or a set of turns designed for mutual inductance with a coil in a separate assembly.

An inductor PCB is a planar inductor used in a filter or a switching converter in place of a discrete wound component. Its job is to store energy in a defined inductance value, and its interactions with anything outside the board are an unwanted side effect rather than the purpose.

The Coupling Requirement

For a coupling application the design is driven by mutual inductance and by the geometry of the gap. A wireless power coil is sized to match the coil in the receiver, and its turns, track width and spacing are chosen to produce the required inductance and coupling at the operating frequency.

The board layer structure matters, because a coil placed close to a solid ground plane has its field partially cancelled by image currents, which reduces the inductance and generates loss. In many designs the coil area is kept free of copper on all other layers for exactly that reason.

Inductance per Unit Area

A planar winding on a single layer produces a modest inductance for its area, because the number of turns is limited by the winding pitch. A spiral of fifteen turns in a few square centimetres yields an inductance in the microhenry range, which is often not enough for a power converter.

Where more inductance is needed, the winding can be distributed across several layers connected in series through vias, with the turns aligned so that their fields add rather than cancel. A planar magnetic built this way replaces a bulky wound component and gives a repeatable, low profile result, while a magnetic coil PCB usually stays on one or two layers because its purpose is coupling rather than inductance per unit area.

Losses and Quality Factor

A planar winding has more resistance than a wound one of the same size, because the copper is thin and the turns are short and wide. At higher frequencies the skin effect makes the situation worse, since the current crowds into the surface of the conductor.

The quality factor of a planar structure is therefore lower than that of a wire wound coil, and that difference matters in resonant applications where a high quality factor keeps the losses down. Where the design needs both the planarity and a good quality factor, thicker copper is used, and sometimes several layers are connected in parallel to reduce the resistance, which brings its own question about current sharing between layers.

Core Materials

An air cored structure has no saturation limit and no core loss, but its inductance is low for the area it occupies. Adding a magnetic core increases the inductance substantially and confines the field, which is what an inductor for a converter needs in order to reach a useful value in a small footprint.

For a magnetic coil PCB used for coupling, a core is often placed on only one side or omitted entirely, because the field must reach the other coil. For an inductor PCB, the core is usually a flat ferrite plate or a shaped piece placed over the winding and bonded to the board, and the gap between the core and the copper becomes a critical dimension.

Thermal Behaviour

Both structures generate heat where the current density is highest, which is in the winding and in the core. Planar copper spreads that heat into the board well, and the copper layers act as a heat sink, which is an advantage over a discrete component mounted on a pad.

Where the loss is significant, the copper weight is increased or the winding is split across layers to reduce the current density. The sizing follows the same current and temperature rule as any other conductor, described in trace width and current calculation, with the additional complication that the resistance changes with frequency.

Where Each Is Used

A magnetic coil PCB appears in wireless charging pads, in current sensors, in planar transformers for isolated supplies and in antenna structures that resemble windings. In each case the board is part of an electromagnetic system rather than a container for one.

An inductor PCB appears in switching converters and in filters where the height of a wound component is not available. Power converters built this way are smaller and flatter than their discrete equivalents, and the layout considerations for the surrounding circuit are those described in DC-DC converter layout and routing.

Interference and Shielding

A planar winding is a source of magnetic field that spreads beyond the structure, and the field couples into anything conductive nearby. For a power inductor, a ground plane under the winding reduces the field that escapes but also reduces the inductance, so the trade must be evaluated rather than assumed.

For a coupling coil, the field is the product, but it still has to be kept away from sensitive circuits. Shielding a planar magnetic usually means a conductive layer or a ferrite plate, and the techniques overlap with the general approach described in EMI suppression design principles.

Manufacturing Considerations

Planar structures need thicker copper than ordinary signal layers, often two to four ounces, which changes the achievable line width and spacing. The winding pitch is therefore a compromise between the inductance required and the etch capability of the fabricator.

Mechanical issues follow. A bonded core adds an assembly step and a height, and its alignment to the winding affects the inductance. Where the board is thin or large, the copper balance of the winding can also affect flatness, so the layer stackup should be balanced around the magnetic layers rather than designed without them.

Choosing Between Them

The question to ask first is whether the structure is meant to couple or to store energy. If it couples, the design is driven by field geometry and gap tolerance, and the copper is a means to that end. If it stores energy, the design is driven by inductance value, saturation and loss, and the planar structure is one implementation among several.

Where an integrated structure cannot meet the requirement, a discrete component is the right answer, and the decision should be made early because it changes the board outline and the assembly process. Combining both approaches on one board is possible, but the planar windings have to be placed so that they do not couple into each other, which usually means physical separation and attention to orientation.

FAQ

Can a planar inductor replace any wound component? Not any. The inductance achievable in a small area is limited, and the resistance is higher, so it suits moderate values at moderate current rather than high inductance or high current designs.

Why does adding a ground plane reduce the inductance? The plane carries an image current that opposes the field, which lowers the inductance and increases the loss. Keeping copper out of the coil area avoids it.

Does the coil need a core? A coupling coil often works without one, while an inductor for a converter usually needs a core to reach a useful inductance without occupying a large area.