Electrical – How to wind an inductor

An inductor fabricated by etching a spiral in a PCB works fine. This has occasionally been used in tight budget high current applications, where EMI radiating from the "coil" is not a major concern, and nor is the precise value of the inductance.

The calculator mentioned by OP in comments is optimistic compared to results obtained in practice.

Yes, 50A in pulses with a sufficient cool-off period between them, on 2 oz quarter inch traces will work fine. If 4 oz copper is an option, that works better, both due to lower resistance and higher thermal capacity.

• One risk of such a design is delamination / gassing of layers in cheaper boards and thinner copper
• Expect perhaps one third to two-third of the inductance that calculator shows, going by actual results I've seen for a 20 Amp 100 microHenry planar PCB inductor
• Coupling with nearby metal is minimal - note that the "coil" is a planar inductor, thus the magnetic lines of force are highly distorted and flattened compared to a circular wire coil profile
• The addition of a squat, rivet-shaped core of suitable material can be very beneficial, especially if the maximum diameter of the spiral can be kept relatively small. As outer diameter increases, efficacy of the core drops drastically.

Disclosure: I've never seen that particular calculator before

You have drawn a differential choke not a common-mode choke - a CM choke has dots at the same end and doesn't easily saturate because current flowing sets up cancelling flux thus nearly entirely preventing saturation. The differential choke in your circuit very easily saturates because the fluxes from the two coils are additive.

The CM choke blocks only common mode currents i.e. currents of the same direction trying to enter and pass through the choke - this is a good example of transformer action. For differential currents (as per the signal you want to pass) the windings are in effect anti-phase and have a low impedance: -

impedance = \$j\omega (L - M)\$ where L is the inductance of a winding and M is the mutual inductance between windings. M is usually very close in value to L therefore the net inductance for differential signals is lower than for common mode signals. For common-mode currents, the impedance is \$j\omega (L + M)\$ i.e. significantly more than for differential currents.

I get the feeling that you are expecting a CM choke to deal with high frequency differential currents differently to low frequency currents. This is theoretically not the case but, in reality, because leakage inductances exist (not all the flux is shared between the two coils), there is a tendency for higher frequencies to receive more attenuation.

For more info read this on page 6