Managing Magnetic Coupling on a Printed Circuit Board
Let’s look at two types of coupling on a PCB. Capacitive coupling or AC coupling is the phenomena associated with two metal conductors separated by a dielectric. This, of course, is how capacitors work. It is also baked into the function of an RF coupler. The electric field on one element finds its way onto the other element due to their proximity and shared boundary.
What is magnetic coupling?
Magnetic coupling is a different animal. A moving electrical charge will generate a magnetic field as a byproduct. The magnetic fields can be quite powerful though their effect drops off rapidly as the victim moves away from the aggressor.
One of the special powers of magnetic fields is that we can’t shield them in the normal way. The flux lines will pass right through layers of copper and almost anything in their path is liable to be affected. Nickel and iron are the two materials that can help shield the magnetics if that is what is required. Then, you’re fighting induced magnetism with actual magnets. Hmm.
While you don’t have to give them a ton of space, the clearance around an inductor or a transformer should be considered in all three dimensions. If I had an inductor for a switch mode power supply on layer one, I would consider a few layers below the inductor to be no-man’s land, especially for any high-speed signal routing.
So we have an induction field close to the conductor and a radiation field that spreads out further. The electric and magnetic currents flow orthogonal to one another with the magnetic field wrapping around the conductor as it goes along. We can figure out the direction of the flow of the magnetic flux using what the physics teachers call the right-hand-rule.
Image Credit: Mr. Burch’s class notes - Current flows towards the North pole while the magnetic field propagates orthogonally to the axis of current flow.
At least one application of capacitive coupling is built into phones and other devices. The touch screen will be based on either a capacitive coupling scheme or a resistive element to capture finger movements. Magnetic coupling, on the other hand, has found its coolest application in wireless charging of those same gadgets.
Getting a Charge Across an Air-gap - A Magnificent Application of Magnetics
The first one, finger tracking, seems quite sensitive while the second seems much more burley. You know some serious coupling is going on when it can recharge a spent battery. It works because there is a rather large coil on both the charging element and another coil on the mobile device. The two devices lock into what is called resonant magnetic coupling.
This frequency alignment allows the power to be scavenged by the receiving unit over a greater distance than typical inductive coupling. The real advance over the days of Nicola Tesla is in efficiency. Resonant transfer works by making a coil ring with an oscillating current. This generates an oscillating magnetic field.
Image Credit: The Royal Institute - Michael Faraday’s lab work, a magnetic field revealed by iron filings on wax paper in the presence of a permanent magnet.
Since the coil is highly resonant any energy placed in the coil dies away over a short period. However, if a second coil is brought near to it, the second coil can pick up most of the energy in much the same way as the secondary coil in a transformer. This focuses the power transfer to the immediate area with a simple circuit.
The thing is that a resonance such as that is not always a good outcome. Improper trace routing can be the genesis of detrimental coupling - noise! Loops in the power and/or ground plane can generate a ringing tone as currents eddy around the slot. Stubs in a transmission line or other reflective geometry can be the source of electro-magnetic interference.
Simple Tips to Deal with Magnetic Coupling
There are a lot of rules baked into PCB design to help diminish the effects of different circuits upon one another. The most relevant is the spacing between elements. Any trace that is put down should consider that so-called B-Field that surrounds the conductor whenever a change in current occurs.
In order to preserve signal integrity, we want to avoid the impedance mis-matches that create obstacles to the current flow. Vias are one of the prime locations for magnetic coupling. Going from a trace into a barrel and back to a trace can be jarring at high data rates.
A little story: Every engineer you work is going to have their own priorities. Some will focus on the power shapes and bypass capacitor placement. Doing well on this will mitigate most other issues. Other EE’s will strive for tighter length matching or ground rails between channels so as to center-cut the timing budget.
I had one gentleman who just couldn’t stand the sight of two vias passing through the same void in the ground plane. The only time that was ok is when the two vias carry signals that form a differential pair. The rest of the time, this was an open invitation for large magnitude signals to mess with their small magnitude cousins.
In order to get the boards out, I went through it layer by layer and separated as many vias as possible. Even with the same-net, we wanted smaller groups to reduce the openings on other copper pour in the area. Reducing the overall load of magnetic interference takes one small act after another. Being aware of the physics of a high-speed bus propagating across a PCB brings you one step closer to successful design work.
Magnetics is nothing new. The pioneers, Faraday, Tesla, Maxwell and others have been studying this natural phenomena for quite some time. Yet, we are still exploring, still learning and still developing new products that put physics to work for the common good.