Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of the current, and follows any changes in current. The magnetic field that is generated when a current is passed through a conductor, typically a wire coil. Inductance is measured in henrys (H).
The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of the current, and follows any changes in current. From Faraday’s law of induction, any change in magnetic field through a circuit induces an electromotive force (EMF) (voltage) in the conductors, a process known as electromagnetic induction. This induced voltage created by the changing current has the effect of opposing the change in current. This is stated by Lenz’s law, and the voltage is called back EMF.
Inductance is defined as the ratio of the induced voltage to the rate of change of current causing it. It is a proportionality factor that depends on the geometry of circuit conductors and the magnetic permeability of nearby materials. An electronic component designed to add inductance to a circuit is called an inductor. It typically consists of a coil or helix of wire.
The term Inductance was coined by Oliver Heaviside in 1886. It is customary to use the symbol L for inductance, in honor of the physical Heinrich Lenz. In the SI system, the unit of inductance is the henry (H), which is the amount f inductance that causes a voltage of one volt, when the current is changing at a rate of one ampere per second. It is named for Joseph Henry, who discovered inductance independently of Faraday.
Inductance of PCB Trace
It’s often said that one solution to reduce transient ringing in a PCB trace is to simply use wider traces. This is true to an extent; making your traces wider will reduce ringing and crosstalk as it reduces the inductance in a trace. However, changing the trace width also changes the impedance of your trace. While ringing and crosstalk cannot be completely eliminated in a perfectly impedance matched transmission line, they can be reduced to the point where they do not cause overshoot or undershoot in the signal level or put a signal in the receiver’s undefined region. You can figure out the best trace width to use in your PCB board if you reframe your PCB trace width calculation as an optimization problem.
How to Optimize PCB Trace Width
The process of optimizing PCB trace width must meet two objectives:
- Minimized Inductance: If you look at the most accurate equations describing trace impedance and inductance, you’ll find that there is a specific width that will produce minimum inductance for a given impedance value and distance to the trace’s ground plane.
- Sufficient Current Carrying Capacity: The current carrying capacity in a PCB is a function of the trace geometry and the desired temperature rise. This can be determined from the IPC-2152 nomograph.
If you need to do determine the trace width that produces minimum inductance for a given desired impedance value, then you need to start with the right trace impedance equations. Let’s take a look at how to do this for microstrips. Note that the process I’ll present here can be easily adapted to striplines, coplanar waveguides, or any other trace geometry.