The voltage over an inductor is not a static value like the voltage drop across a resistor. It is a dynamic, time-dependent reaction to changing current, governed by Faraday’s law of induction. In practical circuit design, calculating and managing the voltage over inductor windings is the difference between a robust power supply and a destroyed switching MOSFET. Whether you are dealing with the steady-state DC resistance drop or the massive transient flyback spikes generated during switch-off events, selecting the right physical component requires understanding both the theoretical math and the physical limitations of the core material.
The Physics: Calculating Voltage Over Inductor Windings
To understand component selection, you must first quantify the electrical stress the part will endure. The instantaneous voltage over an inductor ($V_L$) is defined by the rate of change of current through it:
$V_L = L \times \frac{di}{dt}$
Where $L$ is inductance in Henries, $di$ is the change in current, and $dt$ is the time interval. In a steady DC state, $di/dt$ is zero, meaning the only voltage over the inductor is the minor resistive drop caused by its DC Resistance (DCR). The real engineering challenge occurs during transients.
Worked Numeric Example
Assume you are designing a buck converter using a 100µH inductor. During the switch-on phase, the current ramps from 0A to 2A in 10µs.
- $L = 100 \times 10^{-6}$ H
- $di = 2$ A
- $dt = 10 \times 10^{-6}$ s
$V_L = 100\mu H \times (2A / 10\mu s) = 20V$.
This 20V is the forward voltage required to drive that specific current ramp. However, when the MOSFET turns off, the current attempts to drop from 2A to 0A in nanoseconds. As $dt$ approaches zero, the induced voltage over the inductor theoretically approaches infinity. In reality, it spikes until it finds a path to discharge—often breaking down the drain-source junction of your switching MOSFET if a flyback diode is absent.
For a deeper mathematical breakdown of inductive time constants and transient responses, refer to the foundational guides at Electronics Tutorials.
Inductor Types and Selection Criteria
Not all inductors handle voltage and current stress equally. The core material dictates the saturation limit, while the winding geometry determines parasitic capacitance and insulation breakdown voltage. Here is how to match the physical construction to your specific job.
| Type / Construction | Typical Tolerance | Tempco (ppm/°C) | Core Saturation Behavior | Typical Use Case |
|---|---|---|---|---|
| Shielded Ferrite (Powdered iron/ferrite in epoxy) | ±20% | +100 to +300 | Hard saturation (abrupt inductance drop) | DC-DC switching regulators, high-current buck/boost converters. |
| Toroidal (Continuous wound ring core) | ±10% to ±20% | +50 to +150 | Soft saturation (gradual roll-off) | Audio crossover networks, EMI filtering, linear power supplies. |
| Air Core (Non-magnetic former) | ±5% to ±10% | Near 0 | Cannot saturate (linear indefinitely) | RF tuning, high-frequency resonant tanks, VHF/UHF transmitters. |
| Multilayer Ceramic (LTCC chip inductor) | ±10% to ±20% | +1000+ (Highly variable) | Hard saturation at very low currents | High-frequency signal filtering, RF impedance matching, low-power data lines. |
Which type for which job? If you are handling high $di/dt$ switching node voltages and need to contain EMI, choose Shielded Ferrite. If you are designing an audio filter where low distortion and soft saturation are critical to prevent harsh clipping, choose Toroidal. If your circuit operates at 100MHz+ and core losses would destroy your Q-factor, choose Air Core.
Decoding Physical Markings and Safe Substitution
When repairing a board or prototyping, you rarely have the exact OEM part in your bin. Understanding how to read the physical part and safely substitute it is a critical bench skill.
Reading the Markings
Through-hole and large SMD inductors typically use a 3-digit code similar to ceramic capacitors, but the base unit is microhenries (µH), not picofarads.
- 100: 10 × 10⁰ = 10µH
- 101: 10 × 10¹ = 100µH
- 472: 47 × 10² = 4700µH (4.7mH)
Smaller SMD chip inductors often use color bands (read exactly like 4-band resistors, but yielding µH) or single-letter codes that require looking up the manufacturer's specific datasheet (e.g., Murata or TDK code tables).
How to Substitute Safely
If the exact part is missing, do not just match the inductance value. You must evaluate three parameters to ensure the substitute can handle the voltage and current stresses:
- Inductance ($L$): Match within ±20%. A slightly higher inductance will reduce ripple current but may slow down transient response.
- Saturation Current ($I_{sat}$): The substitute $I_{sat}$ MUST be equal to or greater than the original. If $I_{sat}$ is too low, the core saturates, inductance collapses to near zero, and the resulting massive current spike will destroy your switching IC.
- RMS Current ($I_{rms}$) & DCR: The substitute must handle the continuous thermal load. Look for a part with an equal or lower DC Resistance (DCR) to prevent overheating.
Failure Modes: When Voltage and Current Exceed Limits
Inductors fail in distinct ways depending on whether the abuse is voltage-driven (insulation breakdown) or current-driven (core saturation and thermal runaway). Recognizing the visual symptoms tells you exactly what went wrong in your circuit.
- Insulation Breakdown (Voltage Stress): If the flyback voltage over the inductor exceeds the dielectric strength of the wire enamel, micro-arcs occur between adjacent windings. Visual Symptom: The component smells like burning ozone or fish, the epoxy coating may show localized black blistering, and measuring DCR will show a lower-than-spec resistance due to shorted turns.
- Core Saturation (Current Stress): When peak current exceeds $I_{sat}$, the magnetic domains in the ferrite align completely. The inductor temporarily becomes a piece of straight wire. Visual Symptom: The inductor itself may look perfectly fine, but the driving MOSFET will be cracked or melted due to the massive current spike. In severe cases, the thermal shock can cause the ferrite core to physically crack along its grain.
- Thermal Runaway (RMS Stress): Continuous current exceeds the $I_{rms}$ rating, heating the copper windings. As copper heats up, its resistance increases, generating more heat. Visual Symptom: The outer heat-shrink or epoxy shell becomes brittle, discolored (brown/yellow), and the solder joints on the PCB pads may melt or reflow.
Frequently Asked Questions
How do you safely measure the voltage over an inductor in a high-frequency switching circuit?
Never use a standard single-ended oscilloscope probe with the ground clip attached to the switching node; this will create a ground loop through the probe's parasitic capacitance and blow up your circuit. Instead, use a high-voltage differential probe across the inductor terminals. Alternatively, measure the voltage at the switching node (relative to ground) and the output voltage, then use the oscilloscope's math function to subtract the two channels, yielding the precise voltage over the inductor.
Why does the voltage over an inductor reverse polarity when the circuit opens?
This is dictated by Lenz’s Law. An inductor stores energy in a magnetic field. When the switch opens and current attempts to stop, the magnetic field collapses. The collapsing field induces an electromotive force (EMF) that opposes the change in current. To keep current flowing in the same direction across the newly opened gap, the inductor must make its 'exit' terminal highly positive relative to its 'entry' terminal, resulting in a massive reverse-polarity voltage spike (flyback).
Does the voltage over an inductor determine its core saturation point?
No. This is a common misconception. Current determines core saturation, not voltage. The voltage over the inductor dictates the rate at which the current changes ($di/dt$). A high voltage applied for a long duration will cause the current to ramp up until it eventually hits the core's saturation limit ($I_{sat}$). Therefore, while voltage drives the process, it is the absolute peak amperage flowing through the windings that physically saturates the magnetic core.
