The Core Function: What Is an Inductor in a Circuit?
An inductor is a passive component that stores energy in a magnetic field and inherently resists changes in current. In a DC power circuit, it acts as a kinetic flywheel, smoothing out chopped, high-frequency voltage pulses into a steady, continuous DC current. If you are asking what is an inductor in a circuit doing at a fundamental level, it is managing the di/dt (rate of current change) to protect downstream components and shape power delivery.
Think of a heavy water wheel placed inside a pipe. When the water valve (switch) opens, the wheel takes time to spin up, delaying current flow. When the valve closes, the wheel's physical momentum keeps pushing water forward, maintaining flow even when the source is cut off. This momentum is the inductor's magnetic field.
Topology Breakdown: The Buck Converter LC Filter
To understand inductors in practice, we look at the most common application: the output filter of a step-down (buck) switching regulator. We use an LC (Inductor-Capacitor) topology rather than a simple RC (Resistor-Capacitor) filter because an RC filter wastes power as heat in the resistor. An LC filter stores and transfers energy with minimal I²R losses, making it mandatory for switching power supplies delivering more than 100mA.
- Node SW (Switch Node): Connects the high-side MOSFET, freewheeling diode, and Inductor Pin 1. This is a high-dV/dt square wave node swinging between 0V and VIN.
- Node OUT (Output Node): Connects Inductor Pin 2, Output Capacitor (COUT), and the Load. This should be clean, low-ripple DC.
What Breaks at the Extremes?
Understanding failure modes is critical for debugging. Here is what happens when the inductor is pushed to its physical extremes:
- Shorting the Inductor: Node SW connects directly to Node OUT. The output capacitor and load now see raw 500kHz PWM square waves. Result: Massive voltage ripple, blown output capacitors from excessive RMS current, and fried load ICs.
- Opening the Inductor: The current path is broken. When the high-side MOSFET turns off, the inductor's collapsing magnetic field has no path to discharge. The SW node voltage spikes to hundreds of volts (inductive kickback), instantly avalanching and destroying the MOSFET.
Behavior Matrix: How Component Changes Shift Performance
When tuning an LC filter, changing one variable cascades through the entire system. Below is a behavior table showing how component shifts affect the circuit.
| Parameter Changed | Effect on Ripple Current (ΔIL) | Effect on Transient Response | Effect on Resonant Frequency (fr) |
|---|---|---|---|
| Increase Inductance (L) | Decreases ripple | Slower response (limits di/dt) | Lowers resonant frequency |
| Decrease Inductance (L) | Increases ripple | Faster response | Raises resonant frequency |
| Increase Capacitance (C) | No direct change | Reduces output voltage sag | Lowers resonant frequency |
| Increase Load Current | No direct change (in CCM) | Shifts operating point, may enter DCM at light loads | No change (LC dependent) |
Design Walkthrough: Sizing a 10µH Power Inductor
Let us design the inductor for a 5V to 3.3V buck converter delivering 1A maximum load, switching at 500kHz. Our target inductor ripple current (ΔIL) is 30% of the maximum load, which is 0.3A.
Using the standard volt-second balance equation:
L = (Vout × (Vin - Vout)) / (Vin × fsw × ΔIL)
L = (3.3V × 1.7V) / (5V × 500,000Hz × 0.3A)
L = 5.61 / 750,000 = 7.48µH
We select the next standard value up: 10µH. This slightly lowers our ripple current to ~0.22A, which improves efficiency and reduces output voltage ripple.
Real Component Selection: We choose the Bourns SRN6045TA-100M. It is a semi-shielded 10µH SMD inductor rated for 2.2A RMS and 2.7A saturation current (Isat). At roughly $0.80 on Mouser, it offers excellent thermal performance.
Breadboard Testing: Step-by-Step Verification
You do not need a custom PCB to verify inductor behavior. You can test the LC filter response on a breadboard using a 555 timer as a crude PWM generator. According to All About Circuits, visualizing the phase shift between voltage and current is the best way to internalize inductor theory.
- Build the Driver: Wire an NE555 timer in astable mode to output a 50kHz square wave at 5V. Keep the frequency low (50kHz) to minimize breadboard parasitic capacitance issues.
- Wire the Switch: Connect the 555 output (Pin 3) through a 100Ω gate resistor to the gate of an IRFZ44N N-channel MOSFET. Tie the source to ground.
- Install the LC Filter: Connect the inductor (use a through-hole 10µH radial inductor like the Bourns 78R103C) between the MOSFET drain and your output rail. Place a 100µF electrolytic capacitor and a 0.1µF ceramic capacitor in parallel from the output rail to ground.
- Add the Flyback Diode: Place a 1N5819 Schottky diode with the cathode pointing to the MOSFET drain and the anode to ground. Do not skip this, or the inductor will destroy the MOSFET on the first off-cycle.
- Probe and Verify: Connect Oscilloscope Channel 1 to the MOSFET drain (Node SW) and Channel 2 to the output rail (Node OUT). You should see a 5Vpp square wave on CH1 and a flat ~2.5V DC line on CH2 with a few millivolts of triangular ripple.
Frequently Asked Questions
What is an inductor in a circuit doing to AC vs DC signals?
An inductor passes DC signals with almost zero resistance (only the DCR, or DC resistance of the copper wire, which is typically under 0.1Ω). For AC signals, the inductor presents an impedance that increases with frequency, defined by the formula XL = 2πfL. This is why inductors are used as chokes in power supplies: they block high-frequency switching noise (AC) while freely passing the desired DC current to the load.
Why does an inductor spark when disconnected?
When current flowing through an inductor is suddenly interrupted (like unplugging a relay coil or opening a switch), the magnetic field collapses rapidly. Faraday's law of induction dictates that this induces a massive voltage spike (V = L × di/dt) to keep the current flowing. If the switch contacts are just opening, this high voltage ionizes the air gap, creating a visible spark or arc. In solid-state circuits, this spike will punch through silicon junctions unless clamped by a flyback diode or snubber network. Electronics Tutorials provides excellent oscilloscope captures of this exact inductive kickback phenomenon.
Can I use a ferrite bead instead of an inductor in a power filter?
No, not for a switching regulator output. A ferrite bead is designed to dissipate high-frequency AC energy as heat (it is essentially a frequency-dependent resistor). If you place a ferrite bead in the main power path of a buck converter, it will overheat, saturate at very low DC currents, and fail to store the energy required to smooth the PWM waveform. Use a proper power inductor with a specified saturation current (Isat) for energy storage, and reserve ferrite beads for low-current signal lines or high-frequency EMI filtering on the input side of your board.
