Inductors in Switch-Mode Power Supplies (SMPS)
The inductor is one of the most important components in a Switch-Mode Power Supply (SMPS). It stores energy while the switching transistor is ON and releases that energy when the transistor turns OFF. This continuous charging and discharging of the magnetic field allows an SMPS to convert electrical energy efficiently with very little power loss. Almost every modern electronic device—including computers, televisions, mobile phone chargers, LED drivers and industrial equipment—contains one or more inductors inside its power supply.
Role of the Inductor
In an SMPS, the inductor performs several important functions:
- Stores energy in its magnetic field.
- Smooths the output current.
- Reduces output voltage ripple.
- Transfers energy from the input to the output.
- Improves power conversion efficiency.
Basic Operating Principle
When the switching transistor (usually a MOSFET) turns ON, current flows through the inductor and its magnetic field builds up.
When the MOSFET turns OFF, the magnetic field collapses and the stored energy continues supplying current to the load through the output circuit.
This process repeats thousands or even millions of times per second.
Inductor During One Switching Cycle
| Switch State | Inductor Behaviour |
|---|---|
| MOSFET ON | Current increases and magnetic energy is stored. |
| MOSFET OFF | Stored energy is released to the load. |
| Next Cycle | The process repeats continuously. |
SMPS Converter Types
| Converter | Inductor Function |
|---|---|
| Buck Converter | Stores energy while reducing output voltage. |
| Boost Converter | Stores energy to increase output voltage. |
| Buck-Boost Converter | Can increase or decrease output voltage. |
| SEPIC Converter | Provides non-inverting voltage conversion. |
| Flyback Converter | Uses transformer magnetising inductance for energy storage. |
Important Inductor Specifications
| Specification | Importance |
|---|---|
| Inductance | Determines ripple current. |
| Current Rating | Must safely carry the operating current. |
| Saturation Current | Must exceed the peak current. |
| DC Resistance (DCR) | Lower DCR improves efficiency. |
| Core Material | Affects efficiency and saturation characteristics. |
| Shielding | Reduces electromagnetic interference. |
Typical Applications
| Equipment | Purpose |
|---|---|
| Phone Chargers | Voltage conversion. |
| Laptop Power Adapters | High-efficiency DC conversion. |
| ATX Computer Power Supplies | Output filtering and energy storage. |
| LED Drivers | Current regulation. |
| Solar Power Systems | Power conversion. |
| Electric Vehicles | High-power DC conversion. |
Selecting an SMPS Inductor
- Select the correct inductance recommended by the circuit design.
- Choose a current rating higher than the maximum operating current.
- Ensure the saturation current exceeds the peak inductor current.
- Use a low DCR to reduce copper losses.
- Consider shielded inductors where EMI is important.
- Follow the manufacturer's datasheet recommendations.
Common Faults
| Fault | Possible Cause |
|---|---|
| Overheating | Excessive current or high DCR. |
| Saturation | Undersized inductor. |
| Audible Noise | Loose winding or core vibration. |
| Poor Regulation | Incorrect inductance value. |
| Open Circuit | Broken winding. |
| Cracked Ferrite Core | Mechanical damage. |
Testing an SMPS Inductor
- Measure inductance using an LCR meter.
- Measure DCR with a multimeter or Kelvin meter.
- Inspect the core for cracks.
- Check for overheating or discolouration.
- Monitor ripple current using an oscilloscope.
- Compare measurements with the manufacturer's specifications.
Interesting Facts
- Modern SMPS inductors operate at switching frequencies ranging from tens of kilohertz to several megahertz.
- Lower DCR generally improves power supply efficiency.
- Shielded inductors reduce electromagnetic interference (EMI).
- Many motherboard voltage regulators use large moulded power inductors capable of carrying tens of amperes.
- Inductors in SMPS circuits repeatedly store and release energy during every switching cycle.
Key Points
- The inductor is the main energy-storage component in most SMPS circuits.
- It smooths current and reduces output ripple.
- Correct inductance, current rating and saturation current are essential.
- Low DCR improves efficiency.
- Proper inductor selection is critical for reliable SMPS operation.