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2025/05/29
Switching power supplies, as the core power supply units of modern electronic devices, largely depend on the design and selection of inductors for their performance. Inductors play crucial roles in energy storage, current smoothing, and voltage conversion in switching power supplies, serving as core components for achieving efficient energy conversion.
In the operation of switching power supplies, the primary function of inductors is to store and transfer energy. When the switching transistor is conducting, the inductor stores energy from the input power source; when the switching transistor is turned off, the inductor releases the stored energy to the load. This periodic energy storage and release achieves voltage step-up and step-down conversion.
The magnetic field energy stored in an inductor can be expressed by the following formula:
W = (1/2)LI²
Where:
This formula indicates that the energy stored in an inductor is proportional to the inductance value and the square of the current. In switching power supply design, by controlling the conduction and turn-off time of the switching transistor, the charging and discharging process of the inductor can be precisely controlled, thereby achieving stable output voltage.
In a typical Buck step-down circuit, the energy transfer process is divided into two phases:
In different topologies of switching power supplies, inductors achieve different voltage conversion functions:
Another important function of inductors is maintaining current continuity and smoothness. Due to the characteristic of inductors to resist current changes, they can effectively suppress current pulsations generated by switching actions, providing relatively smooth current to the load.
In continuous conduction mode, the inductor current remains positive throughout the entire switching cycle and does not drop to zero. This operating mode has the following characteristics:
Where D is the duty cycle and T is the switching period.
Under light load conditions, the inductor current may drop to zero within the switching cycle, entering discontinuous conduction mode:
Inductors also undertake important filtering functions in switching power supplies, suppressing high-frequency noise and electromagnetic interference (EMI) generated by switching actions.
Inductors and output capacitors form LC low-pass filters, effectively suppressing switching frequency and its harmonic components:
Properly designed inductors can effectively suppress conducted and radiated EMI:
To better understand the application of inductors in switching power supplies, the following analyzes the inductor selection process in high-frequency switching power supplies through specific design cases.
Inductance Value Calculation:
L = (Vin - Vout) × D × T / (2 × ΔI)
L = (12 - 5) × 0.417 × 2×10⁻⁶ / (2 × 0.9)
L = 3.24μH
Standard Value Selection: Select 3.3μH standard value
Based on the above requirements, select shielded power inductor:
Inductance Value Calculation:
L = Vin × D × T / (2 × ΔIin)
L = 3.3 × 0.725 × 1×10⁻⁶ / (2 × 0.86)
L = 1.39μH
Standard Value Selection: Select 1.5μH standard value
Special considerations for Boost circuits include:
Continuous Conduction Boundary:
Lcrit = Vin × (Vout - Vin) × T / (2 × Vout × Iout_min)
CCM must be maintained even at 10% load
Input Capacitor Requirements:
Due to large input current pulsation, sufficient input capacitance is needed
Output Diode Selection:
Fast recovery diodes or synchronous rectification are required
Through these case analyses, it can be seen that inductor selection requires comprehensive consideration of circuit topology, performance requirements, cost constraints, and other factors, making it a systematic engineering decision process.
Inductor saturation is a critical issue that must be addressed in switching power supply design. When the operating current of an inductor exceeds its saturation current, the inductance value drops dramatically, severely affecting the performance and reliability of the power supply.
Inductor saturation is an inherent characteristic of magnetic materials. When the magnetic field strength exceeds the saturation magnetic field strength of the material, the magnetic permeability drops sharply.
The B-H curve (magnetization curve) of magnetic materials describes the relationship between magnetic flux density B and magnetic field strength H:
Inductance value is directly related to the permeability of the magnetic core:
L = (μ₀ × μᵣ × N² × A) / l
Where:
When the core saturates, μᵣ drops sharply, causing a significant reduction in inductance value.
Electromagnetic interference (EMI) is an important consideration in switching power supply design. Inductors are both sources of EMI generation and important means of EMI suppression. Proper inductor design and selection are crucial for meeting EMC standards.
EMI in switching power supplies mainly originates from:
EMI propagates through the following paths:
Selecting appropriate inductor types is crucial for EMI suppression:
Shielded Inductors:
Completely enclosed magnetic field, small leakage field
Suitable for EMI-sensitive applications
Relatively high cost
Semi-Shielded Inductors:
Partial magnetic field shielding
Balance between performance and cost
Suitable for general applications
Unshielded Inductors:
Low cost, high performance
Require proper layout to reduce EMI
Suitable for applications with less stringent EMI requirements
Proper filter circuit design is key to EMI suppression:
Input Filters:
Common-mode inductor + differential-mode inductor + capacitor
Suppress conducted EMI
Prevent internal noise from propagating to the power grid
Output Filters:
LC filters suppress switching ripple
Reduce high-frequency noise at the output
Improve EMI environment at the load end
Multi-Stage Filtering:
Multi-stage filtering with different cutoff frequencies
Improve high-frequency attenuation characteristics
Avoid limitations of single-stage filtering
PCB layout has significant impact on EMI suppression:
Inductor Layout:
Keep away from sensitive circuits
Avoid magnetic field coupling
Consider thermal requirements
Ground Plane Design:
Complete ground plane
Shortest return path
Avoid ground plane splits
Trace Design:
Shortest connection paths
Avoid forming loop antennas
Control trace impedance
Shielding techniques are effective methods for suppressing radiated EMI:
Magnetic Shielding:
Use high-permeability materials
Shield low-frequency magnetic fields
Suitable for power frequency and low-frequency applications
Electric Shielding:
Use conductive materials
Shield high-frequency electric fields
Requires good grounding
Electromagnetic Shielding:
Combines magnetic and electric shielding
Full-spectrum EMI suppression
Higher cost but better effectiveness
