SMD vs Through-Hole Inductors: A Complete Selection Guide for Engineers
Quick Answer
Choosing between SMD and through-hole inductors — and selecting the right core type — depends on your operating frequency, current rating, PCB space, and EMI requirements. This guide covers every parameter that matters.
Inductor selection is one of the most misunderstood steps in power supply and RF circuit design. Pick the wrong type and you face unexpected saturation, excessive heat, or EMI failures that only appear at full load.
1. SMD vs. Through-Hole Inductors: Which to Choose?
| Parameter | SMD (Surface-Mount) | Through-Hole |
|---|---|---|
| PCB footprint | Very small | Larger |
| Current handling | Up to ~100A | Up to several hundred A |
| Vibration resistance | Moderate | Excellent |
| Automated assembly | Yes (pick-and-place) | Requires insertion |
| Cost at volume | Lower | Higher |
| Best for | Consumer electronics, telecom, compact PSU | Industrial, automotive, high-current apps |
Rule of thumb: Choose SMD for 4-layer+ PCB with automated SMT assembly. Choose through-hole for high-vibration environments or very high current (>30A continuous).
2. Core Material: The Most Overlooked Decision
2.1 Ferrite Core
- Frequency range: 100 kHz – 10 MHz and beyond
- DC bias behavior: Sharp saturation — inductance drops rapidly when current exceeds Isat
- Best for: Switching power supplies (SMPS), EMI filters, high-frequency DC-DC converters
Ferrite is the default choice for most modern switching regulators. Always design with ≥20% headroom below rated saturation current.
2.2 Iron Powder Core
- Frequency range: DC – 100 kHz
- DC bias behavior: Soft saturation — inductance rolls off gradually, tolerates current peaks better
- Best for: Power factor correction (PFC) chokes, output filter inductors, boost/buck stages
2.3 Sendust / MPP / High-Flux Composite Cores
- Frequency range: DC – 500 kHz
- Key advantage: Very low core loss at high DC bias
- Best for: Server power supplies, solar inverters, Class D audio, high-efficiency PFC stages
2.4 Air Core
- No saturation: Inductance stays constant regardless of current
- Frequency range: 1 MHz – GHz range
- Best for: RF circuits, antenna matching networks, high-Q resonant circuits
3. Five Parameters You Must Specify Before Ordering
3.1 Inductance Value (L) and Tolerance
Specify inductance at the operating DC bias current, not just the zero-current value. For ferrite cores, inductance at rated current may be 20–50% lower than nominal. Always request the L vs. I curve from your OEM supplier. Typical tolerance: ±10% (K), ±20% (M), ±5% (J).
3.2 Saturation Current (Isat)
Defined as the DC current at which inductance drops by a specified percentage (typically 20–30% from nominal). Your peak current — including startup surges and load transients — must stay below Isat. A 30% safety margin is the minimum recommended practice.
3.3 RMS Current Rating (Irms) and Temperature Rise
The RMS current rating is thermally limited — the current at which the inductor reaches its rated temperature rise (typically 20°C or 40°C above ambient). In high-ripple applications, RMS current includes both DC and AC components. Calculate both carefully.
3.4 DC Resistance (DCR)
DCR determines conduction loss: P = I² × DCR. For efficiency-critical designs (server PSU, battery management), DCR is often more important than inductance tolerance. Request the DCR vs. temperature curve — DCR increases approximately 0.4% per °C.
3.5 Self-Resonant Frequency (SRF)
Above the SRF, the inductor behaves capacitively. For a switching regulator at 1 MHz, the inductor's SRF must be well above 1 MHz — typically 5–10× the switching frequency. For EMI filter inductors, SRF within the CISPR test range (150 kHz – 30 MHz) will amplify rather than attenuate interference.
4. EMI Filter Inductors: Special Considerations
- Common-mode inductance: Determines attenuation of common-mode noise
- Leakage inductance: Acts as differential-mode inductance for dual filtering
- Insertion loss: The key performance metric — request the dB vs. frequency curve
- Creepage and clearance: Required for mains-connected filters per IEC 60335-1 / EN 60938-2
5. Winding Wire Selection
| Wire Type | AC Loss | Frequency Range | Typical Use |
|---|---|---|---|
| Round enameled (single strand) | Increases sharply above 100 kHz | DC – 100 kHz | Low-frequency power inductors |
| Litz wire (multi-strand twisted) | Minimized skin/proximity effect | 10 kHz – 1 MHz | High-frequency inductors, wireless charging |
| Self-bonding wire | Same as enameled, adds rigidity | DC – 200 kHz | Toroidal inductors, air-core coils |
| Flat/foil winding | Very low DCR | DC – 50 kHz | Ultra-high-current inductors |
For wireless charging coils (WPT) and high-frequency resonant inductors, Litz wire is mandatory. Single-strand wire at 100 kHz+ suffers from severe skin effect, dramatically increasing AC resistance and heating.
6. Inductor Selection by Application
| Application | Recommended Core | Key Spec Priority |
|---|---|---|
| Buck/boost converter (100–500 kHz) | Ferrite (shielded SMD) | Isat, DCR, SRF |
| PFC choke (65–130 kHz) | Iron powder or Sendust | Soft saturation, low loss at DC bias |
| EMI filter (CM choke) | Mn-Zn ferrite | Common-mode inductance, insertion loss |
| Wireless charging (Qi, 100–205 kHz) | Ferrite plate + Litz wire | Q factor, AC resistance, coupling coefficient |
| RF matching / resonant circuit | Air core | Q factor, SRF, temperature stability |
| High-current output filter (>30A) | Composite powder core or gapped ferrite | Isat margin, DCR, thermal resistance |
7. Frequently Asked Questions
Q: What is the difference between saturation current (Isat) and rated current (Irms)?
A: Isat is the magnetic saturation limit — inductance drops beyond this current. Irms is the thermal limit — winding temperature rises beyond this current. Your operating current must stay below BOTH limits simultaneously.
Q: How do I choose between a shielded and unshielded SMD inductor?
A: Shielded inductors have a closed magnetic path, dramatically reducing electromagnetic radiation. Use shielded inductors near analog circuits or signal traces. Unshielded inductors are cheaper and handle higher current for the same size, but radiate more.
Q: What inductance value should I use for a buck converter?
A: Use the formula L = (Vin – Vout) × Vout / (Vin × f × ΔIL), where ΔIL is the desired peak-to-peak ripple current (typically 20–40% of Iout). For a 12V-to-5V converter at 500 kHz with 2A output and 30% ripple: L ≈ 9.7 μH (round to 10 μH).
Q: Can I use the same inductor for 50 Hz line frequency and 100 kHz switching frequency?
A: No. Inductors for 50 Hz use laminated silicon steel or nanocrystalline cores. Inductors for 100 kHz switching use ferrite cores. A 50 Hz core at 100 kHz would overheat within seconds due to catastrophic core loss.
Q: What Q factor should I target for RF inductors?
A: Q > 50 is a common minimum for RF matching networks. High-performance designs target Q > 100. For resonant wireless power transfer (WPT), Q > 100 on both coils is needed for efficiency above 85%.
Q: How do I specify a custom inductor for OEM production?
A: Provide: inductance value and tolerance, Isat (at what % drop), Irms (at what temperature rise), maximum DCR, SRF minimum, operating frequency range, physical dimensions, operating temperature range, and applicable safety certifications (UL, CE, AEC-Q200).
8. Summary: Inductor Selection Checklist
- Form factor confirmed: SMD or through-hole based on assembly process and current level
- Core material selected based on frequency, saturation behavior, and loss budget
- Inductance value calculated at operating DC bias (not just zero-current nominal)
- Isat verified with ≥20% margin above peak current (including transients)
- Irms verified against continuous RMS current at maximum ambient temperature
- DCR budgeted for efficiency and thermal targets
- SRF confirmed to be well above operating frequency (minimum 5× recommended)
- Wire type specified (Litz wire for >50 kHz high-current applications)
- Shielded vs. unshielded selected based on EMI sensitivity of surrounding circuits
- Safety certifications confirmed: AEC-Q200, UL, CE, RoHS as required
About Chengpin Technology
Chengpin Technology specializes in custom inductor coils, transformers, and wireless charging coils for industrial, medical, consumer electronics, and automotive applications. Our engineering team supports design-in from specification to sample approval, with full documentation for international certifications.
Custom inductor capabilities: Inductance range 0.1 μH – 100 mH | Current range 0.1A – 200A | Core types: ferrite, iron powder, Sendust, MPP, air core | Wire options: enameled, Litz, self-bonding, foil | Certifications: RoHS, REACH, AEC-Q200.
Contact us for a custom inductor specification review and sample request.
References: IEC 62368-1 | CISPR 32 | IEC 61558-2-6 | Würth Elektronik: Trilogy of Magnetics