A PFC (Power Factor Correction) inductor in an active boost converter typically ranges from 100 µH to 2 mH and handles 2A to 20A+ RMS. Its primary job is to store energy during the MOSFET's on-time and release it to the output, forcing the AC input current to track the AC voltage waveform and pushing the power factor above 0.95. If you are replacing or designing one, the core material and saturation current (Isat) dictate survival; getting it wrong guarantees a blown switching MOSFET.
Core Material Comparison: Which Type for Which Job?
The single most critical decision when specifying a PFC inductor is the core material. The inductor must handle a massive DC bias (the rectified AC current) without saturating, while minimizing core losses at high switching frequencies (typically 65 kHz to 150 kHz for modern GaN/Si PFC controllers). Below is a data-dense comparison of the four dominant core technologies used in modern power supplies.
| Core Material | Initial Permeability (µi) | Saturation Flux (Bsat) | Tempco & Losses | Typical Application |
|---|---|---|---|---|
| Sendust (Kool Mµ) | 26 - 125 | ~1.05 T | Near-zero magnetostriction, soft saturation curve, moderate core loss. | 100W - 1kW CCM Boost PFC. The industry standard for cost/performance. |
| MPP (Molypermalloy) | 14 - 550 | ~0.75 T | Lowest core losses of all powder cores, highly stable over temperature. | Aerospace, medical, and high-efficiency (>98%) server power supplies. |
| High Flux | 14 - 160 | ~1.50 T | Highest DC bias capability, higher core losses than MPP/Sendust. | High-current, low-line (85VAC) industrial PFC stages where size must be minimized. |
| Gapped Ferrite (E/EI Core) | 50 - 200 (effective) | ~0.35 T (at gap) | Sharp saturation knee, high fringing flux losses near the gap, lowest material cost. | CrCM (Critical Conduction Mode) PFC, low-cost consumer electronics, high-frequency GaN designs. |
Selection Rule of Thumb: Choose Sendust toroids for continuous conduction mode (CCM) designs between 150W and 800W where acoustic noise (magnetostriction) must be low. Choose gapped ferrite E-cores for transition-mode (CrCM) designs or when automated PCB mounting (bobbin style) is required. For a deeper look at magnetic design trade-offs, refer to the Coilcraft power magnetics design guides.
Decoding Markings and Reading the Datasheet
Unlike small 0805 SMD inductors that use three-digit EIA codes (e.g., '101' for 100 µH), through-hole PFC inductors rarely stamp their electrical values directly on the core. Because they are often custom-wound or highly specialized, physical markings usually fall into two categories:
- Manufacturer Part Numbers: A string like
7443551100(Würth Elektronik) orDO3316P-104(Coilcraft). The physical part will only show this alphanumeric code, a date code, and sometimes a polarity dot indicating the start of the winding. - Custom OEM Codes: In mass-market ATX power supplies, you will often see a manufacturer's internal drawing number (e.g.,
PFC-L300-RevB) printed on a piece of Kapton tape wrapped around the windings.
When you pull the datasheet for that part number, you must differentiate between two critical current ratings. Confusing these is the most common reason a repaired PFC stage blows up on the bench:
- Isat (Saturation Current): The DC current at which the inductance drops by a specified amount (usually 20% or 30%). In a PFC boost converter, if your peak current exceeds Isat, the inductor turns into a low-value resistor, current spikes uncontrollably, and the PFC MOSFET dies instantly.
- Irms (Thermal/RMS Current): The continuous AC+DC RMS current that causes the inductor's temperature to rise by 40°C. Exceeding Irms won't kill the MOSFET immediately, but it will slowly melt the winding enamel over hours of operation.
Safe Substitution: When the Exact Part is Missing
If you are repairing a board and the original PFC inductor is burnt or unavailable, you cannot simply drop in any inductor with the same microhenry rating. Follow this strict substitution hierarchy to ensure safe operation:
1. Match the Topology and Conduction Mode
A PFC inductor designed for Critical Conduction Mode (CrCM) operates with zero-current switching and massive peak-to-peak ripple. A CCM inductor operates with a high DC bias and low ripple. Never substitute a small CrCM bobbin inductor into a high-power CCM circuit; it will saturate immediately.
2. The Isat Margin Rule
Your substitute's Isat must be at least 20% higher than the calculated peak input current at the lowest AC line voltage (usually 85VAC). If the original part had an Isat of 12A, your substitute must be 12A or higher. You can safely use a 15A part, provided it physically fits on the PCB.
3. The Gapped vs. Ungapped Trap
Never substitute a gapped core with an ungapped ferrite core. Standard ferrite materials (like PC40 or PC95) saturate at roughly 0.39 Tesla. Without a physical air gap to increase the reluctance of the magnetic circuit, even 1 Ampere of DC bias will saturate a standard toroidal ferrite core. Powdered iron cores (Sendust, MPP) have distributed 'micro-gaps' built into the material, making them inherently safe for high DC bias without a macroscopic air gap.
4. DCR and Thermal Matching
Ensure the DC Resistance (DCR) of the substitute is equal to or lower than the original. A higher DCR will increase I²R copper losses, leading to thermal runaway inside the power supply enclosure. For more on thermal derating, consult Würth Elektronik's magnetics application notes.
Failure Modes and Visual Diagnostics
When a PFC stage fails, the inductor is rarely the root cause—it is usually the victim of a control loop failure, a shorted diode, or a MOSFET fault. However, diagnosing the physical state of the inductor tells you exactly what went wrong.
| Failure Mode | Visual & Olfactory Symptoms | Root Cause & Fix |
|---|---|---|
| Magnetic Saturation | Inductor looks perfectly pristine. No burn marks. However, the PFC MOSFET is shattered or shorted drain-to-source, and the gate resistor is charred. | The control IC lost regulation, or the inductor was undersized for low-line (85VAC) peak currents. Replace MOSFET, verify current sense resistor, and confirm inductor Isat. |
| Thermal Runaway | Darkened, brittle wire enamel. The bobbin plastic may be warped or melted. Strong smell of burning phenolic or varnish. Multimeter shows normal DCR. | Operated above Irms rating, or inadequate airflow. The skin effect at high frequencies may be causing excess AC copper loss. Rewind with Litz wire or upgrade to a larger core. |
| Gap Fringing Overheating | On gapped E-cores, the copper wire directly adjacent to the center leg gap is blackened and melted, while the rest of the winding looks fine. | Fringing flux from the air gap is inducing massive eddy currents in the nearby copper. Fix by moving the gap to the outer legs, or adding a copper shield/foil around the gap. |
| Mechanical Fracture | Visible hairline crack on the ferrite E-core, usually near the center leg or the clamping tab. Audible buzzing or ringing during operation. | Overtightened metal mounting clamp, or severe thermal shock. Replace core, use a compliant silicone pad under the clamp, and verify proper potting. |
Worked Example: Sizing a PFC Inductor for a 300W Supply
Let's walk through the math for sizing a PFC inductor for a 300W continuous conduction mode (CCM) boost converter. This demonstrates why we care so much about the 85VAC low-line condition.
Assumptions:
Output Power (Pout) = 300W
Efficiency (η) = 0.92
Power Factor (PF) = 0.99
Minimum AC Input (Vin_rms) = 85VAC
Switching Frequency (fsw) = 100 kHz
Target Ripple Current (ΔI) = 20% of peak input current
Step 1: Calculate Input Currents
First, find the maximum input power required:
Pin = Pout / η = 300W / 0.92 = 326W
Next, calculate the maximum RMS input current at the lowest line voltage:
Iin_rms = Pin / (Vin_rms × PF) = 326W / (85V × 0.99) = 3.87A
Now, find the peak input current (which occurs at the peak of the AC sine wave):
Iin_peak = Iin_rms × √2 = 3.87A × 1.414 = 5.47A
Step 2: Determine Inductance Value
We want the ripple current (ΔI) to be 20% of the peak current:
ΔI = 0.20 × 5.47A = 1.09A
The worst-case inductance requirement occurs at the peak of the AC line, where the duty cycle (D) is at its minimum. At 85VAC, the peak rectified voltage (Vin_peak) is 120V. Assuming a 390V DC output bus:
D = (Vout - Vin_peak) / Vout = (390V - 120V) / 390V = 0.69
Using the standard boost inductor formula L = (Vin_peak × D) / (fsw × ΔI):
L = (120V × 0.69) / (100,000 Hz × 1.09A) = 759 µH
Step 3: Select the Core and Verify Isat
We need an inductor of roughly 750 µH.
More importantly, the saturation current (Isat) must handle the absolute maximum peak current plus a safety margin.
Isat_min = Iin_peak + (ΔI / 2) = 5.47A + 0.54A = 6.01A
Applying a 20% safety margin for transient loads and component tolerances:
Target Isat = 6.01A × 1.20 = 7.2A
Final Selection: We would specify a Sendust toroidal inductor (or a gapped ferrite E-core) with a nominal inductance of 750 µH, an Irms rating of at least 4.5A, and a hard Isat rating of 8A or higher. If you substitute a 750 µH inductor that only has a 5A Isat rating, the core will saturate at low AC line voltages, the current sense resistor will see a massive voltage spike, and the PFC controller will either trip into overcurrent protection or destroy the MOSFET.
Understanding the interplay between core material, DC bias, and thermal limits transforms the PFC inductor from a mysterious black cylinder into a highly predictable, calculable component. Always verify your substitutions against the Isat margin, and never trust a physical marking without pulling the manufacturer datasheet.
