Flyback Transformer: Complete OEM Selection Guide, Design Parameters, and Custom Manufacturing
Quick Answer
A flyback transformer is a coupled energy-storage inductor used in isolated DC-DC and AC-DC switching power supplies. Unlike a conventional transformer that transfers energy simultaneously through both windings, a flyback transformer stores energy in the magnetizing inductance during the primary switch ON interval and releases it to the secondary during the switch OFF interval. Selecting and specifying a flyback transformer for OEM production requires defining: operating mode (DCM/CCM), switching frequency, primary magnetizing inductance, turns ratio, isolation voltage, core material, winding construction, and insulation class.
1. What Is a Flyback Transformer? How It Differs from a Conventional Transformer
A flyback transformer is fundamentally a coupled inductor with an intentional air gap in the magnetic circuit. It provides primary-to-secondary voltage transformation and galvanic isolation — but its magnetic behavior is closer to a gapped inductor than to a conventional transformer.
| Parameter | Conventional Transformer | Flyback Transformer |
|---|---|---|
| Energy transfer | Simultaneous: primary delivers, secondary receives | Sequential: primary stores energy, secondary delivers it |
| Core purpose | Transfer energy between windings | Store energy during ON, release during OFF |
| Air gap | Minimal or none | Intentional air gap (required) |
| Magnetizing current | Undesirable; minimized | Fundamental to operation; carries all stored energy |
| Typical power range | Any | Below ~200 W (most common) |
The air gap is critical: it lowers the effective permeability of the magnetic path, allowing the required magnetizing inductance to be set with a practical number of turns, and increases the core's usable energy-storage capability before saturation.
2. Flyback Transformer Operating Modes: DCM, BCM, and CCM
| Characteristic | DCM | BCM | CCM |
|---|---|---|---|
| Magnetizing current reaches zero | Yes — for a finite interval | Yes — at cycle boundary | No |
| Peak current for same power | Highest | Intermediate | Lowest |
| Core demagnetizes fully each cycle | Yes | Approximately yes | No |
| Transformer complexity | Lower | Moderate | Higher (DC bias design) |
| Typical application | < 30 W, simple supplies | LED drivers, PFC | 30–200 W, lower stress |
Specify the operating mode to your transformer OEM before requesting a quotation. A transformer designed for DCM is not directly interchangeable with one optimized for CCM.
3. Key Design Parameters: What to Specify for a Flyback Transformer
3.1 Primary Magnetizing Inductance (L_m)
Magnetizing inductance controls the rate of current rise during the ON interval: ΔI = (V_in × D) / (L_m × f_sw)
- Higher L_m → lower peak current → lower conduction losses → larger core required
- Lower L_m → higher peak current → smaller core possible → higher conduction and switching losses
Specify L_m at the operating frequency, not at 1 kHz. At 100 kHz+, measured inductance can differ from 1 kHz values by 10–30%.
3.2 Turns Ratio (N_p : N_s)
The turns ratio sets the secondary voltage based on the reflected voltage: V_out = (N_s / N_p) × V_reflected, where V_reflected = V_in_min × D_max / (1 − D_max). It also determines peak voltage stress on the primary switch and secondary diode.
3.3 Switching Frequency
| Frequency Range | Core Material | Notes |
|---|---|---|
| 50–100 kHz | Standard MnZn ferrite | Most common; good balance of loss and size |
| 100–300 kHz | High-frequency MnZn ferrite (3C95/3F3-class) | Smaller cores; GaN/SiC topology compatible |
| 300 kHz–1 MHz | High-frequency ferrite or powder iron | Very small size; requires careful loss management |
3.4 Air Gap
The air gap sets the magnetizing inductance: L_m ≈ (μ₀ × N_p² × A_e) / l_g. A larger gap lowers L_m (more turns needed), reduces core loss, but creates fringing field that induces losses in adjacent copper conductors. Never leave the gap specification to chance — specify target L_m ± 10% and let the transformer OEM set the gap accordingly.
3.5 Leakage Inductance
Leakage inductance is the portion of primary inductance not coupling to the secondary. It causes voltage spikes on the primary switch at turn-off, requiring RCD snubbers that dissipate 1–3% of converter power. Target: leakage inductance < 2–3% of magnetizing inductance. Achieved through primary-secondary-primary interleaved winding construction.
3.6 Isolation Voltage
| Safety Standard | Application | Isolation Requirement |
|---|---|---|
| IEC 62368-1 | IT, AV, consumer electronics | 3 kVAC (reinforced), 1 min |
| IEC 60601-1 | Medical equipment (BF/CF) | 4 kVAC (2×MOPP) |
| UL 1012 / UL 508C | Industrial power supplies | Per working voltage class |
| IEC 62477 | Power electronic converters | Per installation category |
4. Core Material and Geometry
| Application | Frequency | Recommended Core Material |
|---|---|---|
| Standard AC-DC adapters, offline supplies | 65–100 kHz | MnZn ferrite (3C90 / PC40 class) |
| High-efficiency adapters, USB-C PD | 100–300 kHz | High-frequency MnZn ferrite (3C95 / PC95 class) |
| GaN-based converters | 300 kHz–1 MHz | Low-loss ferrite or powder iron |
| Industrial DIN-rail supplies | 50–150 kHz | MnZn ferrite — Class H insulation |
| EV on-board charger auxiliary supply | 100–500 kHz | High-frequency ferrite — automotive qualified |
5. Winding Construction
For switching frequencies above 100 kHz, Litz wire is strongly recommended. Skin depth in copper at 100 kHz is approximately 210 μm — a standard 0.5 mm wire uses only its outer shell for current flow, wasting most of its cross-section.
| Construction | Leakage Inductance | Interwinding Capacitance | Best for |
|---|---|---|---|
| Simple P then S | High | Low | Low-cost, non-EMI-critical |
| S–P–S sandwich | Low | Medium | High-efficiency, lower voltage stress |
| Fully interleaved (P–S–P–S) | Very low | Higher | High-power, high-efficiency designs |
A Faraday shield (grounded copper foil between primary and secondary) significantly reduces conducted EMI — essential for converters targeting CISPR 22 / EN 55032 Class B conducted emission limits.
6. Insulation System
| Class | Maximum Temperature | Typical Application |
|---|---|---|
| Class B (130°C) | 130°C | Consumer adapters (low ambient) |
| Class F (155°C) | 155°C | Industrial power supplies, moderate ambient |
| Class H (180°C) | 180°C | Outdoor, automotive, high-ambient industrial |
7. Applications: Where Flyback Transformers Are Used
- Consumer Electronics / USB-C PD Chargers: 65–300 kHz, IEC 62368-1 isolation, maximum efficiency at 25% load (DoE Level VI / EU CoC Tier 2), miniaturized PQ or EFD core
- Industrial Power Supplies: 85–264 VAC or 100–370 VDC input, Class H insulation, IEC 62477 or UL 508C, –40°C to +70°C
- LED Drivers (Isolated): BCM or DCM with constant current output, power factor > 0.9, Class A efficiency ≥ 90% above 25 W (EU 2019/2020)
- EV Auxiliary Supplies: 250–800 VDC input, AEC-Q200 qualification, CISPR 25 EMC, ISO 16750-3 vibration/shock
- Medical Devices: IEC 60601-1 (2×MOPP = 4 kVAC), leakage current < 10 μA (CF) / < 100 μA (BF), reinforced creepage ≥ 8 mm
- Telecom and Networking (PoE): IEC 62368-1, isolation 1.5–3 kV
- IoT and Smart Home: Standby power < 0.5 W (ErP Lot 26 / DoE Level VI), extremely cost-sensitive, ultra-compact
8. Flyback vs. Forward Converter: When to Use Which
| Criterion | Flyback | Forward Converter |
|---|---|---|
| Power range | Best below ~150–200 W | Better above 100 W |
| Multiple output rails | Excellent — transformer secondaries | Possible but adds complexity |
| Component count | Fewer (single active switch) | More (free-wheeling diode, output inductor) |
| Output ripple | Higher (DCM discontinuous output current) | Lower (continuous output inductor current) |
| Efficiency | Good for low-power; optimizable | Often higher at medium-high power |
General rule: Flyback for < 150 W, multiple outputs, cost-sensitive, or space-constrained. Forward converter for > 100 W single-output high-efficiency designs.
9. Frequently Asked Questions
Q: What is the difference between a flyback transformer and a flyback converter?
A: The flyback converter is the complete power supply topology — it includes the MOSFET, control IC, snubber, secondary rectifier, output capacitors, and the flyback transformer. The flyback transformer is the single magnetic component within that converter. When an OEM requests a "flyback transformer," they want the magnetic component only — not the complete converter board.
Q: Why does my flyback transformer have an air gap? Won't that reduce inductance?
A: The air gap is intentional and essential. Without a gap, the ferrite core would saturate at a very low magnetizing current. The gap reduces effective permeability, allowing the transformer to carry higher magnetizing current before saturating. It reduces inductance per turn — but that is compensated by adding turns or selecting a larger core. The gap is what makes the flyback transformer function as an energy-storage element.
Q: What is the reflected voltage in a flyback converter, and why does it matter?
A: Reflected voltage (V_r) is V_out × (N_p / N_s). Peak MOSFET voltage stress = V_in_max + V_r + V_spike. Typical target: V_r = 90–135 V for 230 VAC input designs using a 600 V MOSFET. Setting V_r too high requires a higher-rated, more expensive switch.
Q: How do I reduce the voltage spike caused by leakage inductance?
A: Three approaches: (1) Reduce leakage inductance through interleaved winding construction; (2) Add an RCD snubber — clamps the spike but dissipates 1–3% efficiency as heat; (3) Use active clamp flyback (ACF) topology — recycles leakage energy and enables ZVS, but requires more complex gate drive. The transformer OEM can target a specific leakage inductance by controlling winding geometry.
Q: Can I use the same flyback transformer across 85–264 VAC input?
A: Yes — this is one of the flyback's practical advantages. The transformer must be designed for worst-case: at Vin_min (85 VAC ≈ 115 VDC rectified), duty cycle is maximum and primary peak current is highest. At Vin_max (264 VAC ≈ 375 VDC), duty cycle is minimum — verify the controller can operate at low duty cycle without instability.
Q: What certifications does a flyback transformer need for CE marking?
A: The transformer is evaluated as part of the end-product certification — not independently CE marked. The transformer OEM must provide: isolation test certificate (hi-pot report), creepage/clearance dimensions, insulation material data sheet, and RoHS/REACH declaration. Standards depend on end product: IEC 62368-1 (IT/AV), IEC 60601-1 (medical), IEC 62477 (industrial converters).
Q: What is the minimum information needed for a flyback transformer custom quotation?
A: (1) Input voltage range, (2) output voltage(s) and current(s), (3) switching frequency, (4) operating mode (DCM/BCM/CCM), (5) isolation standard, (6) maximum PCB footprint (L × W × H), (7) target production volume and timeline. An experienced OEM can calculate all other parameters from these inputs.
10. Custom Flyback Transformer Specification Checklist
- Input voltage range confirmed (min and max, AC or DC)
- Output voltage(s) and rated current confirmed
- Switching frequency confirmed (at operating temperature and load)
- Operating mode confirmed: DCM, BCM, or CCM
- Target primary magnetizing inductance (L_m) specified ± tolerance at operating frequency
- Turns ratio calculated and confirmed
- Peak primary current calculated (at Vin_min, full load)
- Leakage inductance target specified (% of L_m or absolute value in μH)
- Isolation standard confirmed: IEC 62368-1 / IEC 60601-1 / UL 508C / other
- Creepage and clearance distances confirmed per working voltage and pollution degree
- Core material: standard MnZn ferrite / high-frequency ferrite / other
- Core shape preference or footprint limit (EE, ETD, PQ, EFD, RM)
- Wire type: standard magnet wire / Litz wire / foil winding
- Insulation class: B (130°C) / F (155°C) / H (180°C)
- Multiple outputs: secondary winding count, voltage, and current for each
- Faraday shield required: Yes / No (needed for Class B EMI targets)
- Operating temperature range confirmed
- Target production volume (prototype / NPI / mass production monthly)
- Certification documentation required: isolation test report / material data sheet / RoHS
About Chengpin Technology — Custom Flyback Transformer OEM Manufacturer
Chengpin Technology specializes in the design and manufacture of custom flyback transformers for OEM customers across consumer electronics, industrial power supplies, medical devices, LED drivers, and EV auxiliary systems. Our engineering team works from your power supply schematic — or from your electrical specifications alone — to deliver a complete custom flyback transformer with full electrical characterization data.
Custom flyback transformer capabilities: Power range 1 W to 300 W | Switching frequency 50 kHz – 1 MHz | Isolation up to 6 kVAC (IEC 62368-1 reinforced; IEC 60601-1 2×MOPP available) | Insulation class B through H | Wire: Litz wire (0.04–0.3 mm strand), foil winding, standard enameled copper | Winding: interleaved, split bobbin, Faraday shield | Certifications: RoHS, REACH; isolation test report included with every prototype | Lead time: engineering prototype 7–14 days; production tooling 4–6 weeks | MOQ: 50 pcs (prototype).
Contact Chengpin Technology with your flyback transformer specification — receive a technical review and sample quotation within 48 hours.
Website: www.jyinductor.com | www.inductor-coil.com
References: IEC 62368-1 | IEC 60601-1 | IEC 60664-1 | IEC 62477-1 | SolidMagnetics Flyback Transformer Design Guide (2026) | Coilcraft Guide to Flyback Transformers | WC Magnetics Flyback Converter Design Guide (2026)