Designing a USB Type‑C Flyback Planar Transformer with Frenetic’s Planar Tool

Planar magnetics are becoming a key enabler for compact, high‑efficiency USB Type‑C power adapters and embedded power stages. This article walks through a complete 100 W USB Type‑C flyback transformer design using Frenetic’s Planar Tool, highlighting what design engineers and component buyers should take away for their own planar transformer projects.

The focus is not only on how to reproduce the demonstrated design, but also on how to interpret core choices, layer stacks, gaps, and parasitics in a way that makes design decisions more robust and procurement discussions more concrete.

Key features and benefits

The Frenetic showcased design is a 100 W USB Type‑C flyback transformer implemented as a planar magnetic on an ER41 core with a hybrid PCB and copper‑stamp concept. The objective is to combine good volumetric efficiency with controlled losses and predictable parasitic elements for accurate simulation.

Main characteristics of the example transformer:

• 100 W flyback transformer for USB Type‑C power stage
• ER41 ferrite core, standard geometry, with customized gap
• Ferrite material: Ferroxcube 3C95 (power ferrite for high‑frequency operation)
• Single air gap of 0.2 mm to set the primary inductance
• Primary winding: 21 turns, implemented as two parallels across six copper layers
• Secondary winding: 3 turns, one turn per layer, one parallel
• Primary copper thickness: approximately 1 oz copper in each layer
• Secondary copper thickness: approximately 14 oz equivalent (implemented via hybrid approach to remain practical and cost‑effective)
• Simulated total loss around 1.6 W at the evaluated operating condition
• Leakage inductance with respect to the primary around 2.7 µH
• Lumped primary capacitance around 85 pF, leading to a relatively high self‑resonant frequency for this power level
• LTspice model export including frequency‑dependent parasitic elements

From a power‑density perspective, using an ER41 planar core with multi‑layer copper allows the magnetic structure to be integrated with the PCB while still keeping losses and temperatures manageable, assuming proper thermal design of the power stage.

Practical benefits for engineers and purchasers

• Predictable manufacturing: Using a standardized ER41 geometry with well‑defined PCB copper patterns makes it easier to communicate requirements to PCB suppliers and magnetics manufacturers.
• Lower profile: A planar transformer can help meet height constraints in slim adapters and low‑profile embedded power supplies.
• Controlled parasitics: Simulating and tuning leakage inductance, capacitance and self‑resonant frequency in the tool reduces the number of lab iterations.
• Direct simulation models: The LTspice export shortens the path from magnetics design to full converter verification.
• Hybrid construction flexibility: Splitting the structure into two multi‑layer PCBs and copper stamps lets purchasing balance cost, manufacturability and current capability.

Typical applications

The demonstrated 100 W USB Type‑C flyback transformer is representative of a broader class of medium‑power offline and DC‑DC applications where planar magnetics provide clear advantages.

Typical end uses include:

• USB Type‑C power adapters in the 60–120 W segment
• Embedded USB‑PD power stages on main boards (notebook motherboards, docking stations, all‑in‑one PCs)
• Auxiliary supplies and system power rails in industrial and telecom equipment where board height is limited
• Compact chargers, adapters and power modules for consumer electronics where efficiency and form factor are critical
• Isolated DC‑DC stages in EV or industrial systems where PCB‑integrated transformers simplify assembly

In many of these applications, the transformer is a cost‑ and risk‑driving component; being able to rapidly evaluate trade‑offs in core selection, layer arrangement and parasitics is essential for both design and sourcing decisions.

Technical highlights

This section focuses on the key technical steps in the demonstrated design: core and material choice, gap and inductance setting, winding configuration, and parasitic evaluation.

Core and material selection

The design starts with the choice of an ER41 core, a relatively large E‑core format suitable for 100 W‑class isolated power stages.

Key points in the example:

• Core geometry: ER41, standard dimensions as provided by the manufacturer.
• Capability to customize: The tool allows dimension customization if needed for special mechanical or electrical constraints.
• Ferrite material: Ferroxcube 3C95, a widely used power ferrite optimized for high‑frequency operation with relatively low core losses in the target frequency range.

From a design‑in perspective, choosing a standard ER41 geometry and a mainstream ferrite material makes second‑sourcing easier, because several manufacturers provide functional equivalents or similar performance classes.

Gap configuration and primary inductance

An air gap is introduced to set the desired primary inductance and to avoid core saturation at high peak currents.

In the example:

• A single 0.2 mm gap is used in the core.
• The gap value is tuned so that the resulting primary inductance matches the requirements of the USB Type‑C flyback design specification.
• The tool directly links gap definition to the resulting inductance and magnetizing current, which is critical for ensuring proper current‑mode control behavior and limiting peak flux.

For engineers, this means that iterating on gap and inductance can be done digitally before committing to hardware, reducing the risk of hitting saturation under worst‑case input voltage or load.

Winding area and layer arrangement

The planar transformer relies on defined copper areas on the PCB to form the primary and secondary windings.

In the demonstrated setup:

• The winding window is defined relative to the core and the gap position, including distances to the top and bottom and to the lateral edges.
• Two windings are used: primary and secondary.
• Primary winding: 21 turns total, split into two parallels, distributed across six copper layers (three layers per parallel).
• To reach 21 turns with six layers, each layer carries seven turns, and the parallels are combined at the pins.
• Primary copper thickness is set to approximately 1 oz per layer, suitable for the 2.5 A‑class primary current in the given operating point.
• Secondary winding: 3 turns total, one turn per layer, one parallel, requiring three copper areas or layers.
• The secondary carries about 18 A at the simulated operating point, driving the need for much higher copper cross‑section (about 14 oz equivalent), which is not practical as a pure PCB implementation.

Because 14 oz copper is usually not economical or manufacturable in standard PCB processes, the design assumes a hybrid implementation combining conventional PCBs with copper stamps for the high‑current secondary.

Hybrid planar implementation

The video suggests a hybrid implementation to make the design realistic and cost‑effective.

Conceptually:

• The transformer can be built as two four‑layer PCBs sandwiched together, providing enough layers for the primary turns and necessary insulation.
• The secondary uses copper stamps or busbars instead of extremely thick PCB copper, reducing cost and widening the choice of PCB suppliers.
• All elements are sandwiched into the core window, maintaining the planar form factor.

For purchasing teams, this hybrid approach is important: instead of requiring exotic PCB processes, you can combine standard PCB services with stamped copper parts sourced from metal fabricators.

High‑voltage clearances and layer spacing

A key advantage of a dedicated planar design environment is the ability to control clearances and creepage distances.

In the example:

• Distances between the copper areas and the core/window edges are explicitly defined for high‑voltage clearances on the primary side, which is connected to the rectified mains.
• Layer‑to‑layer spacing is adjustable, helping to meet insulation, creepage and clearance norms for isolation.
• The tool allows reordering of layers to implement interleaving while maintaining required distances.

For safety‑critical or mains‑connected designs, these constraints must later be cross‑checked against the relevant insulation standards and manufacturer datasheet recommendations for the chosen core and PCB materials.

Operating conditions and harmonic‑based loss simulation

The example imports a CSV file containing the operating conditions for the primary and secondary currents.

Key data and steps:

• Primary current at the inspected operating point is about 2.5 A.
• Secondary current is about 18 A during conduction intervals.
• The tool supports harmonic‑based analysis of the current waveforms, taking up to 10 harmonics (maximum setting in the example) into account.
• The engineer can reduce the number of harmonics to speed up intermediate simulations and increase it for final, more accurate runs.

Using harmonic‑based loss calculation is particularly relevant in flyback designs where currents are far from sinusoidal; properly accounting for high‑frequency components helps avoid underestimating copper and core losses.

Losses, leakage inductance and parasitic capacitance

After running the simulation, the tool presents a view of core and winding losses as well as key parasitic parameters.

In the example:

• Total simulated losses (core plus windings) are around 1.6 W at the evaluated operating point.
• Leakage inductance referred to the primary is approximately 2.7 µH.
• Effective lumped capacitance of the transformer referred to the primary is around 85 pF.
• Self‑resonant frequency is derived from the inductance and capacitance and is kept sufficiently high for the target switching range.
• The difference between primary inductance and magnetizing inductance corresponds directly to the leakage inductance.

For a 100 W flyback, leakage inductance of a few microhenries can be acceptable or even beneficial for snubber design and clamp circuits, but must be kept under control to avoid excessive voltage spikes and losses. Likewise, primary capacitance in the order of tens of picofarads helps maintain a high self‑resonant frequency, limiting adverse resonances with switching edges.

LTspice model export

One of the most practical features highlighted in the video is the ability to export a complete transformer model for LTspice.

The demonstrated workflow:

• Generate an LTspice model file directly from the Frenetic Planar Tool.
• Assign a meaningful name, for example $\text{flyback\_USB\_100W}$flyback_USB_100W.
• Open the exported text file in LTspice and use the “Create Symbol” function.
• LTspice automatically creates a symbol linked to the model, which includes parasitic elements extracted from the simulation (leakage inductance, magnetizing inductance, capacitances and frequency‑dependent losses).

This workflow closes the loop between magnetics design and converter simulation, enabling engineers to validate soft‑switching conditions, snubber behavior, loop stability and efficiency with a magnetics model that reflects the planned physical implementation.

Design‑in notes for engineers

This section summarizes practical guidance for engineers who want to adapt the demonstrated approach to their own designs.

Selecting core size and material

• Start from system power and topology: for a flyback at around 100 W, an ER41‑class core is a realistic starting point, but for lower power levels an ER28 or ER35 might be sufficient.
• Choose a ferrite material with proven loss performance in your switching frequency range and temperature range; 3C95 is a typical choice for mid‑frequency, mid‑power designs, but refer to the manufacturer datasheet for exact loss curves.
• If the design shows excessive core losses in simulation, consider a larger core or a material with lower loss at the operating flux density.

Defining the gap and inductance

• Use the simulator to iterate on the air gap until the primary inductance meets the converter design target, ensuring that the peak flux remains within the material limits across line and load variations.
• Remember that the primary inductance sets both the current ripple and the magnetizing current in a flyback; verify that the resulting $I_{\text{peak}}$Ipeak​ is compatible with your switch and current‑sense components.
• Once the gap is fixed, simulate worst‑case temperature and frequency combinations using the ferrite loss curves from the manufacturer datasheet.

Layer arrangement and interleaving

• Plan the primary and secondary layers to reduce leakage inductance while satisfying insulation requirements. Interleaving primary and secondary layers (for example P‑S‑P‑S…) can significantly reduce leakage.
• In the example, interleaving is chosen to balance leakage and capacitance; too much interleaving can increase capacitance and EMI, while too little increases voltage spikes.
• Use the tool’s layer‑reordering capability to quickly explore configurations; check how each arrangement impacts leakage and capacitance.

Handling high currents in planar secondaries

• For high‑current secondary windings, thick PCB copper quickly becomes impractical; consider copper stamps, busbars, or parallel PCB traces in multiple layers.
• A hybrid solution with two four‑layer PCBs and separate copper stamps, as shown in the example, is often a good compromise between manufacturability and current handling.
• Coordinate early with PCB and metal suppliers on achievable copper thicknesses, tolerances and stacking options, and reflect these constraints in your layer definitions.

Clearances, creepage and insulation

• Explicitly define distances from copper to core and from traces to the edge of the window, especially for primary circuits tied to the rectified mains.
• Use layer‑to‑layer spacing, insulating prepregs and dedicated clearance slots to meet the required isolation category for your application (for example reinforced insulation according to the relevant safety standard).
• After configuring the geometry in the tool, cross‑check key distances against system safety standards and the transformer manufacturer’s design rules.

Simulation‑driven optimization

• Import realistic current waveforms (or harmonics) via CSV from your power stage calculations or preliminary simulations.
• Start with a reduced number of harmonics to speed up iterations during early design, then run full 10‑harmonic simulations for final verification.
• Evaluate total losses, core temperature rise and winding temperatures using your system cooling assumptions; if necessary, adjust core size, copper distribution or interleaving to redistribute losses.

Using the LTspice model in the converter design

• Insert the exported transformer model into your flyback schematic instead of using simplified ideal inductors.
• Verify switch voltage stress, clamp circuit operation, and EMI filter interaction with the more realistic parasitic behavior.
• Perform sensitivity analyses by varying leakage inductance and capacitance within realistic tolerances to understand worst‑case behavior before freezing the layout and BOM.

Considerations for purchasing and sourcing

• Keep as many parameters as possible tied to standard materials and processes (standard core families, widely available ferrite grades, PCB copper thicknesses that mainstream suppliers support).
• Define, in documentation, which elements are mandatory (core type, material, gap, inductance) and which are negotiable (exact PCB stack‑up, copper stamp manufacturing route) to give suppliers some flexibility.
• Ask potential suppliers whether they can work directly with exported geometry or Gerber data from tools like Frenetic’s Planar Tool, which can reduce interpretation errors.

Conclusion

By following the workflow demonstrated with the Frenetic Planar Tool, design engineers can move from a high‑level USB Type‑C flyback specification to a manufacturable planar transformer design with quantified losses and parasitics. The ER41‑based 100 W example highlights how core selection, gap definition, layer arrangement and hybrid PCB‑plus‑stamp construction come together to deliver a low‑profile transformer that remains practical for production.

With harmonic‑based simulations and direct LTspice model export, it becomes much easier to integrate realistic magnetics behavior into converter‑level simulations, reducing the risk of late‑stage surprises related to leakage, capacitance or thermal performance. Engineers and procurement specialists can use this approach as a template for future planar magnetics in adaptors, embedded supplies and other compact power designs, while still relying on exact values “according to manufacturer datasheet” when specifying ratings and safety parameters.

Source

The information in this article is based on the USB Type‑C flyback planar transformer design example and workflow demonstration from Frenetic’s Planar Tool, as presented in the referenced video, together with the manufacturer’s datasheet information for core and material characteristics.

References

1. Designing a USB Type‑C Flyback Planar with Frenetic – YouTube
2. Frenetic YouTube channel