How to Specify the Fine-Wire Secondary of a Fast-Charger (GaN / USB-PD) Transformer

GaN and USB-PD have pushed AC-DC transformers into the region of "higher frequency, smaller volume, higher power density", and the rules for the secondary winding have changed with them. This article is written for the design and sourcing engineers of fast-charger transformers: why the secondary moves towards fine wire and multiple strands, when a TIW with reinforced insulation is the right choice, and which values in the selection are correctly stated as "per the Approval Document".

Why GaN / USB-PD push the transformer towards high frequency and fine wire

GaN devices have lower switching losses, which lets the flyback topologies commonly used in fast chargers (mostly QR quasi-resonant flyback, or active clamp flyback) raise the switching frequency. At a higher frequency, both the inductance and the core volume needed for the same power shrink — this is the main lever by which GaN / USB-PD fast chargers reach high power density; in practice the frequency can reach the hundreds of kHz to 1 MHz range.

But once the frequency goes up, the price is paid in the winding: AC copper loss at high frequency (skin effect and proximity effect) becomes one of the main losses. The skin effect concentrates the current at the outer edge of the conductor, so the effective cross-section shrinks and the high-frequency resistance rises; the skin depth of copper decreases as the frequency rises, on the order of 0.21 mm at 100 kHz and about 66 µm at 1 MHz. In other words, at fast-charger operating frequencies a large part of the cross-section of a thick single-strand wire is simply "not conducting".

Why the secondary moves to fine wire and multiple strands

The fast-charger secondary is low-voltage, high-current, and operates at high frequency. With a thick single-strand wire, the skin and proximity effects make the high-frequency resistance far higher than the DC resistance, and both heating and efficiency suffer. The common industry practice is to switch to multiple fine strands wound in parallel or to stranded wire (including the litz concept), so that each strand's diameter is smaller than one skin depth and the current is shared across more strands, thereby holding down the high-frequency AC copper loss; a common rule of thumb is to keep the single-strand diameter below about twice the skin depth.

Note that the proximity effect cannot be solved by reducing the wire diameter alone; it also has to be addressed through winding layering and the winding arrangement — that is implementation know-how, and we leave it at that. For the sourcing side, the point is that the secondary needs a wire that "can be made in fine diameters and can be supplied in multi-strand / stranded form".

Reinforced insulation and high power density: the margin-free advantage of TIW

Primary-to-secondary in a fast charger requires reinforced insulation. The traditional approach relies on sandwich winding plus margin tape on both sides to build up the creepage and clearance distances, but the margins eat into the winding window and pull against the goal of "miniaturisation and high power density".

Triple insulated wire (TIW) builds the reinforced insulation into the conductor itself. Under the component-level evaluation of IEC 62368-1 Annex J, with TIW the winding-to-winding creepage and clearance are greatly simplified and are no longer the bottleneck of the winding window; the margins can be dropped and the window fully wound — particularly advantageous for the small, high-power-density designs of fast chargers. One reminder: the core is treated as a conductive part, so creepage and clearance from the winding to the core and to the terminals must still be kept per the safety standard; not everything goes to zero. The actual evaluation conditions and wire diameter ranges of each model are always per the Approval Document; the details of the standard are not expanded here.

For the differences between TIW, ordinary enamelled wire and FIW, and the relationship between thermal class and the overall insulation system, see /news/tiw-vs-fiw-vs-enamelled-wire and /news/wire-vs-system-thermal-class.

How the DAH JIN TLW series maps to the fast-charger fine-wire secondary

DAH JIN is a manufacturer of triple insulated wire (not a trading company), with its own factory in Guiren, Tainan, Taiwan — made in Taiwan. For the fast-charger secondary's requirement of "fine wire + multiple strands + reinforced insulation", several characteristics of the TLW series apply directly:

• The TLW series (TLW-B / TLW-BB) covers a continuous wire diameter range from fine to thick, including the fine-wire segment that fast chargers need.
• TLW-B7S is a 7-strand stranded form, matching exactly the practice of sharing current across multiple strands / stranded wire at high frequency to hold down AC copper loss; it is a component of the DJ-130 insulation system (System Class 130(B)).
• TLW-F is Class F (155 °C) thermal class, evaluated as Reinforced under VDE 40008834 (IEC 62368-1 Annex J), suited to small, high-power-density designs with tight thermal margins.
• TLW-BB and TLW-B7S provide reinforced insulation as TIW, supporting compact, margin-free, full-window winding.

The table below maps common fast-charger secondary requirements to the DAH JIN models that can correspond to them; specific values are always per the Approval Document.

• Multi-strand / stranded current sharing at high frequency — Approach：7-strand stranded wire；DAH JIN models：TLW-B7S；Specification：Single-strand diameter and strand count per the Approval Document
• Reinforced insulation, margin-free full window — Approach：TIW reinforced insulation；DAH JIN models：TLW-BB / TLW-B7S / TLW-F；Specification：Evaluation conditions per the Approval Document
• Tighter thermal margin — Approach：Class F (155 °C)；DAH JIN models：TLW-F；Specification：Thermal class and wire diameter per the Approval Document
• Fine-wire segment — Approach：Fine wire diameter；DAH JIN models：TLW series；Specification：Wire diameter per the Approval Document

For the selection steps (how wire diameter, strand count, thermal class and insulation system are decided together) see /news/tiw-selection-guide; for the mapping between thermal class and system insulation see /news/wire-vs-system-thermal-class. For the actual wire diameter, strand count and insulation grade of a fast-charger secondary, refer to the Approval Document or contact our sales team.