Data-Center and AI-Server Power: Why the Main Power Stage Uses Planar and Litz, and Where Triple Insulated Wire Fits

Power supply units (PSUs) for AI servers and data centers keep climbing in power, and design teams are often asked the question in the form "can this triple insulated wire (TIW) be used in an AI server?" That framing lumps an entire PSU together; the honest answer is stage by stage. The main power path (PFC, main LLC, resonant inductor) is the territory of planar transformers (planar) and litz wire (litz), not of any single-strand TIW; triple insulated wire belongs on the small isolation transformers of the auxiliary / standby / bias (aux / standby / bias) and gate-drive stages. This article breaks a PSU into its stages, explains stage by stage which magnetics each one uses and where TIW finally comes into play, and gives power, magnetics, safety and procurement engineers an honest line of comparison; the division of labour among the main-stage magnetics themselves is covered in the companion article "AI-Server Main Power: The Territory of the Main LLC and the Resonant Inductor" in this series.

1. First, break a PSU into stages

The big-picture backdrop at rack level is rising voltage. NVIDIA's technical blog explains that today's AI factory distributes 54 VDC within the rack, and that to support 1 MW-class IT racks an 800 VDC high-voltage DC architecture will be introduced from 2027; this is a rack-level power-distribution trend and does not by itself determine which wire any given transformer uses. What actually decides where a wire lands is the staging inside the PSU.

A server PSU can be roughly divided into two paths, each with different magnetics requirements:

• Front-end AC-DC (PFC) — Function：Power factor correction; regulates the intermediate bus voltage；Typical magnetics：PFC inductor / choke (wound magnetics, mostly heavy-gauge copper or litz wire)；Single-strand TIW category?：No
• Back-end isolated DC-DC (main LLC) — Function：Primary/secondary isolation and regulation; delivers the main output；Typical magnetics：Main power transformer + resonant / magnetizing inductors；Single-strand TIW category?：No (planar + litz, see §2)
• Auxiliary / standby / bias (aux / standby / bias) — Function：The "housekeeping" supply for the control circuitry; comes up before the main output；Typical magnetics：Low-power isolation transformer (wound, e.g. flyback)；Single-strand TIW category?：Yes, a common fit (see §3)
• Gate drive (gate-drive) — Function：Isolated transfer of drive and bias to the power-stage switches；Typical magnetics：Wound gate-drive transformer；Single-strand TIW category?：Yes, a common fit (see §4)

The key point up front: the main power path (PFC, main LLC, resonant inductor) is not the category for DAH JIN's single-strand triple insulated wire; where TIW really belongs is the two kinds of small isolation transformer in the lower half of the table. The stages are taken in turn below.

2. Why the main power stage uses planar and litz

Take, as an example, an Infineon application note for a 12 kW PSU written for AI data centers and servers. Its main power path is two-stage: the front end is an interleaved three-level flying-capacitor totem-pole PFC (providing power factor correction and regulating the intermediate DC bus), and the back end is an isolated DC-DC LLC resonant converter (delivering an isolated, regulated output of about 50 VDC). Designs of this class reach a peak efficiency of 97.8% and a power density above 113 W/in³.

The crux is how the magnetics are built. The note states that the main LLC transformer uses a "hybrid planar" structure: it is built around a planar-magnetics main transformer (primary and secondary implemented as multilayer PCB windings), with Kapton insulation foil between primary and secondary to obtain the withstand voltage and a low parasitic capacitance, while the magnetizing and resonant inductors are wound with litz wire and stacked on top of the planar transformer structure. In other words, the main power stage achieves primary/secondary isolation through the insulation foil between PCB layers and through creepage and clearance design, not by winding in a single-strand triple insulated wire; and the high-frequency resonant / magnetizing inductors use litz wire to keep AC resistance down, which is likewise not the single-strand TIW category.

The reasoning behind this design choice is straightforward: the main power stage has to balance high frequency, high power density, strong heat dissipation and soft switching (ZVS), and the low profile, predictable interlayer parameters and heat dissipation of a planar transformer, plus the low high-frequency loss of litz-wound inductors, line up exactly with those needs. That is why the honest answer to "should the main power stage of an AI server use triple insulated wire" is no; the isolation and windings at that stage follow a different construction. Academic and industry LLC designs for data centers (such as high-density LLC research with integrated planar magnetics and LLC AC-DC reference designs for servers) point consistently to the same planar-plus-litz combination.

3. The first place for triple insulated wire: the auxiliary / standby / bias supply

Apart from the main output, every PSU has a "housekeeping" supply. OCP's solid-state transformer (SST) design specification lists it as a separate component (Auxiliary power) and defines its role: it is the first step at power-on, operates with the highest level of availability, and is independent of the operating status of the main equipment. The standby / bias supply of a server PSU is the same kind of thing: it feeds the controllers, communication and protection circuits before the main output is established.

This supply is low in power, but it still has to achieve safety isolation across primary and secondary (the flyback is a common topology), and what it uses is a small wound isolation transformer. To achieve reinforced insulation on such a small transformer with a single winding wire, triple insulated wire is the usual means, and this is the application level that DAH JIN's four triple insulated wire types (TLW-B, TLW-BB, TLW-B7S, TLW-F) genuinely match.

For this stage, the first-hand basis DAH JIN can offer is a fixed set of certification conditions, not a claim of "already designed into a particular AI server": all four types are listed within the IEC 62368-1 Annex J assessment scope of VDE 40008834 Anlage 200; the specific values for working voltage, peak voltage and test frequency are always per the Approval Document and the VDE certificate; wire diameter 0.10–1.00 mm (TLW-B7S is a 7-strand construction with 0.10–0.30 mm per strand); thermal class 130(B), with TLW-F at Class F (155°C). The design-side practice is to take the actual operating point of your own auxiliary / standby transformer and compare it against the Annex J conditions stated in the Approval Document: within the conditions, the wire is selectable; if any item falls outside, a separate assessment is needed. This article does not back-calculate a transformer working voltage from the 800 VDC rack bus, nor does it claim that DAH JIN triple insulated wire has been designed into any specific data-center power supply. For the boundary between wire thermal class and insulation-system thermal class, see Wire thermal class vs. insulation-system thermal class.

4. The second place: the gate-drive transformer

The switches of SiC / GaN power stages need to be driven with isolation, and drive signals and bias are commonly carried across primary and secondary by a wound gate-drive transformer. Industry product data for gate-drive transformers shows that, to bring such transformers to a reinforced-insulation grade, the corresponding versions are wound with triple insulated wire on the windings to meet the reinforced-insulation requirements of standards such as IEC 61558 and IEC 60601; Texas Instruments (TI) application material likewise treats "the isolated bias supply for isolated gate drivers" as a design topic in its own right. These are third-party application facts, cited to establish that "the gate-drive transformer is a reasonable place for TIW"; the withstand-voltage figures in them belong to those vendors' catalogues and are not DAH JIN's conditions.

Back to DAH JIN itself: when selecting triple insulated wire for the small isolation transformers of the gate-drive / bias stage, the first-hand conditions that can be cited are the same set as in §3: all four types are within the Annex J (IEC 62368-1) assessment scope, working voltage / peak voltage / test frequency per the Approval Document, wire diameter ≤1.00 mm (TLW-B7S strands ≤0.30 mm), 130(B). What Anlage 200 states is a test frequency; this article does not extrapolate it to the actual switching frequency of SiC / GaN.

5. Engineering rule: classify the stage first, then talk about the wire

The one-page conclusion for power and magnetics engineers, in order, is "first place the component in its stage, then discuss whether TIW applies":

1. Main power path (PFC, main LLC, resonant / magnetizing inductors): planar transformer and litz wire; primary/secondary isolation is achieved by the insulation between PCB layers and by spacing design. This stage is not the single-strand triple insulated wire category; there is no need, and no reason, to think of TIW here.
2. Small isolation transformers of the auxiliary / standby / bias and gate-drive stages: these are the wound isolation transformers, and this is where triple insulated wire fits. To select TIW, take the transformer's actual working voltage, peak voltage and test frequency and compare them against the Annex J certification conditions stated in the Approval Document (with the wire diameter within 0.10–1.00 mm and TLW-B7S strands within 0.10–0.30 mm): within the conditions, selectable; if any item is outside, a separate assessment is needed.
3. No back-calculation, no extrapolation: do not back-calculate a transformer operating point from the rack bus voltage (54/800 VDC); the medical Annex L (2 MOOP / 2 MOPP) has its own set of conditions, is tied only to TLW-BB and TLW-B7S, and its values are likewise per the Approval Document. For the first branch of selection (B 130°C vs F 155°C), see the Triple insulated wire selection guide.