ESS / Solar Inverters: Triple Insulated Wire Goes in the Auxiliary Power Supply, Not the Main Power Stage

Design teams working on photovoltaic (PV) and energy storage system (ESS) inverters often ask one question: "Can this triple insulated wire (TIW) be used in an inverter?" The question lumps the whole inverter together; the honest answer is stage by stage. The main power path (the DC/DC boost inductor, the DC/AC grid-side filter inductor — and many grid-tied models have no main isolation transformer at all) is wound with large cross-section conductors and litz wire, not with any single-strand triple insulated wire. Where TIW does belong is the small isolated flyback transformer of the auxiliary / bias power supply. This article breaks an inverter down into its stages, explains which magnetic component each stage uses and where TIW finally comes in, and gives power, magnetics, safety and procurement engineers an honest reference line. It gives direction and criteria only; all DAH JIN product values are "per the Approval Document".

1. First, break a PV / ESS inverter into its stages

An inverter's job is to convert between DC and AC: a solar inverter turns the DC from PV panels into grid-synchronised AC; the power conversion system (PCS) of an energy storage system has to be bidirectional, converting battery DC to AC and rectifying AC back to charge the battery. Both applications fall under IEC 62109 for product safety (see §4), and Part 2 explicitly brings inverters that "may be intended for use in conjunction with batteries or other forms of energy storage" into scope.

Take the common solar string inverter as an example. TI's application note on auxiliary power supplies breaks it into several power conversion stages: the first is the DC/DC stage, which generates a stable DC bus voltage from the PV string's DC input; an inverter stage (DC/AC) then converts that DC bus into AC for feeding into the grid. Another TI document on string inverter power topologies likewise divides it into three major power blocks — a unidirectional DC/DC converter stage, a bidirectional DC/DC stage and a bidirectional DC/AC inverter stage — and notes that in cost-sensitive designs the front-end DC/DC is usually a non-isolated boost topology.

Laid out by "where the magnetics sit", an inverter has roughly the two paths below, with completely different magnetic component requirements:

• Front-end DC/DC (boost / MPPT) — Function：Builds a stable DC bus from the PV string or battery；Typical magnetic component：Boost inductor / choke (high current, large core)；Single-strand TIW category?：No (see §2)
• DC/AC inverter stage — Function：Converts the DC bus into grid-synchronised AC；Typical magnetic component：Grid-side filter inductor (typically an LCL filter with two inductors)；Single-strand TIW category?：No (see §2)
• Main isolation transformer (if an isolated topology is used) — Function：Primary/secondary isolation and voltage adjustment；Typical magnetic component：High-power wound or planar transformer；Single-strand TIW category?：No (large cross-section windings, see §2)
• Auxiliary / bias power supply (aux / bias) — Function："Housekeeping" power for control, sensing and gate drive；Typical magnetic component：Small isolated flyback transformer；Single-strand TIW category?：Yes, the usual location (see §3)

The key point first: the main power path (boost inductor, LCL filter inductors, main isolation transformer) is not the product category for DAH JIN's single-strand triple insulated wire; the real home of TIW is the small isolation transformer in the last row. The following sections take each stage in turn.

2. Why the main power stage is no place for single-strand TIW

Two facts about the main power stage put it out of range of single-strand triple insulated wire from the outset.

First, many grid-tied inverters have no main isolation transformer at all. Single-phase and three-phase transformerless string inverters are common industry practice, and the literature states plainly that these topologies "have no galvanic isolation" — in other words, there is no main-power wound transformer between the PV side and the grid side; the magnetics of the main power stage are the boost inductor and the grid-side filter inductor (the common LCL filter itself contains two inductors). With no main isolation transformer, the question "should the main power windings use TIW?" never arises.

Second, main power inductors are high-current, large-core power magnetics whose windings use large cross-section conductors. A high-current inductor winding has to trade DC resistance against high-frequency AC resistance; the four practical choices in engineering are solid wire, litz wire, flat foil and formed foil. Solid single-strand wire conducts only in its outer skin at high frequency and incurs large AC copper losses, so high-frequency, high-current inductors switch to litz wire (many fine strands sharing the current) or foil winding to bring losses down. Triple insulated wire is positioned the opposite way: it is a relatively fine single-strand enamelled conductor with a thick three-layer insulation, whose value is achieving Reinforced insulation with a single wire and dispensing with creepage distance, so that low- to mid-power small transformers can be made smaller. Putting this fine single-strand, thick-insulation wire on the high-current winding of a main power inductor fails on both current-carrying capacity and high-frequency copper loss — and no brand or Approval Document can make up for that.

Even where an inverter uses an isolated topology and really does have a main isolation transformer, that is a high-power transformer with large cross-section windings, again not a place for single-strand TIW. So the honest answer to "should the main power stage of a solar / ESS inverter use triple insulated wire?" is — no. This matches the position taken in this series' article on data centre / AI server power supplies: the main power stage belongs to large cross-section conductors (litz / foil / planar); TIW sits in a different stage.

3. Where triple insulated wire goes: the flyback transformer of the auxiliary / bias power supply

Beyond the main power stage, every inverter has a "housekeeping" power supply. TI's application note defines it clearly: the auxiliary power supply is "designed to provide power to the control, signal-chain, sensing and gate-driver devices", and it "typically comes in the form of an isolated flyback controller that converts voltage from the DC bus or AC grid (or both)" into what the internal components need to operate; without it, none of the electronics is powered and the whole system fails. The document also points out that devices such as gate drivers "depend on the correct bias voltage in order to properly drive the FETs".

Two characteristics of this auxiliary supply decide where TIW goes: low power, but with safety isolation across primary and secondary. How low? The auxiliary power reference design cited in the same TI document takes 350 V to 1500 V in and delivers only about 150 W — a thin slice compared with a main power stage of several kilowatts to several hundred kilowatts. And what it uses is precisely a small wound isolation transformer (flyback): achieving Reinforced insulation on such a small transformer with a single winding wire is where triple insulated wire is the industry's usual tool — third-party flyback transformer construction guides and research on PV inverter auxiliary supplies both note that TIW is used on the secondary winding of a flyback transformer where primary/secondary reinforced isolation is required. This is the application level that DAH JIN's four triple insulated wire types (TLW-B, TLW-BB, TLW-B7S, TLW-F) genuinely correspond to.

For this stage, the first-hand basis DAH JIN can offer is a fixed set of certification conditions, not a claim of "already used in a particular inverter": all four types are listed within the IEC 62368-1 Annex J evaluation scope of VDE 40008834 Anlage 200; specific values such as working voltage, peak voltage and test frequency are per the Approval Document and the VDE certificate; wire diameter 0.10–1.00 mm (TLW-B7S is a 7-strand stranded conductor with 0.10–0.30 mm strands); thermal class 130(B), with TLW-F in Class F (155 °C). The design-side practice is to take the actual operating point of your own auxiliary / bias flyback transformer and check it against the Annex J conditions stated in the Approval Document — within the conditions, the wire is a candidate; if any item falls outside, a separate evaluation is needed. This article does not back-calculate a transformer operating point from PV string or DC bus voltages, nor does it claim that DAH JIN triple insulated wire has been used in any specific inverter. For the boundary between wire thermal class and insulation system thermal class, see Wire thermal class vs. insulation system thermal class.

4. The safety framework: how IEC 62109 treats isolation

Product safety for inverters follows IEC 62109. Part 1 (general requirements) covers protection against electric shock, energy, fire, mechanical and other hazards for PV power conversion equipment, including insulation systems (basic / supplementary / reinforced) and the creepage distances and clearances for the corresponding voltage levels and pollution degrees, and applies up to 1500 V on the DC side and 1000 V on the AC side; Part 2 (particular requirements for inverters) explicitly covers grid-connected and stand-alone inverters and includes those that "may be intended for use in conjunction with batteries or other forms of energy storage" — so an ESS PCS falls within the same framework.

Two levels need to be kept apart here (in outline only; details per the standard text): IEC 62109 is the product safety standard for the finished inverter, deciding what grade of isolation the equipment needs and on which barrier it sits; the TIW Approval Document is a material-level (wire-level) isolation evaluation, assessed under UL 2353 / IEC 62368-1 Annex J (ICT / audio-video) and IEC 60601-1 (medical, per the Approval Document). The two are not the same level and cannot substitute for one another. The correct design practice: the end product passes IEC 62109, and where a given auxiliary / bias flyback transformer needs Reinforced insulation at the winding, choose a triple insulated wire whose Approval Document conditions cover that operating point, as one of the means of achieving Reinforced insulation. Which grade of isolation is required, and what withstand voltage and spacings go with it, is determined by the end product's IEC 62109 classification, not reverse-engineered from a wire catalogue. DAH JIN's corresponding values are per the Approval Document and are not re-listed here.

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

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

1. Main power path (boost inductor, LCL grid-side filter inductors, and the main isolation transformer if there is one): large cross-section conductors — litz wire or foil winding where frequency and current are high, to keep copper loss down; transformerless models do not even have a main isolation transformer. This stage is not the category for single-strand triple insulated wire; there is no need, and no reason, to think of TIW here.
2. Small isolation transformer (flyback) of the auxiliary / bias power supply: this is the wound isolation transformer, and this is where triple insulated wire goes. To select TIW, take the transformer's actual working voltage, peak voltage and test frequency and check them against the Annex J certification conditions stated in the Approval Document (with wire diameter within 0.10–1.00 mm, or strand diameter within 0.10–0.30 mm for TLW-B7S) — within the conditions, the wire is a candidate; if any item falls outside, a separate evaluation is needed.
3. No back-calculation, no extrapolation: do not back-calculate the transformer operating point from PV string or DC bus voltages; for medical applications, Annex L (2 MOOP / 2 MOPP) has its own set of conditions, bound only to TLW-BB and TLW-B7S, with values likewise per the Approval Document. For the first selection fork (B 130 °C vs F 155 °C), see the Triple insulated wire selection guide.