This article is written for power-supply and magnetics engineers, safety-compliance engineers and purchasing engineers. Inside one power supply you will meet several "wound magnetic components"—the PFC inductor, the common-mode choke, the POL (point-of-load) buck inductor and the IBC (intermediate bus converter)—and they are often lumped into one question: "do these need triple insulated wire (TIW)?" This article settles all four "do not use" components at once: it is not that they are not good enough, but that their electrical position is simply not where TIW earns its keep. The criterion is a single sentence, stated first; all values of DAH JIN products are always "per the Approval Document".
This article is written for power-supply and magnetics engineers, safety-compliance engineers and purchasing engineers. Inside one power supply you will meet several "wound magnetic components"—the PFC inductor, the common-mode choke, the POL (point-of-load) buck inductor and the IBC (intermediate bus converter)—and they are often lumped into one question: "do these need triple insulated wire (TIW)?" This article settles all four "do not use" components at once: it is not that they are not good enough, but that their electrical position is simply not where TIW earns its keep. The criterion is a single sentence, stated first; all values of DAH JIN products are always "per the Approval Document".
1. The one-sentence principle: TIW earns its keep "across the safety barrier"
First make clear what TIW is and why it exists; the four components then become easy to judge.
The value of TIW is that it is a winding wire type-tested under the safety standards and evaluated as reinforced insulation. In the classification used by safety standards of the IEC 62368-1 type, insulation is divided into four kinds—functional, basic, supplementary and reinforced; of these, functional insulation provides no protection against electric shock, while reinforced insulation is a single insulation system "equivalent to double insulation", providing two levels of protection against electric shock.
The place that genuinely requires "reinforced (or double) insulation" is the safety barrier between the primary side (connected to the mains, at hazardous voltage) and the secondary side (accessible to the user, SELV / low voltage). In wound magnetics this barrier appears between the primary and secondary windings of an isolation transformer—and TIW is the means of achieving reinforced insulation there with the wire itself.
Conversely: a wound component that does not cross this primary-to-secondary safety barrier at all has no requirement for "reinforced insulation", and therefore no use for TIW. Its windings only need ordinary winding wire suited to their own potential domain (enamelled wire, or basic / functional insulation). All four components below fall on the "does not cross the barrier" side.
2. The electrical position of the four components
Look at where each of the four magnetics sits in the circuit, and you will find that they are all "on the same side of the barrier" or "downstream of the barrier".
PFC inductor (boost choke)—a single winding on the rectified-mains (primary) side. The boost PFC inductor of a continuous-conduction-mode (CCM) converter is an energy-storage inductor with a single winding, operating directly from the rectified mains voltage (that is, on the primary / mains side). All of its turns sit in one potential domain; there is no such thing as "primary versus secondary" inside the component, so there is no requirement for reinforced insulation across a barrier. What it needs is winding wire that withstands that potential, that current and that temperature—not triple insulation across a safety barrier.
Common-mode choke—an EMI filter component at the AC input, with L and N both on the primary side. The subject here is the AC-line-filter common-mode choke at the power-supply input (often paired with the PFC stage). It consists of two windings on one magnetic core, placed in series with the Line and the Neutral conductor respectively, to attenuate common-mode noise—a functional EMI-filtering task, not the establishment of safety isolation between primary and secondary. L and N are both in the same potential domain on the AC-input side; what the windings need between them is functional / basic insulation to handle the L-N differential-mode voltage and surges, not reinforced insulation across a safety barrier. It "crosses lines" but does not "cross the barrier", so it has no use for TIW.
POL (point-of-load)—a non-isolated buck converter, entirely on the SELV / low-voltage secondary side. A POL converter is usually implemented as a non-isolated buck (step-down) topology that steps the on-board intermediate bus voltage (commonly 12 V) down further to the lower voltages required by loads such as microprocessors, ASICs and DSPs; it has no transformer, its input and output are not isolated, and they share a common ground. Since there is no isolation barrier and the whole converter sits on the SELV / low-voltage secondary side, its buck inductor is a single-winding power inductor, and reinforced insulation is naturally not an issue.
IBC (intermediate bus converter)—in the low-voltage bus domain, with safety isolation mostly already done upstream. An IBC performs one stage of conversion on the intermediate bus voltage (commonly 48 / 53 V to 12 V in telecom and data centers); both the bus and the output are in the low-voltage domain. The key point: in a system fed by an AC/DC switch-mode power supply (SMPS), the safety isolation required by the system is usually already provided by the front-end SMPS, and the IBC itself often needs no further safety isolation—hence the industry's wide use of non-isolated IBCs; even where an isolated IBC is used, its isolation is mostly functional (within an already low-voltage bus domain, for breaking ground loops and the like), not the main primary-to-secondary safety barrier. The safety barrier was crossed back at the AC front end; it never reaches the IBC, so TIW has no place there.
3. Comparison table: component → electrical position → why no TIW is needed
Safety-compliance and purchasing engineers can locate their case directly in this table. All table values are "per the Approval Document".
- PFC inductor (boost choke) — Electrical position：Single winding on the rectified-mains (primary / mains) side, one potential domain；Why no TIW is needed：No primary-to-secondary barrier inside the component to cross; what it needs is winding wire suited to that potential, not reinforced insulation across a barrier
- Common-mode choke (AC-input EMI filter) — Electrical position：Two windings in series with L and N respectively, both in the same potential domain on the AC-input (primary / mains) side；Why no TIW is needed：Its task is common-mode noise suppression (functional EMI); winding-to-winding insulation is functional / basic, not reinforced insulation across a safety barrier
- POL (point-of-load, non-isolated buck) — Electrical position：Entirely on the SELV / low-voltage secondary side; input and output not isolated, common ground；Why no TIW is needed：No isolation barrier to cross; single-winding buck inductor, reinforced insulation not involved
- IBC (e.g. 48 V → 12 V) — Electrical position：Low-voltage bus domain; system safety isolation mostly already provided by the front-end AC/DC SMPS；Why no TIW is needed：The IBC itself is mostly non-isolated, or its isolation is functional (within the low-voltage domain); not the main primary-to-secondary safety barrier
The table in one sentence: these four magnetics are either single windings within one potential domain (PFC, POL), functional filtering within one potential domain (common-mode choke), or in the low-voltage domain downstream of the safety barrier (IBC). None of them needs reinforced insulation across a barrier between its windings, so none of them is TIW territory.
4. So where is TIW used? Back to DAH JIN's home ground
Since none of these four uses it, the true place of TIW becomes all the clearer: isolation-transformer windings that must cross the primary-to-secondary safety barrier and achieve reinforced insulation with the wire itself—for example the isolation transformer of the AC front end, and the small isolation transformers for auxiliary / bias supplies and gate drive, which are small but still have to provide reinforced isolation across a high-voltage barrier. To decide "which side" your own design belongs to, start with Does your product need TIW?.
In the DAH JIN product line, the four external models of triple insulated wire are TLW-B, TLW-BB, TLW-B7S and TLW-F, all type-tested under the safety standards and evaluated as usable for reinforced insulation; the applicable models, wire-diameter ranges and all other values are always defined by the Approval Document, and this article does not repeat specific figures. DAH JIN is a wire manufacturer (not a trading company), producing at its own factory in Guiren, Tainan, Taiwan. For which model to choose and which conditions to check, see the TIW selection guide; for definitions of terms see the TIW glossary, and for individual cases consult sales to have them defined per the Approval Document.
Frequently Asked Questions
The PFC inductor and the common-mode choke are both connected to the mains at a fairly high voltage—why do they not use triple insulated wire?
A high voltage does not by itself mean that reinforced insulation is needed. What TIW solves is reinforced insulation "across the primary-to-secondary safety barrier". The PFC inductor is a single winding sitting entirely in one potential domain on the mains (primary) side; the two windings of the common-mode choke, L and N, are also both on the same AC-input side and perform functional EMI filtering. Neither crosses the primary-to-secondary barrier, so neither needs TIW.
POL and IBC are on the low-voltage side—is it because "the voltage is too low" that they do not use it?
The more precise statement is "they sit downstream of the safety barrier". A POL is a non-isolated buck with input and output sharing a common ground, entirely on the SELV / low-voltage secondary side, with no isolation barrier; an IBC is in the low-voltage bus domain, where the system's safety isolation is usually already provided by the front-end AC/DC SMPS, and the IBC itself is mostly non-isolated or provides only functional isolation. Where no winding crosses the main safety barrier, TIW has no place.
The two windings of a common-mode choke also need insulation between them—how is that different from the reinforced insulation of TIW?
The difference is "whom it is for". The insulation between the two windings of a common-mode choke is functional / basic insulation, handling the differential-mode voltage and surges between L and N, both on the primary side; what TIW provides is reinforced insulation, equivalent to double insulation, used for protection against electric shock between primary and secondary (hazardous versus accessible). The former is not a safety barrier against electric shock; the latter is, and the two are not interchangeable.
Which component in my design is the one that should use DAH JIN's triple insulated wire?
Locate first—sort every magnetic component by "does it cross the primary-to-secondary safety barrier or not". Those that do (such as the primary-to-secondary windings of an isolation transformer, or small isolation transformers that must cross a high-voltage barrier) are TIW territory; those that do not (PFC, common-mode, POL, IBC) can use ordinary winding wire. Once the need is confirmed, the model and wire-diameter range are always per the Approval Document and can be defined with sales.