Reinforced insulation in a small transformer is fundamentally a space problem. The two accepted ways to solve it are triple insulated wire and margin tape, and the choice changes winding window, cost, leakage inductance and how easily the part can be automated.
1. What reinforced insulation actually requires
Reinforced insulation means the barrier must withstand the same test as double insulation: for a 250 V working voltage under IEC 62368-1 that is typically a 3 kV hipot plus defined creepage and clearance. Inside a bobbin, that barrier sits between primary and secondary, and it must be continuous around the winding — not just at the pins.
2. Margin tape
Margin tape is insulating tape built up at both ends of each winding, creating a physical distance between the primary and secondary edges. Advantages: uses standard enamelled copper wire, which is cheaper and available in any gauge; simple to hand-wind; predictable creepage because the margin is a fixed width.
Disadvantages: the margin consumes winding window, often 15–25% of the width per side, which pushes the core size up; the winding must stop short of the tape, so the copper utilisation falls; and the tape layers add cost and a process step that must be inspected.
3. Triple insulated wire
TIW is a single conductor with three concentric insulation layers, rated as a reinforced barrier by itself. Advantages: the whole window is usable, so a smaller core can meet the same power; no margin tape process; the winding can be automated more easily; creepage is satisfied by the wire rather than by geometry.
Disadvantages: cost per metre is several times that of enamelled wire; the insulation adds diameter, so fewer turns fit per layer; and the wire is stiffer, which matters on small bobbins and on toroids.
| Criterion | Margin tape | Triple insulated wire |
|---|---|---|
| Window utilisation | Poor (15–25% lost per side) | Good |
| Material cost | Low | High per metre |
| Core size for the same power | Larger | Smaller |
| Automation | Hard | Easy |
| Leakage inductance | Higher (wider separation) | Lower |
| Best fit | Low volume, large cores, high turns | High volume, small cores, low profile |
4. The cost crossover
At low volume with a large core and many turns, margin tape is usually cheaper: the tape is inexpensive and the core is not the cost driver. As volume rises and the design shrinks toward a smaller core — typical of a 30 W adapter or a compact LED driver — TIW wins because the saved window lets the design move down one core size, which removes more cost than the wire adds.
5. Hybrid approaches
Two practical compromises exist. First, use TIW for the secondary only, where the turn count is low and the wire cost is small, and margin tape for the primary. Second, use a sectional bobbin that physically separates the windings; the creepage is then built into the former and neither tape nor TIW is needed, at the cost of a more expensive bobbin.
6. Verification checklist
- Confirm the barrier rating of the TIW grade — not all triple-insulated wire is rated for reinforced insulation at your working voltage.
- Check the actual creepage with the finished winding, including the solder joint area.
- Run hipot and partial discharge after impregnation, since the process can affect the barrier.
- Confirm the pin-side clearance, which is often the real limit rather than the winding itself.
Shaanxi SiRui Technology winds isolation transformers with both approaches and selects between them on total cost at the customer’s volume, using Magnetic Core series bobbins and cores sized to the barrier requirement rather than to the catalogue.
Send your working voltage and volume to sales@sirui-tech.com.