Potting compound for inductive components: coils, chokes and transformers
Coils, chokes, and transformers place different demands on a potting compound than a flat printed circuit board: They combine heat generation, high electric field strengths, and delicate, fine windings in a single component. This article shows what really matters when choosing the material – and which potting compound is suitable for which application.
To the point: Which potting compound is suitable for coils and transformers?
In short: For most inductive components, an elastic two-component silicone is the best choice because it remains low-stress over a wide temperature range and does not damage delicate windings. Use soft silicone gels for sensitive or high-voltage windings, a thermally conductive variant for high-loss chokes, and epoxy only when mechanical rigidity is required and temperature fluctuations are limited. In every case, a bubble-free potting is crucial.
Why inductive components have special requirements
Four stresses act simultaneously – and they sometimes pull the choice of materials in opposing directions.
1. Heat from copper and core losses
Electrical losses in the winding and hysteresis losses in the core generate heat that must be dissipated. A potting compound can conduct this heat away via the housing – provided it is thermally bonded and sufficiently thermally conductive. However, the downside is important: a full potting compound can also impede heat dissipation, for example, if convection at the winding head previously provided cooling. Therefore, for highly stressed chokes, a thermally conductive compound or a combination with a thermal interface to the heat sink (see Thermal Interface Materials) is recommended.
2. Electric field strength and partial discharge
High local field strengths occur in layer and winding insulation. Air inclusions, voids, or bubbles are the critical weak points: partial discharges , slowly degrading the insulation system over operating hours until it fails. Therefore, the absence of bubbles in high-voltage transformers is not merely cosmetic, but essential for service life. Measurement is performed via partial discharge testing according to IEC 60270; air and creepage distances, as well as the pollution degree, are determined by the insulation coordination according to IEC 60664-1. A dense, full encapsulation achieves pollution degree 1, thus allowing for more compact designs with shorter creepage distances. For high-frequency switching transformers, also consider the reduction factors for high frequencies according to IEC 60664-4.
3. Mechanical and thermal cycling
Potting compound, copper wire, ferrite or iron core, and winding body have very different coefficients of thermal expansion. With each temperature cycle, these materials work against each other. A hard, rigid compound transfers these stresses directly to the fine winding wire and its varnish insulation – resulting in wire breakage and insulation cracks. Soft, elastic silicones and gels, on the other hand, decouple with minimal stress and simultaneously dampen vibrations and mechanical resonances. For fine windings with small wire diameters, a low Shore hardness is therefore almost always the safer choice.
4. Humidity, condensation and media
Potting compound hermetically protects the winding from moisture, condensation, salt spray, and aggressive media. Silicones are particularly robust here due to their resistance to hydrolysis and weathering across wide temperature ranges. The long-term thermal stability of an insulation system is classified according to IEC 60085's , while the surface tracking resistance is determined by the CTI (Comparative Value for Thermal Inductance) according to IEC 60112.
Silicone, polyurethane or epoxy?
All three chemicals are potted – but they differ significantly for inductive components:
- Silicone – widest temperature range (typically -50 to +200 °C), permanently elastic, low-stress, partial-discharge resistant. First choice for thermal switching, high voltage and fine windings.
- Polyurethane – tough and elastic, good mechanical and chemical protection, medium temperature range. Suitable when toughness and abrasion resistance are paramount.
- Epoxy – hard, high-strength, chemically resistant, but rigid: The difference in the coefficient of thermal expansion puts stress on windings during temperature changes. Suitable for mechanically highly stressed, dimensionally stable potting blocks with limited temperature range.
A detailed comparison can be found under Potting Compounds & Electrocasting Resins: Epoxy, Silicone or PU , and for a fundamental distinction under Potting vs. Encapsulation.
Correctly matching the Shore hardness to the coil
The most important selection criterion for inductive components is not the brand, but the hardness:
| Requirement | Recommendation | Products (Examples) |
|---|---|---|
| Fine/sensitive winding, high voltage, repairable later | Soft gel / soft silicone (very low Shore) | SILISIL RTV MD-Gel, MD-Soft 10, PP-Soft 00, MF-Soft 12 |
| Standard potting with thermal cycling and vibration | Elastic silicone (medium Shore) | SILISIL RTV MF-Flex 20, PC-Flex 20, PRO-Cast 45 |
| Mechanically robust, dimensionally stable | Higher fill/firmer silicone | SILISIL RTV MF-Dura 35, MF-Ultra 50 |
| Transparent, high-strength encapsulation | Clear high-performance silicone | BLUESIL ESA 7250, BLUESIL RTV 3132 |
Heat dissipation in chokes and transformers
Where power loss occurs, thermal interfaces are crucial. A thermally conductive potting compound lowers the winding temperature and extends the service life – however, thermal conductivity alone is of little use if the heat path to the housing or heat sink is poor. For connection to cooling surfaces, thermal pastes such as DOWSIL 340 the potting compound. Higher thermal conductivity also usually means a higher fill level and therefore higher viscosity – this must be taken into account when dispensing and degassing. We would be happy to select a suitable, easily dispensed thermally conductive compound for your specific application.
Application: bubble-free is mandatory
Especially with inductive components, the potting compound creeps between closely spaced winding layers – the air trapped there later becomes the starting point for partial discharge. These factors determine the outcome:
- Clean and preheat the assembly; seal the winding body and housing.
- Mix the 2K components exactly in the prescribed ratio and homogenize completely.
- Degassing in a vacuum is not optional for tightly wound components.
- Pour slowly in one spot to allow air to escape from the coils; observe the pot life.
- Cure or temper according to the data sheet; wait for complete curing.
The complete procedure is described in the step-by-step instructions for electronic potting; typical error patterns and their causes can be found under Avoiding potting errors.
You can determine how much material you need for your component and how the mixing quantities for component A and B are divided directly with our potting and mold making calculator.
Unsure about your choice?
Coils, chokes, and transformers differ significantly in voltage range, power dissipation, and winding design. If you provide us with the component details, operating temperature, voltage, and installation situation, we will recommend the appropriate type and provide you with a sample. Contact us or write to info@silitech.ch.
