High-voltage transformers — used in power supplies, ignition systems, medical equipment, industrial drives, and test instruments — require encapsulation that addresses two requirements simultaneously: electrical isolation adequate for the operating voltage, and thermal management sufficient to keep the winding temperature within the insulation system’s thermal class. These two requirements drive the potting compound specification in opposite directions. The highest-dielectric-strength epoxy systems tend to be unfilled or lightly filled, with low thermal conductivity. The highest-thermal-conductivity systems use high filler loading that can reduce dielectric strength and create internal stress during thermal cycling that may compromise isolation integrity. Selecting the right encapsulant for a high-voltage transformer requires explicit analysis of both requirements and an understanding of where each is the binding constraint.
The Electrical Isolation Requirement
High-voltage transformer encapsulation must maintain electrical isolation between primary and secondary windings, between windings and the case, and between turns within each winding at the operating voltage — plus adequate margin for transient overvoltages, voltage spikes from switching, and any withstand test voltage.
The minimum required insulation distance through the encapsulant is determined by the working voltage and the dielectric strength of the potting compound. For a compound with dielectric strength of 20 kV/mm, a working voltage of 5 kV requires a minimum insulation thickness of 5 kV ÷ 20 kV/mm = 0.25 mm — nominally. In practice, safety factors of 5 to 10 are applied, requiring 1.25 to 2.5 mm insulation thickness at 5 kV. These safety factors account for: variation in compound quality, the effect of voids on local dielectric strength reduction, the reduction in dielectric strength at operating temperature, moisture absorption effects, and long-term dielectric degradation.
Partial discharge. At voltages approaching the local dielectric strength of any void in the compound, partial discharge occurs within the void — a low-energy discharge that does not immediately cause failure but erodes the surrounding insulation progressively. For high-voltage transformer encapsulation, partial discharge inception voltage (PDIV) must be above the highest transient voltage the transformer may experience. Void elimination through vacuum potting is the primary control for partial discharge in transformer encapsulation.
Dielectric strength at operating temperature. Dielectric strength of epoxy decreases with temperature. At 100°C, many standard epoxies show 15% to 25% reduction in dielectric strength from the ambient value. The insulation distance calculation must use the dielectric strength at the maximum operating temperature, not at ambient.
If you need dielectric strength data at operating temperature, partial discharge inception voltage, and thermal cycling data for high-voltage transformer potting compounds, Email Us — Incure provides formulation-specific high-voltage electrical characterization data for transformer encapsulation applications.
The Thermal Requirement
Transformer winding losses — copper losses from winding resistance and core losses from eddy currents and hysteresis — generate heat that must flow from the winding wire to the transformer case or a heat sink. The thermal path from wire to case passes through: the wire enamel insulation, the potting compound filling the winding space, and the transformer case wall.
The temperature rise across the potting compound layer is: ΔT = Q × (thickness / thermal conductivity × area), where Q is the heat generated in watts, thickness is the distance from the hot wire to the case wall, and area is the effective cross-section for heat flow. For a winding dissipating 5W, compound thickness of 5 mm, area of 10 cm², and thermal conductivity of 0.2 W/m·K (unfilled epoxy):
ΔT = 5W × (0.005 m ÷ (0.2 W/m·K × 0.001 m²)) = 5 × 25 = 125°C
This 125°C temperature rise across the compound, added to case temperature, would push winding temperature far above acceptable limits for most insulation classes. Using filled compound at 1.5 W/m·K reduces this to 17°C — a dramatic improvement.
For high-power density transformers, thermally conductive potting is not optional — it is required to keep winding temperature within the rated insulation class.
Balancing Dielectric and Thermal Requirements
The apparent conflict between high electrical isolation (favors unfilled or lightly filled, lower dielectric constant compound) and high thermal conductivity (favors highly filled compound) resolves differently depending on the operating voltage and power level:
High-voltage, low-power transformers (signal-level isolation, >10 kV, <10W). The dielectric requirement is binding; thermal conductivity is less critical because power levels are low. Low-filled or unfilled epoxy with maximum dielectric strength is appropriate. Void elimination is the primary process requirement.
Medium-voltage, medium-power transformers (1 kV to 10 kV, 10 to 1000W). Both requirements are significant. Alumina-filled epoxy at 60% to 70% loading achieves thermal conductivity of 1 to 2 W/m·K while maintaining dielectric strength above 15 kV/mm — adequate for most medium-voltage applications. This range is the most common specification for industrial power supply and drive transformers.
Low-voltage, high-power transformers (<500V, >1 kW). Thermal management is binding; dielectric requirements are easily met. Highly filled alumina or BN compound at maximum thermal conductivity is appropriate.
Vacuum Potting Process for High-Voltage Transformers
Void-free encapsulation is non-negotiable for high-voltage transformer potting. Any void adjacent to a conductor at high voltage is a partial discharge initiation site. Vacuum potting — filling the transformer housing under vacuum to eliminate air from all winding interstices before compound is introduced — is the standard process for high-voltage transformer encapsulation.
The process sequence: pre-dry the wound transformer assembly at 80°C to 100°C to remove absorbed moisture from wire enamel and bobbin; place in a vacuum chamber and evacuate; introduce degassed compound under vacuum; release to atmospheric pressure and allow compound to penetrate all void space; cure.
Cure temperature compatibility with the winding materials — wire enamel temperature rating, bobbin material, and any pre-installed cores — sets the maximum cure temperature. Most transformer windings tolerate 120°C to 150°C cure; some thermoplastic bobbins are limited to 85°C.
Contact Our Team to discuss high-voltage transformer potting compound selection, dielectric strength at operating temperature, vacuum potting process development, and thermal class qualification for your transformer program.
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