A home wallbox isn’t a fast charger—no liquid cooling or megawatt thermal engineering needed. Its real challenge is quieter: lasting 10+ years outdoors, passively cooled, sealed, with no fan or service. This mix—low heat density, sealed enclosure, passive cooling, long life—demands specific materials, unlike DC fast chargers. This guide covers where heat builds, which materials fit, and what to ask suppliers.
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FEHONDA | Thermal Materials Application Guide
Where a home wallbox actually heats up, which thermal material belongs at each spot, and what to ask for before you buy
Residential AC charging piles come in a few familiar ratings: 3.5 kW and 7 kW on single phase, and 11 kW or 21 to 22 kW on three phase. The 3.5 and 7 kW units dominate the home market; three-phase 11 and 22 kW units appear in homes with a three-phase supply and in light commercial and workplace installations.
The thermal load is modest. A 7 kW single-phase unit passes 32 A, and the losses that matter are contact and conduction loss in the relay, on-resistance in the switching devices, the auxiliary power supply, and the communication module. Total internal dissipation is small in absolute terms, and the heat flux at any single point is far lower than in a DC power module.
The usual arrangement is that units at 7 kW and below rely on natural convection, while 11 kW and above may add a fan. In practice many 11 and 22 kW units are still designed for natural convection, using a finned or ribbed housing and sometimes an aluminium backplate to spread heat. The engineering preference is strong, and for good reason.
Figure 1. Natural convection versus fan-assisted cooling in a wallbox.
A home wallbox has four places worth attention. None of them is individually severe, and the correct response to all four is modest and targeted rather than heavy-handed.
Figure 2. Where a home AC charging pile generates heat.
| Hot spot | Why it heats | Thermal character | Sensible response |
| Control board | Carries the auxiliary supply, Logic and sensing; heat is distributed rather than concentrated | Low flux, spread across the board | Spread heat to the housing wall through a conformable interface; protect the board surface |
| MOSFETs and relay | Conduction loss in the switching devices, and contact and coil loss in the relay carrying the full charge current | The relay is often the largest single contributor inside a 7 kW unit | A heat path to the housing or a small heatsink; the relay also benefits from mechanical retention |
| Small power module | The auxiliary flyback or buck supply feeding the control electronics | Steady, moderate, continuous whenever the unit is powered | A local interface to the housing or a small finned sink |
| Communication chip | Wi-Fi, Bluetooth, 4G or Ethernet module | Low dissipation, but thermally sensitive and often mounted near the top of the enclosure where warm air collects | Usually adequate with board-level spreading; check that it is not sitting in a warm pocket |
Note that the largest single source is often the relay rather than the semiconductors, which surprises people arriving from power electronics. And because total dissipation is low, the temptation is to specify a high-conductivity material everywhere. At these fluxes that buys little — contact and long-term stability matter more than the conductivity figure.
Figure 3. Five material applications inside a home wallbox.
| Material | Conductivity | Where it goes | Why it is the right choice there |
| Thermal grease (thin coat) | 1.5 to 6.0W/(m·K) is sufficient | Between MOSFETs, small power ICs and a small aluminium heatsink or the housing boss | At these power levels a thin grease bond line gives the lowest interface resistance. What matters is not the conductivity figure but that the grease does not dry out, crack or powder over years of thermal cycling |
| Ultra-soft thermal pad | Around 2.0 W/(m·K) | Between the power board and the internal cooling ribs or wall of the housing | Fills the variable gap to the housing at low compression force, so it bridges the tolerance stack-up without loading the board or its solder joints. Doubles as a cushion against vibration and handling |
| Thermal gel or putty | 3.8 to 8.0 W/(m·K) | Irregular or odd-shaped gaps, and around components of differing height | Dispensable, so a small amount fills geometry a pre-cut pad cannot match. Use sparingly and only where the shape demands it |
| RTV thermal sealant | 1.0 to 1.5 W/(m·K), UL94 V-0 | Housing seams, cable gland entries, and the main control cavity | Outdoor units need heat transfer and weather sealing in one material. A thermally conductive RTV carries modest heat while sealing against rain and dust, and the flame rating matters in an enclosure that is never opened |
| Two-part potting compound | 0.8 to 2.0 W/(m·K), low viscosity | Relay cavity and connector terminal cavity, where heat or contamination concentrates | Insulates and locks the termination against vibration and moisture. Use it locally and deliberately — see the caution below |
Do not pot a home wallbox in full. A fully potted unit cannot be serviced — a failed relay or communication module cannot be replaced, turning a component failure into a whole-unit return. Local potting of the relay or terminal cavity gives the insulation and retention benefit while leaving the rest openable. This is a case where using less material produces a better product.
FEHONDA supplies thermal conductive potting adhesive, thermally conductive silica, thermally conductive gel, ultra-soft thermal pads, composite TIM constructions and thermally conductive RTV sealants for residential and light-commercial AC charging piles, with thermal conductivity grades spanning 1.5 to 6.0 W/(m·K).
Our sales and engineering support teams help with grade selection, dispensing and compression trials, sealant selection, compliance documentation, custom die-cutting, OEM packaging and product customisation. Send us your enclosure drawing, the gap range at each position, your target market and production volume, and we will return a shortlist, sample recommendations and lead time. Please submit your request via our custom service form to ensure a faster response.
📩 Contact our technical team: sales@fehonda.com
Far less than a DC charger. Losses come from the relay carrying the full charge current, the switching devices, the auxiliary supply and the communication module. The heat flux at any single point is low, which is why natural convection is usually sufficient and why contact and long-term stability matter more than peak conductivity.
Units at 7 kW and below generally do not, and many 11 and 22 kW units are also designed for natural convection using a finned housing or an aluminium backplate. A fan adds a wear item, noise, and a dust and moisture path that has to be filtered and maintained. It becomes worthwhile at higher power, in hot climates, or in a very small enclosure.
Lower than most people assume. Around 1.5 to 3.6 W/(m·K) covers thermal grease on MOSFETs and small power ICs, and around 2.0 W/(m·K) is typical for a board-to-housing pad. Above those levels the extra conductivity rarely changes the result, because at low flux the limiting factor is contact and bond line, not bulk conductivity.
Grease where the surfaces are flat and you can hold a thin bond line, such as a MOSFET under a small heatsink. A pad where the gap varies or the surfaces are not flat, such as a board against a ribbed housing wall. Where the shape is irregular, a small amount of gel or putty will reach geometry a pre-cut pad cannot.
Use a thermally conductive RTV sealant rated UL94 V-0 at the housing seams, cable gland entries and control cavity. It seals against rain and dust while still carrying heat, unlike a non-conductive sealant that insulates the joint.
No. Local potting of the relay cavity or terminal cavity gives insulation, retention and moisture protection where it is needed, while leaving the rest serviceable. A fully potted wallbox cannot be repaired in the field, turning a component failure into a whole-unit return.
Cycling or ageing data rather than a single conductivity figure, plus continuous operating temperature range. For outdoor units add dielectric strength and the UL94 rating with its certified thickness; for European markets, RoHS and REACH declarations with supporting test reports.
FEHONDA's new materials are used in a variety of applications around the world, providing effective solutions for thermal conductivity, shielding and more for products. Our sales and engineering support teams are ready to help you with any questions you may have, including quotations, product application support and product customization. Please submit your request via our custom service form to ensure a faster response.
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