Key takeaways
- A sealed enclosure cannot use internal air as a heat path, so the heat has to leave through the wall or a cold plate.
- A filtered enclosure trades dust ingress for filter maintenance, and a blocked filter is a thermal failure.
- External fins add surface area without breaking the IP rating, but they need a drainage path and a clean airflow path.
- Condensation is a real risk in a sealed unit that cycles between cold nights and warm operation.
The enclosure decides the cooling route
An EV charger or an inverter is installed outdoors, in a garage or in a plant room. The enclosure has to keep rain, dust and insects out, and it has to let heat out. Those two requirements pull in opposite directions. A sealed box protects the electronics and traps the heat. A vented box cools easily and lets contamination in.
The practical answer is usually one of four routes: open ventilation, filtered ventilation, a sealed enclosure with external fins, or a sealed enclosure with a liquid cold plate. Each route has a different heat sink, a different maintenance plan and a different IP rating. The choice is made at the enclosure level, not at the heat sink level.
The heat sink design follows. An external fin stack can be bolted to the outside of a sealed box, with a thermal path through the wall. A cold plate can sit inside the sealed volume and carry heat to a radiator or a chiller. An internal sink with a fan only works if the enclosure is vented, and then the fan and the fins become a dust maintenance item.
IP rating and airflow
The IP code describes protection against solid objects and water. The first digit covers solids and the second covers water. A higher rating usually means a tighter seal, which means less natural air exchange and a greater reliance on conduction through the enclosure wall.
A vented enclosure can use a low IP rating and a fan, but it needs a filter and a maintenance interval. The filter adds pressure drop, and a blocked filter reduces the airflow. The thermal design should include a blocked filter case, not only a clean one, because the unit may run for months between services.
A sealed enclosure avoids the filter but needs a pressure equalisation element. The element lets air pass slowly to equalise pressure without letting water in. Without it, the seal sees a pressure difference at every thermal cycle, and a cable gland or a lid gasket can leak. The element is small, but it belongs in the thermal and sealing design.
Thermal density and surface area
Thermal density is the loss divided by the volume or the external surface area of the unit. A compact charger with a high loss and a small case is harder to cool than a larger unit with the same loss, because the case has less area to reject heat. The external surface, not the internal volume, is the limit for a sealed natural convection unit.
A sealed box rejects heat from all of its faces, but the mounting face and the shaded faces contribute little. The effective area is often much smaller than the total. A wall mounted unit loses the back face, and a unit in a recess loses the sides. The installation instructions should say how much clearance is needed, and the thermal rating should assume the worst orientation.
External fins add area without breaking the seal. They work best when the fins are vertical and open at both ends, and when the unit is not packed against a wall. A fin stack on the top face of a box can trap a warm layer, and a fin stack on the bottom face can collect debris. The mounting orientation and the fin direction are part of the thermal specification.
Condensation, coating and potting
A sealed unit is not a dry unit. Moisture can enter during assembly, through a cable or through a breathing element, and it can condense on a cold surface when the unit powers up. The water film can bridge creepage distances, corrode a contact or short a low voltage circuit.
Conformal coating protects a circuit board, but it is not a substitute for a drainage path or a desiccant. A coating has to cover the edges and the connectors, and it must be compatible with the thermal interface. A coated surface under a thermal pad can add resistance if the coating is thick or uneven.
Potting fills the volume and protects the components, but it adds mass and changes the thermal path. A potted unit conducts heat to the enclosure through the compound, so the compound's conductivity and the fill level matter. Potting also makes repair impossible, which is a service decision as much as an engineering one.
Vibration, mounting and service
A charger or an inverter may be mounted on a wall, a pole or a vehicle. The mounting has to carry the mass of the heat sink and the enclosure through vibration and shock. A heavy external sink on a thin panel can fatigue the panel or loosen the fasteners, so the bracket and the panel thickness belong in the design.
The heat sink should be supported at more than one point, and the fasteners should have a positive lock or a thread insert in a thin wall. A thermal pad between the sink and the enclosure can act as a spring, but it can also allow movement. A rigid thermal path with a controlled flatness is usually better for a vibration environment.
Service access matters too. A filter needs to be reachable without removing the unit from the wall. A fan needs a replacement interval and a part number. A cold plate needs a drain and a bleed point. A design that ignores service will be maintained by whatever method is available, and that is usually worse than the specified one.
Liquid cooling and cold plates
A liquid cold plate is a heat sink with channels instead of fins. It carries heat to a radiator or a chiller, and it can handle a much higher loss density than an air cooled sink in the same volume. The cold plate can be machined, brazed or extruded, and the channel geometry sets the pressure drop and the heat transfer.
The cold plate is only part of the system. The pump, the hoses, the fluid, the reservoir and the radiator all add mass, cost and failure modes. A leak inside an electrical enclosure is a serious fault, so the fluid path is usually kept outside the electronics volume, with a sealed interface through the wall.
The fluid matters. Water has a high specific heat but freezes and corrodes. A glycol mix lowers the freezing point and changes the viscosity and the heat transfer. The maintenance plan has to include a fluid check and a filter, and the pressure drop must stay within the pump curve.
What to put in the specification
State the enclosure route, the IP rating and the installation orientation. Give the loss at the worst case, the ambient, the allowable component temperature and the expected duty cycle. Add the altitude, the solar load and the clearance to nearby surfaces.
Specify the heat sink material and finish, the mounting interface, the flatness and the interface material. For an external sink, add the drainage path, the fastener sealing and the corrosion protection. For a cold plate, add the fluid, the pressure drop, the test pressure and the leak acceptance.
Finally, state the test. A thermal test in the final enclosure, at the worst orientation and with a blocked filter where applicable, is the only result that represents the installed unit. A bench rating in still air is a starting point, not a guarantee.
| Route | IP approach | Maintenance | Best for |
|---|---|---|---|
| Open ventilation | Low IP, screened openings | Clean the screen and the fan | Indoor plant rooms with clean air |
| Filtered ventilation | Medium IP, filtered inlet | Replace the filter on a schedule | Sheltered outdoor units with service access |
| Sealed, external fins | High IP, sealed shell | Clean the fins and the drain path | Outdoor wall units with a moderate loss |
| Sealed, liquid cold plate | High IP, sealed fluid path | Check fluid, pump and filter | High density chargers and inverters |
The IP rating is only valid for the assembled unit as tested. A heat sink or a gland does not carry a rating on its own.
Frequently asked questions
Can a sealed enclosure cool without a fan?
Yes, if the loss is low enough and the external surface is large enough. Heat leaves through the wall by convection and radiation. External fins add area without breaking the seal. The limit depends on the ambient, the orientation and the internal temperature.
Does a higher IP rating reduce cooling?
It usually does, because a tighter seal reduces air exchange and rules out a direct fan path. The heat has to leave through the wall or a cold plate. A high IP unit needs more external surface or a liquid path for the same loss.
How often should a charger filter be replaced?
The interval depends on the site, the dust load and the filter. Set it from the installation environment and check it at commissioning, not from a generic schedule. A blocked filter case belongs in the thermal design, because the unit may run for months between services.
Is a cold plate better than an external fin sink?
A cold plate handles a higher loss density in a smaller volume, but it adds a pump, fluid, hoses and a maintenance plan. An external fin sink is simpler and has no fluid leak path. The choice depends on the loss, the enclosure and the service model.
Sources and further reading
- IEC 60529:1989+AMD1:1999+AMD2:2013, Degrees of protection provided by enclosures (IP code)
- IEC 61851-1:2017, Electric vehicle conductive charging system. General requirements
- JESD51-2A, Integrated circuits thermal test method environmental conditions. Natural convection (still air)
Qumek Engineering Desk
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