Resonant Tank Housing Sheet Metal for EV Converters

• 1
  What the housing actually does in a resonant tank
• 2
  Material choices and where each one stops working
• 3
  From blank to enclosure: the sheet metal sequence
• 4
  Why CNC machining gets added to a sheet metal part
• 5
  Where a sheet metal housing is the wrong answer
• 6
  Choosing wall material and thickness for a resonant tank housing

An LL resonant tank sits between the switching bridge and the transformer in an LLC or CLLC converter. The inductor and the resonant capacitor carry a near-sinusoidal current at 100 kHz to 500 kHz. The box around them is not a container. It sets the magnetic boundary, the thermal path and the mechanical datum for the whole power stage.

Three jobs run at the same time. First, the wall acts as a shield that keeps switching fields inside the tank and keeps external fields out. Second, the shell moves heat from the inductor winding and the capacitor body to the cold plate or the chassis. Third, the housing holds the core, the busbar and the mounting feet in position so the gap and the leakage inductance stay repeatable.

Those jobs pull in different directions. A thicker wall shields better and spreads heat faster, but it is heavier and harder to bend to a tight radius. A thinner wall saves mass and lowers eddy-current loss in the shell itself, yet it warps more during welding. Most EV designs land between 1.0 mm and 3.0 mm for the main shell.

The housing also has to survive the build environment, not just the bench. Potting compound, thermal gap filler and conformal coat all add mass and cure at temperature. A shell that is flat when it leaves the brake may bow after two cure cycles if residual stress was never relieved.

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  ShieldingContinuous metal path around the tank, no long slots near the winding.
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  ThermalShort, wide conduction path to the cold plate.
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  DatumMachined faces that locate the core and the busbar.

Aluminium dominates this part family. Alloys such as 5052, 6061 and 6063 give thermal conductivity in the 150-210 W/m·K range, form well and keep mass low. 5052 is the better choice for deep drawn or heavily bent shells because it work-hardens less than 6061. 6061 is easier to machine when the housing needs tight bores, threaded inserts or a lapped sealing face.

When magnetic shielding matters more than mass, cold-rolled steel or galvanized steel enters the picture. A steel shell blocks low-frequency fields that aluminium does not. The cost is weight and corrosion control. Galvanized steel needs cut edges protected, and a mixed aluminium-steel assembly needs isolation washers to avoid galvanic corrosion at the fasteners.

Thickness is a trade, not a default. Shells of 1.0-1.5 mm are common where eddy-current loss in the housing must stay low and the part is small. Larger tanks that carry a heavy core and get potted often move to 2.0-3.0 mm so the flange stays flat through welding and cure. Ask what the wall is really doing before copying a thickness from an older program.

Post-forming heat treatment is the step most drawings leave out. Stress relief after bending and before final machining keeps sealing faces flat and stops the part from moving during the first thermal cycle. If the sealing face is called out at 0.1 mm flatness over 200 mm, the sequence matters as much as the alloy.

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  5052 aluminiumBest formability for deep shells, moderate strength.
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  6061-T6 aluminiumMachinable, good for inserts and sealing faces.
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  SPCC / galvanized steelBetter low-frequency shielding, heavier, needs edge protection.
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  Not idealVery thin walls under 0.8 mm on potted, high-vibration tanks.

A typical housing runs through laser cutting or fine blanking, then bending, then welding, then machining. Each step leaves a mark on the next one. Cut edges from laser cutting carry a small heat-affected zone. If those edges become a sealing surface later, they need machining, not sanding.

Bending sets the corner radii and the flange angles. A press brake with the right punch radius keeps the inner radius consistent, which matters when the core sits against that wall. Over-bending to compensate for springback is normal, but it should be measured on the first article, not guessed.

Welding is where flatness usually dies. A continuous TIG seam puts more heat into the part than a stitch weld, and the shrinkage pulls the flange. For housings that will be potted, a controlled stitch pattern with a chill block behind the flange often holds flatness better than a full seam. Leak-tight parts still need a full seam, so plan a stress relief and a finish-machining pass after welding.

Machining comes last for a reason. Once the shell is welded, the mounting feet, the core seat and any threaded holes are cut in one setup so they share a datum. That is how a 4,000 mm maximum processing size machine can hold a long housing straight while the bores stay on center.

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  CutLaser or fine blank; leave stock on faces that get machined.
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  FormPress brake or draw; control inner radius and springback.
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  JoinTIG, laser or spot weld; plan for shrinkage.
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  MachineOne setup for feet, core seat and threads.

Pure sheet metal cannot hold every feature a resonant tank needs. The core seat usually has a flatness and a position tolerance tighter than a formed wall can deliver. Threaded inserts, dowel holes, busbar slots and sealing grooves all want machined geometry. This is why the housing is a hybrid: formed shell plus machined interfaces.

A 5-axis machining center earns its place on these parts. Angled ports, sloped sealing faces and features on more than one side can be cut in a single setup, so the position error between them stays small. On a part with a core seat on one face and a connector flange on another, that single-setup approach is the difference between a stack of tolerances and one datum.

Tolerances need to match the function. A mounting hole can sit at ±0.1 mm and still work. The core seat and the sealing groove often need ±0.005 mm and a fine finish. Putting tight tolerances everywhere raises cost and does not improve the converter. Mark the features that actually set the magnetic gap and let the rest breathe.

Surface finish follows the same logic. A sealing face at Ra 0.8-1.6 μm holds an O-ring or a gasket. A conductive grounding pad may need a brighter finish and a masked anodize so the coating does not insulate the joint. Specify the finish per face, not per part.

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  Single setupKeeps core seat, ports and feet in one coordinate frame.
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  Threaded insertsMachined or installed after welding, not through a thin wall.
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  Sealing groovesMachined after stress relief so flatness holds.

A sheet metal shell is not always the right call. When the tank carries very high current and the wall itself would see strong eddy currents, a non-metallic or a laminated structure can be better. When the housing must also be a pressure vessel or a liquid-cooled jacket with internal channels, a casting or a machined body usually wins.

Very small tanks with complex internal ribs are another poor fit. Forming those ribs in sheet metal takes several dies and still leaves tolerance stack-up. A machined or die-cast body handles that geometry in fewer steps. The same applies when the wall needs a thick boss for a high-load fastener; a formed wall cannot carry it without a welded insert.

Thermal path is the last boundary. If the inductor heat has to cross a long, narrow flange before it reaches the cold plate, the sheet metal housing will run hotter than a design with a direct machined pad. Adding a machined heat spreader inside the shell can fix it, but at that point the part is a hybrid assembly, not a simple box.

None of this makes sheet metal a weak choice. It makes it a specific one. For a potted resonant tank with moderate current, a formed and machined aluminium housing is often the lightest and cheapest way to hold the core, shield the field and get heat out. Outside that window, look at other processes first.

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  Consider alternatives whenLiquid-cooled jackets, internal ribs or heavy bosses are required.
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  Stay with sheet metal whenThe tank is potted and current is moderate.