Key takeaways
- An inverter or UPS busbar is a current path, a mechanical frame and a heat path at the same time.
- Loop inductance is set by the area between the outgoing and return conductors, so lamination and overlap do more than extra copper.
- Capacitor landings and device pads decide current sharing and joint temperature as much as the bar section does.
- Specify the ambient inside the enclosure, the ripple current, the fault level and the cooling method before a bar is sized.
Two current paths, one assembly
A busbar in an inverter or an uninterruptible power supply is not a single conductor. It is the short, wide path between the DC link capacitor bank, the switching devices and the AC output. In a double conversion UPS the same assembly also carries the rectifier output and the bypass path. The shape is usually a laminated stack or a set of formed bars bolted into a compact frame.
The electrical job is to move a large current through a small space with the lowest practical inductance and the least heat. The mechanical job is to hold the devices, the capacitors and the output terminals in a fixed relationship so the assembly can be built and tested as one unit. The thermal job is to conduct heat away from the joints and into the enclosure or the heat sink.
Those three jobs compete. A wider bar lowers resistance and helps cooling, but it takes space and adds mass. A thinner dielectric lowers inductance, but it raises the electric field. A shorter path reduces loss, but it can put a joint where there is no room for a wrench. The design is the set of compromises that satisfies the switching margin, the temperature rise and the build.
Why the laminate matters in a switching circuit
When a power device turns off, the current in the commutation loop falls in a short time. The busbar inductance produces a voltage in proportion to how fast the current changes, and that voltage adds to the DC link voltage across the device. A smaller loop gives a smaller overshoot, which leaves more margin against the device rating and reduces the snubber or clamp needed.
The loop area is the thing to control. If the outgoing and return conductors are far apart, the loop is large and the inductance is high. If they are bonded close together with a thin dielectric between them, the loop is small. The effect is strongest when the two paths are wide, run in parallel and carry current in opposite directions, because their fields partly cancel.
Lamination also fixes the spacing. A stack that is bonded and cured has the same layer separation in every unit, so the inductance is repeatable from the first build to the last. That repeatability is useful when the switching behaviour has to be verified once and then trusted across a production run.
- Keep the DC positive and DC negative conductors adjacent over the whole commutation path.
- Overlap the capacitor landing with the device pads so the current does not have to turn through a narrow neck.
- Keep the distance between the capacitor bank and the switching devices as short as the layout allows.
- Separate a control or signal layer from a high dv/dt layer with a screen or a greater dielectric thickness.
- Avoid abrupt width changes, which add local inductance and create a hot spot.
Cooling the bar, not just the device
Heat in the busbar comes from three places: the resistance of the conductor, the resistance of each joint and the heat radiated or conducted from the devices. In a compact inverter the joint contribution can be significant, because a bolted or clamped joint has a contact resistance that rises if the faces are rough, oxidised or under-torqued.
The bar can be cooled by natural convection inside the enclosure, by forced air, or by conduction to a heat sink through a thermal interface. Conduction is the most direct path when the bar is close to a metal surface, but it needs a controlled flatness and a pad or paste that does not dry out. Forced air is effective when the bar has fins or when the airflow passes over the joint faces, but the air path has to be designed rather than assumed.
A temperature rise test on a complete assembly is more useful than a calculation on the bar alone. The test shows the joint temperature, the effect of the enclosure and the interaction with the devices. If a calculation is used for the first design, it should be checked against a test before the drawing is released.
A busbar is part of the thermal circuit
Treat the bar, the joint and the mounting surface as one path. A cold bar with a hot joint is a joint problem, not a bar problem.
Sizing for continuous current, ripple and fault
The continuous current sets the conductor section, but it is not the only current. A switching inverter has a ripple component at the switching frequency, and a UPS may carry harmonic current from the load. The effective heating current is higher than the fundamental, so the section has to be chosen for the root mean square current at the expected ambient inside the enclosure.
The fault current sets the bracing and the joint duty. A short circuit produces a large electromagnetic force between conductors, and the supports have to hold the bar until the protection clears the fault. The force rises quickly as the current rises and as the distance between conductors falls, so a compact laminated stack can see high internal forces even though the external clearance is small.
The table below lists the inputs that a supplier needs before sizing a bar. Without them, a quote is based on a guess about the enclosure, the cooling and the protection.
| Input | Why it matters | Where it comes from |
|---|---|---|
| Continuous current | Sets the conductor section and the loss | Load schedule and rating |
| Ripple and harmonic current | Raises the heating current above the fundamental | Switching design and load profile |
| Ambient inside the enclosure | Sets the allowable temperature rise | Thermal survey or enclosure test |
| DC link voltage | Sets the dielectric thickness and clearance | Circuit design |
| Prospective fault current | Sets the bracing and joint duty | Protection study |
| Fault clearing time | Sets the thermal duty during a fault | Breaker or fuse curve |
| Capacitor bank layout | Sets the loop area and current sharing | Mechanical layout |
| Device package and pad geometry | Sets the termination and the heat path | Device datasheet |
| Cooling method | Sets the thermal model and the finish | Enclosure design |
The values are project specific. A bar sized for one enclosure and cooling method should not be reused at another rating without a new check.
Capacitor landings and device pads
A capacitor bank is often built from several devices in parallel. The landing on the busbar has to spread the current evenly between them, or the nearest capacitor carries more current and ages faster. A wide, symmetric landing with a short path to each capacitor terminal does more for sharing than a thicker bar.
The device pads have the same problem. A pad that is narrower than the package terminal adds resistance and creates a hot spot at the edge of the contact. The pad should cover the terminal, support the washer and leave enough creepage to the next layer. If the device is pressed into the bar, the hole and the interference have to suit the laminate and the insertion force.
The assembly order matters at the termination. If the bar is bolted to the devices before the stack is aligned, the laminate can be twisted and the joint faces can be preloaded unevenly. The mounting should locate the bar first, then allow the fasteners to clamp it without bending it.
- Keep the capacitor landing symmetric so parallel devices share current.
- Match the device pad to the terminal and keep the washer fully supported.
- Leave a defined creepage distance from the pad to the layer edge.
- Specify the torque and the fastener, and mark the joint after tightening.
Build, test and first article
The first article of an inverter busbar should confirm the current path, the insulation and the mechanical fit. Continuity from the capacitor landing to the device pad checks the conductor. A layer-to-layer insulation resistance and a withstand voltage check the dielectric. A partial discharge measurement is useful on higher voltage classes, because a void or a sharp edge can pass a short test and fail in service.
The joint checks matter as much as the electrical ones. A millivolt drop test shows the contact resistance at the specified torque. A thermal or power cycle on a sample shows whether the joint and the laminate stay stable as the assembly heats and cools. Dimensional checks should cover the pad positions, hole pattern, stack height and mounting face flatness.
For production, the routine test can be shorter: continuity, insulation, dimensions and a visual check of the plating and the edges. The full set belongs on the first article and on any change to the laminate, the plating or the termination.
What to send for a useful quote
Send the circuit diagram, the mechanical layout, the device datasheets and the capacitor datasheet. Add the current profile, the ambient, the cooling method, the fault level and the protection curve. State the voltage class and the pollution degree.
If the design is still moving, send the concept and mark the fixed interfaces: the capacitor terminals, the device pads, the output studs and the mounting points. A supplier can then price the stack and flag the features that will change the number. That is more useful than a single price built on an assumed layout.
- Single line diagram and commutation loop sketch
- Mechanical layout with device and capacitor positions
- Current profile, ripple and fault level
- Ambient inside the enclosure and the cooling method
- Voltage class, pollution degree and altitude
- Termination type and torque requirement
- Annual volume and release size
Frequently asked questions
Why are inverter busbars laminated rather than made from separate bars?
Lamination holds the outgoing and return conductors close together with a thin dielectric, which reduces the loop area and the inductance. It also fixes the spacing so the inductance is repeatable and the assembly can be fitted as one part. Separate bars are easier to change but usually have a larger loop and looser spacing.
How do I reduce voltage overshoot across a switching device?
Reduce the commutation loop inductance by overlapping the outgoing and return paths, shortening the distance between the capacitor bank and the device, and widening the current path. Check the result with a double pulse test or a switching test on the assembly, because the layout and the device package both affect the overshoot.
Should the busbar be cooled by air or by conduction?
It depends on the enclosure and the joint temperature. Conduction to a heat sink gives a direct path but needs flatness and a stable thermal interface. Forced air works when the airflow reaches the bar and the joints. Natural convection is often enough at lower current. A temperature rise test on the assembly is the practical check.
What current should I use to size an inverter busbar?
Use the root mean square current at the expected ambient inside the enclosure, including ripple and harmonic content, then check the fault current and clearing time for the bracing and joint duty. A section chosen on the fundamental current alone can run too hot.
Sources and further reading
- IEC 60664-1, Insulation co-ordination for equipment within low-voltage supply systems
- IEC 62040-1, Uninterruptible power systems, general and safety requirements
- Qumek inverter and UPS busbar design and first article reviews
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Designing a busbar for an inverter or UPS?
Send the layout, current profile, fault level and cooling method. The engineering desk will review the loop, the terminations and the first article plan.
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