Influence of Busbar Thickness on Current Sharing Performance

Recently, during customer testing of busbars, it was found that busbars with identical shape but different thicknesses exhibit inconsistent current sharing performance. This paper analyzes the underlying causes as follows:

Core Conclusion

Changing the overall thickness of the busbar leads to a notable variation in the current difference between branch AB and branch AC. The thinner the busbar, the more severe the current imbalance between the two branches.

1.Simplified Busbar Topology Model

Current flows into the busbar at point A, with two output terminals on the right: terminal B in the middle and terminal C on the far right. Two conductive branches are formed:

• Branch 1 (A → B): Short conductive path.
• Branch 2 (A → B → C): Relatively long conductive path.

The two branches share a single integrated busbar. The current imbalance is caused by different path lengths inside the same busbar, rather than parallel connection of separate busbars.

2. Performance under DC and Power Frequency Operating Conditions

DC Resistance Formula

here:

t = busbar thickness, L = branch length, W = busbar width.

The resistance of each branch is inversely proportional to the busbar thickness. A reduction in thickness increases the resistance of all branches and amplifies the absolute resistance difference between the long and short branches.

Case Analysis

Assume the length of path A-B is L1, and the length of path A-C is L2=2L1 . The cross-sectional area is defined as S=W* t.

• For a thicker busbar with thickness t1: R1=ρ*L1/S1 , R1=ρ*2L1/S1 , and the resistance ratio R2/R1=2 .
• For a thinner busbar with thickness t2=0.5t1 : S_2=0.5S1 , R1'=2R1 , R2'=2R2 . The resistance ratio remains 2.

However, the absolute resistance difference increases significantly: the resistance difference of the thicker busbar is Δ R=ρ*L1/S1 , while that of the thinner busbar rises to ΔR'=2ρ*L1/S1 .

In parallel circuits, current is inversely proportional to resistance. A larger resistance difference results in a greater current deviation between branches and worse current sharing.

In summary:

• Increasing busbar thickness reduces the overall resistance of branches and narrows the resistance difference, achieving better current balance.
• Reducing busbar thickness raises the overall branch resistance and widens the resistance difference, leading to a much larger current gap between terminal B and terminal C.

3. Performance under Medium and High Frequency Operating Conditions

For inverters, switching power supplies, energy storage converters and other equipment operating at kilohertz frequencies, skin effect, internal eddy current and stray inductance will jointly affect current distribution. The impact of thickness on current sharing is far more drastic than that under DC conditions.

3.1 Constraint of Skin Effect

The skin depth δ of alternating current is very small at high frequencies.

• When the busbar thickness t >> δ , increasing thickness barely expands the effective conductive area, so the AC impedance difference between the two branches changes slightly.
• When the busbar thickness is close to or smaller than the skin depth, the effective conductive area drops sharply as thickness decreases. Eddy current loss of the long path (A-C) rises dramatically, causing a sharp increase in its AC impedance. Most current will flow to branch A-B, resulting in severe current imbalance.

3.2 Amplified Difference of Stray Inductance

The longer conductive path (A-C) encloses a larger magnetic flux area, so its stray inductance L2 is greater than L1 of the short path. A thinner busbar means a smaller cross-sectional area, which increases the overall stray inductance of each branch and further widens the inductance difference between the two paths.

High-frequency circuits feature a large current change rate di/dt. The voltage difference caused by inductance will drive more current to the short branch A-B and aggravate current imbalance.

3.3 Positive Feedback of Temperature Rise

Thinner busbars generate more heat under operation. The long path A-C has a higher current density and operating temperature. The resistivity of copper increases with temperature, which further raises the resistance of branch A-C. Consequently, more current flows to branch A-B, and the current deviation keeps expanding.

4. Engineering Summary

4.1 For a single integrated busbar with one input (A) and two outputs (B & C), thickness is a key factor affecting current sharing.

•  
  • A thicker busbar lowers the overall resistance and inductance of branches, reduces impedance deviation, and realizes well-balanced current between B and C.
  • A thinner busbar enlarges the impedance difference between branches, and the output current of B is much higher than that of C.
  • For high-frequency switching equipment, the deterioration of current sharing caused by thickness variation is far more obvious than that under DC or power frequency conditions.
  • Optimization Solutions
• Appropriately increase busbar thickness to reduce branch impedance difference.
• Optimize layout and shorten the length difference between the two conductive paths.
• Connect copper bars or copper foils with identical specifications in series at terminals B and C to match branch impedance and improve current sharing.

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