Generally speaking, a multi-layer bus bar consists of multiple, usually planer, conductors insulated from one another with an appropriate dielectric material.  The conductors are typically formed to accommodate component installation and arranged to promote satisfactory circuit performance.

That said, multi-layer bus bars can be relatively simple or very complex depending on the number of conductors, required withstand voltage rating, current rating, component terminals and any other stipulation incurred by the specific application.

1.  Conductor Element Design
When designing your conductor elements you must carefully consider your  current handling requirement(s) and electrical layout

Current handling
As a general rule 0.000375 in². of copper cross section per 1 ampere is recommended for sufficient current handling.  The table below lists the recommended minimum conductor widths required for three common copper sheet thicknesses (1/8”, 1/16” & 1/32”) when applied to a broad range of currents.

Rated Current (Amperes)	Recommended Copper Conductor Cross Section inches2	Minimum Recommended Conductor Width
		Thickness 0.125"	Thickness 0.0625"	Thickness 0.0312"
10 A	.00375 in2	N/A	N/A	0.120 in
20 A	.00750 in2	N/A	0.120 in	0.240 in
50 A	.01875 in2	0.150 in	0.300 in	0.600 in
100 A	.0375 in2	0.300 in	0.600 in	1.200 in
200 A	.075 in2	0.600 in	1.200 in	2.400 in
300 A	.1125 in2	0.900 in	1.800 in	3.600 in
400 A	.150 in2	1.200 in	2.400 in	4.800 in
500 A	.1875 in2	1.500 in	3.000 in	6.000 in

 

It is recommended that the bus bar designer make their conductor elements as wide (assuming > the minimum cross section) as practical.  Doing so will help reduce the overall inductance of the bus assembly.  For high speed switching applications, bus inductance is considered to be highly undesirable and will result in excessive “over-shoot voltage” which will, to some level, compromise system efficiency and can potentially damage semiconductors.

The approximate inductance (L=nano Henries) of a 2 conductor bus bar assembly can be estimated using the following formula:

L=32 d lC/wC

where:

32 = 32pico Henries/mil
d = Distance between conductors
lC = Conductor Length
wc = Conductor Width

This formula assumes the 2 conductor’s geometries match exactly and that the supply and return current paths are routed so that they directly oppose each other.

 

2.  Layout
When laying out a bus bar assembly, aside from mechanical fit (size) and making the appropriate electrical connections, both of which are application specific, the designer, typically, has three other layout dependent characteristics to take into consideration. Each being as important as the other and more or less difficult to achieve depending on the application.  These are:

• Optimize inductance (typically the lowest possible)
• Promote satisfactory current distribution (particularly important when paralleling components)
• Maintain proper strike and surface creep distances between polarities

Optimizing Inductance:
As mentioned in the previous section, in order to achieve optimal (low) inductance, circuit paths must be routed so that equal amounts of current passing through one plane are directly opposed by the other plane.  In essence, the two planes should be identical mirror images of one another.  In practice, this is typically impossible to achieve due to the necessity of offset component mounting holes or other mechanical obstacles; however, one should attempt to mirror the planes as closely as possible. 

In most instances, if a satisfactory level of inductance can not be achieved with the bus bar alone,  bypass capacitors can be added in close proximity of the switching devices (semiconductors) to minimize the resulting overshoot voltage (V=Ldi/dt).

If you have questions regarding optimizing inductance of a bus bar assembly or the use of bypass capacitors please contact our sales department or applicable International Sales Representative.

Current Distribution:
When paralleling components, proper current distribution is a paramount concern.  The impact of improperly distributed current goes without saying. 

In most cases, the designer’s goal is to equally distribute load current among multiple like components (semiconductors, capacitors, resistors…etc).   To do this, the designer must match the bus bar resistance and inductance to each of the components.  The most apparent solution is by creating geometrically identical paths to each of the like components.  When this isn’t achievable, one may employ another technique commonly referred to as the “First-In Last-Out” method.  Using this technique, the component given the lowest impedance “In” path would be assigned the highest impedance “Out” path.  This approach is particularly attractive when paralleling more than two like components. 

If you have questions regarding current distribution within a bus bar assembly or would like to know more about paralleling components please contact our sales department or applicable International Sales Representative.

Strike and Surface Creep:
Probably one the most overlooked aspects of designing a bus bar is maintaining proper strike and creep clearance between polarities.  Usually this occurs in the regions where multiple terminal components are installed.  For example: The terminals of an IGBT module are designed to meet strike and creep requirements associated with its rated voltage.  The IGBT module designer has gone to great lengths to satisfy these requirements by using the appropriate spacing and molded elements like elevated islands, barriers or repeating barriers.   The photo below demonstrates the use of these elements.

The common mistake of many laminated bus designers is to not plan for similar features on their bus bar assembly.  Simply attaching this unit to a flat bus plane having no barriers between its mating terminals would spell disaster. 

That said, not all laminated bus bar manufactures have the ability to include similar features; however,  CEI has spent many years developing it’s Large Scale Molding capability and can incorporate these necessary features as well as a variety of other reliability enhancing elements into your next laminated bus bar design.

If you have questions regarding creep and strike or CEI’s large Scale Molding capability, please contact our sales department or applicable International Sales Representative.