Design Guide Formulas
Calculating conductor size is very important to the electrical and mechanical properties of a bus bar. Electrical current-carrying requirements determine the minimum width and thickness of the conductors. Mechanical considerations include rigidity, mounting holes, connections and other subsystem elements. The width of the conductor should be at least three times the thickness of the conductor.
Capacitance of the bus arrangement depends upon the dielectric material and physical dimensions of the system. Capacitance varies only slightly with frequency change, depending on the stability of the dielectric constant. This variation is negligible and therefore is omitted in this analysis:
Because of skin effect phenomena, inductance and resistance are dependent on frequency. At high frequency, currents tend to flow only on the surface of the conductor. Therefore the depth of penetration of the electromagnetic energy determines the effective conducting volume.
Maintaining a low inductance results in a low characteristic impedance and greater noise attenuation. When minimum inductance is a design objective, consider these tips:
- 1. Minimize the dielectric thickness.
- 2. Maximize the conductor width.
- 3. Increase the frequency.
There are two types of inductance to be determined: internal inductance, which is a result of flux linkages within a conductor, and external inductance, which is determined by the orientation of the two current carrying conductors.
To calculate the DC conductor resistance, the following formula applies (Resistance at 20°C):
As current travels across a conductor, it loses voltage. This is caused by the resistivity of the conductor. The losses are referred to as voltage drop. Use this formula to calculate the voltage drop across the conductors:
In the design of laminated bus bars, you should consider maintaining the impedance at the lowest possible level. This will reduce the transmission of all forms of EMI (electromagnetic interference) to the load.