Application of Inductance in Power Distribution Lines

    The problem caused by inductance in the power cord is more serious than resistance. The rapidly changing current causes a voltage deviation between the power supply and the logic circuit it supplies through the inductance of the power supply. These voltage offsets change more rapidly and much larger than those caused by wiring impedance.

    Unfortunately, the sensing line circuit cannot respond quickly enough to correct wiring inductance.

    Here are three methods to solve the inductance problem of power wiring:

    Using wiring with lower inductance

    Using logic circuits to eliminate interference from power source noise

    Reduce the variation of power supply current

    Because inductance is a logarithmic function of diameter, it is almost impossible to reduce wiring inductance simply by using thicker wires.

    The following equation represents the inductance of two parallel power distribution lines:

    Among them: X=warp length, IN

    H=average distance between two lines, IN

    D=Line diameter, IN

    L=Inductance, NH

    According to the above formula, even a wire with a very large diameter has a large inductance. Using a wide, flat, and parallel distribution line is much better than using circular leads. The distribution line with the lowest inductance uses multiple parallel flat ribbon cables to place the power and ground on different layers.

    The following equation represents the inductance of parallel stacked flat cables.

    Where: X=length of flat cable, IN

    H=spacing between two flat cables, IN

    W=width of flat cable, IN

    N=number of flat cables (2 for single ground single power supply, 3 for dual ground single power supply, etc.)

    L=Inductance, NH

    In fact, differential transmission can resist power fluctuations and interference. For communication between printed circuit boards, differential drivers and receivers would be very effective due to the lack of methods to provide low-cost, low impedance power distribution. The cost and additional space required for differential transmission are usually much smaller than the cost and space required for improving power distribution cables.

    The final method to reduce the impact of inductance on power supply lines is to reduce the amplitude of changing currents. Note that we are using the term ‘variable current‘. Although it is not possible to reduce the average current flowing through the power line. But it can completely reduce the rate of change of the current. The next section will demonstrate how to use board level bypass capacitors to achieve this goal.