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Digital Circuit Boards: Mach 1 GHz by Ralph Morrison

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4.15 Power Dissipation

At high clock rates, it can be assumed that most of the logic traces will be series terminated in their characteristic impedance. Consider the series terminated case where current flows until the wave associated with each logic transition makes one round trip. The energy stored in the electric field is

4.7 4.7

where n is the number of logic transitions per second, V is the power supply voltage, d is the average trace length, ε is the dielectric constant, c is the velocity of light, and Z is the characteristic impedance of the average line. This energy is eventually dissipated in driver resistances, in termination resistors, in the dielectric, in the copper along the paths of transmission and finally in radiation. The energy lost to radiation is usually very small compared to the heating losses. For n = 1012/s, V = 3.5 V, d = 3 cm, a board will dissipate about 10 W on logic traces. This is in addition to the dissipation in active components.

N.B.
Stored field energy is dissipated in terminations. When there are no terminations, stored energy is dissipated in radiation and IR losses from multiple reflections.

The heat generated in internal layers of the board must have a thermal path to outer conductive layers. If this is not done, the temperature rise on internal layers can cause the board to warp. If the outer layer is a ground or power plane then vias that ...

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