Primary and induced currents from cable discharges

Analysis of cable discharge events (CDE) starts with the basic wire-to-ground pulse and continues, for shielded cables, with induced currents on the signal lines. These situations can be classified and analyzed through step response and impulse response. The essential behaviors can be derived from the uniformly charged shield being grounded to produce induced current on the data lines at the opposite end. This effect is called the W-pulse and is shown to be a kind of step response for both loads. From this and similar results, the effects on shielded cable lines of the commonly-applied IEC 61000-4-2 platform pulse are derived by convolution. Finally, the induced currents resulting from a charged shielded cable above a ground plane are deduced. Because the coupling from shielded cables to internal lines is almost all inductive, we have bipolar induced pulses with zero charge integral. Thus the bipolar data line response to these shielded cable pulses is a significant hazard for pulsed latchup. We discuss adequacy of the low-voltage IEC direct pin pulse (often called direct pin zap or DPZ) for assessing pulsed latchup on external ports, shielded in particular. Finally, it is shown why having connected cables as part of the regular IEC platform test also helps to test for cable discharge latchup hazards.