In systems where power is distributed over long wires, severe transients are generated by load steps (abrupt changes in load current). Negative load steps happen when load current drops from a high value to a low value. Negative changes in current (dI/dt) cause the wire's parasitic inductance to generate a positive-going high voltage spike which can cause damage to neighboring devices powered from the same wire. High values of dI/dt are produced by fast load switching, such as caused by relays, switch contacts and solid state load switching. Corroded connections between a power source and load can lead to an abrupt interruption of current flow, and a high value of dI/dt. The best example of this condition is automotive load dump, where there is a sudden break in the connection to the battery caused by vibration and corroded terminals.
Load dump causes a voltage surge that stays elevated for hundreds of milliseconds (see Figure 1). The amplitude of the transient, according to the Society of Automotive Engineers (SAE), may be as high as 125V. A typical load dump profile has a rise time of 5 milliseconds and decays exponentially with a time constant of 200ms. In industrial systems similar events can be caused by regeneration in solenoids and motors.
Figure 1: Typical Load Dump Waveform
Electronic circuits have become more prevalent in automobiles, and they must be reliable. In addition, sophisticated consumer electronics such as smart phones, laptops, MP3 players, GPS, and data entry devices that charge through automobile cigarette lighters must also protect their products from both repetitive transients and unexpected voltage spikes. Inadequate protection from high voltage transients leads to degraded performance or failure and costly replacement.
These transients pose a difficult challenge for engineers focused on protecting sensitive electronics. Historically, this protection was achieved using bulky capacitors, TVS diodes and fuses, but this discrete solution consumes a lot of real estate, and may be impractical.