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Two-Terminal Transient Suppressor Disclosure Number: IPCOM000010946D
Publication Date: 2003-Feb-03
Document File: 6 page(s) / 427K

Publishing Venue

The Prior Art Database


Key Words: Transient Voltage Supressor, Two-terminal Device, Load Dump, Avalanche Mode

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Two-Terminal Transient Suppressor  


Key Words: Transient Voltage Suppressor, Two-terminal Device, Load Dump, Avalanche Mode

The automobile’s transient voltage suppressor functions to protect these FETs and other devices from voltage spikes that exceed their maximum rated voltages, thus avoiding damage that could otherwise occur to these devices.

FIG. 1 illustrates a simplified block diagram of an automotive system 100 that includes a transient voltage suppressor 108.

During normal operations, alternator 104 charges battery 102, and switch 110 is closed, meaning that the battery cable connects battery 102 and alternator 104.  Accordingly, the charge stored in the alternator magnetics is absorbed by battery 102.  At times, alternator 104 could be supplying 60 to 100 amps to battery 102, depending on the battery’s state of charge.

During a load dump, a voltage spike of 80 volts or more over several milliseconds could be encountered, and all subsequent circuitry must be able to withstand this load dump or potentially be damaged.

Rather than requiring automobile electronics 106 to be capable of absorbing excessive charges associated with a load dump, other systems provide a transient suppressor 108 for that purpose.  Desirably, transient suppressor 108 will begin diverting excess charge at a voltage (referred to herein as the “clamp voltage”) that is higher than the battery voltage, but lower than a voltage that will damage electronics 106.  For example, in a 14-volt system, a typical clamp voltage specification might require suppressor 108 to divert the charge when the voltage is between 24 and 58 volts.

In a proposed 42 volt system, voltage transients could occasionally reach 120 volts or higher.  It would be impractical for the industry to specify that all automotive circuits must be redesigned to be able to handle these high transients.  Accordingly, automotive transient voltage suppressors in 42-volt systems will likely have a clamp voltage specified in a range of about 44 to 58 volts. 

Unfortunately, the clamp voltage range specified for 42-volt systems is significantly narrower than the range specified for 14-volt systems.  Accordingly, neither the zener diode nor MOV technologies would be capable of consistently keeping their breakdown voltages within the required range.  Specifically, because a load dump’s high energy could cause extremes in device temperatures, the breakdown voltage of the zener diode could easily exceed the specified maximum clamp voltage.  In addition, a MOV would be too resistive to be used in a 42-volt system, by the time the transient reached 100 amps, the automobile’s circuitry would likely have experienced voltages that greatly exceed the circuitry’s capabilities.  Thus, the MOV voltage suppressor is too resistive to be useful in a 42-volt system.

FIG. 2 illustrates cost effective two-terminal voltage suppressor 208 using a clamp circuit.  The diodes have a near zero temperature coefficient for break...