Use transient voltage suppression diodes to ruggedize circuits and maintain electrical integrity
state (called avalanche breakdown) to absorb the large current and clamp the voltage to a safe level. TVS diodes are available as unidirectional or bidirectional P-N junction devices. Despite the names, most unidirectional TVS diodes suppress voltages in both polarities. The difference is that unidirectional types have asymmetrical voltage-current (V- I) properties, while bidirectional TVS diodes have symmetrical V-I properties (Figure 8). Bidirectional TVS diodes are well-suited for protecting electrical nodes with signals that are bidirectional or both above and below the ground voltage. Top-tier parameters, packaging, and placement define TVS performance TVS diodes are defined by many high-level specifications. Among them are: ■ Nominal reverse working maximum voltage (V RWM ): Also called reverse standoff voltage, this is the maximum operating voltage of a TVS diode when it is “OFF” ■ Breakdown voltage (V BR ): The voltage at which avalanche breakdown occurs in a TVS diode, resulting in low impedance
CLASSIFICATION
SUCCESSFUL ESD PERFORMANCE
Class 1
0 V to 1,999 V
Class 2
4 2,000 V to 3,999 V
Class 3
4,000 V or greater
Figure 6: There are three levels of ESD sensitivity classifications per MIL-STD-883 Method Number 3015. (Image source: Eaton)
Note that EFTs are often characterized simply by two paired numbers: their rise time to peak value (t 1 ), and the pulse duration until the transient falls to 50% of peak value (t 2 ). The 8/20 microsecond (µs) transient is a common pulse in industrial applications. The magnitude of transient- voltage ESD a circuit or system must withstand depends on the application. Three classes are defined by MIL-STD-883, which is widely used by the industry, as well as military and aerospace systems (Figure 6). TVS devices solve the problem To meet various requirements and protect their systems, designers can use TVS diodes. TVS diodes are silicon overvoltage protection devices that work based on the diode avalanche-breakdown principle. They are installed parallel
with the normal circuit to protect internal components from short- duration (transient) and medium/ high voltages (Figure 7).
Figure 7: The TVS diode is placed across the input, between the line being protected and the system ground. (Image source: Eaton) In normal, non-transient operation, TVS diodes maintain a high impedance and do not interfere with power or signal transmission through equipment. However, when a TVS diode experiences an instantaneous, high-energy shock across its terminals, it protects downstream circuit elements by rapidly entering a low-impedance
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