AN9768 Datasheet, PDF(1/8 Page) Littelfuse – Transient Suppression Devices and Principles

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AN9768 Datasheet, PDF (1/8 Pages) Littelfuse – Transient Suppression Devices and Principles

Transient Suppression Devices and Principles

Application Note

January 1998

AN9768

[ /Title

(AN97

68)

/Sub-

ject

(Tran-

sient

Sup-

pres-

sion

Device

s and

Princi-

ples)

/Autho

r ()

/Key-

words

(TVS,

Tran-

sient

Sup-

pres-

sion,

Protec-

tion,

High-

reli-

ability,

High

Reli-

ability,

Mil,

Transient Suppression Devices

There are two major categories of transient suppressors: a)

those that attenuate transients, thus preventing their

propagation into the sensitive circuit; and b) those that divert

transients away from sensitive loads and so limit the residual

voltages.

Attenuating a transient, that is, keeping it from propagating

away from its source or keeping it from impinging on a

sensitive load is accomplished with ï¬lters inserted in series

within a circuit. The ï¬lter, generally of the low-pass type,

attenuates the transient (high frequency) and allows the signal

or power ï¬ow (low-frequency) to continue undisturbed.

Diverting a transient can be accomplished with a

voltage-clamping type device or with a âcrowbarâ type

device. The designs of these two types, as well as their

operation and application, are different enough to warrant a

brief discussion of each in general terms. A more detailed

description will follow later in this section.

A voltage-clamping device is a component having a variable

impedance depending on the current ï¬owing through the

device or on the voltage across its terminal. These devices

exhibit a nonlinear impedance characteristic that is, Ohmâs

law is applicable but the equation has a variable R. The

variation of the impedance is monotonic; in other words, it

does not contain discontinuities in contrast to the crowbar

device, which exhibits a turn-on action. The volt-ampere

characteristic of these clamping devices is somewhat

time-dependent, but they do not involve a time delay as do

the sparkover of a gap or the triggering of a thyristor.

With a voltage-clamping device, the circuit is essentially

unaffected by the presence of the device before and after the

transient for any steady-state voltage below the clamping

level. The voltage clamping action results from the increased

current drawn through the device as the voltage tends to

rise. If this current increase is greater than the voltage rise,

the impedance of the device is nonlinear (Figure 1). The

apparent âclampingâ of the voltage results from the

increased voltage drop (IR) in the source impedance due to

the increased current. It should be clearly understood that

the device depends on the source impedance to produce the

clamping. One is seeing a voltage divider action at work,

where the ratio of the divider is not constant but changes.

However, if the source impedance is very low, then the ratio

is low. The suppressor cannot be effective with zero source

impedance (Figure 2) and works best when the voltage

divider action can be implemented.

1000

100

Z = 1â¦

NONLINEAR Z (Î± = 5)

Z = 10â¦

Z = 500â¦

IMPEDANCE

LINEAR

Z = 1â¦

0.01

0.1

100

CURRENT (A)

LINEAR IMPEDANCE: I =

NONLINEAR IMPEDANCE (POWER LAW): I = KVÎ±

1000

FIGURE 1. VOLTAGE/CURRENT CHARACTERISTIC FOR A

LINEAR 1â¦ RESISTOR AND NONLINEAR

VOC

VZV

ï£«

ï£¬

ï£

Z----V----Z--+--V---Z----S--ï£¸ï£·ï£¶

VOC

FIGURE 2A. VOLTAGE CLAMPING DEVICE

VOC

SCR

VZV

FIGURE 2B. CROWBAR DEVICE

FIGURE 2. DIVISION OF VOLTAGE WITH VARIABLE

IMPEDANCE SUPPRESSOR

10-102

▷