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Transient Voltage Suppression Diodes

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Transient Threats – What Are Transients?

Voltage Transients are deﬁned as short duration surges of

electrical energy and are the result of the sudden release

of energy previously stored or induced by other means,

such as heavy inductive loads or lightning. In electrical

or electronic circuits, this energy can be released in a

predictable manner via controlled switching actions, or

randomly induced into a circuit from external sources.

Repeatable transients are frequently caused by the

operation of motors, generators, or the switching of

reactive circuit components. Random transients, on the

other hand, are often caused by Lightning and Electrostatic

Discharge (ESD). Lightning and ESD generally occur

unpredictably, and may require elaborate monitoring to be

accurately measured, especially if induced at the circuit

board level. Numerous electronics standards groups have

analyzed transient voltage occurrences using accepted

monitoring or testing methods. The key characteristics of

several transients are shown in the table below.

VOLTAGE

CURRENT

RISE-TIME DURATION

Lighting

25kV

20kA

10 μs

1ms

Switching

600V

500A

50μs

500ms

EMP

1kV

10A

20ns

1ms

ESD

15kV

30A

<1ns

100ns

Table 1. Examples of transient sources and magnitude

Characteristics of Transient Voltage Spikes

Transient voltage spikes generally exhibit a “double

exponential” wave, as shown below for lightning and ESD.

Vp/2

Figure 1. Lightning Transient Waveform

100%

90%

10%

30n

60n

tr = 0.7 to 1.0ns

Figure 2. ESD Test Waveform

The exponential rise time of lightning is in the range

1.2μsec to 10μsec (essentially 10% to 90%) and the

duration is in the range of 50μsec to 1000μsec (50% of

peak values). ESD on the other hand, is a much shorter

duration event. The rise time has been characterized at less

than 1.0ns. The overall duration is approximately 100ns.

Why are Transients of Increasing Concern?

Component miniaturization has resulted in increased

sensitivity to electrical stresses. Microprocessors for

example, have structures and conductive paths which

are unable to handle high currents from ESD transients.

Such components operate at very low voltages, so

voltage disturbances must be controlled to prevent device

interruption and latent or catastrophic failures.

Sensitive microprocessors are prevelant today in a wide

range of devices. Everything from home appliances, such

as dishwashers, to industrial controls and even toys use

microprocessors to improve functionality and efﬁciency.

Most vehicles now also employ multiple electronic

systems to control the engine, climate, braking and, in

some cases, steering, traction and safety systems.

Many of the sub- or supporting components (such as

electric motors or accessories) within appliances and

automobiles present transient threats to the entire system.

Careful circuit design should not only factor environmental

scenarios but also the potential effects of these related

components. Table 2 below shows the vulnerability of

various component technologies.

DeviceType

Vulnerability (volts)

VMOS

30-1800

MOSFET

100-200

GaAsFET

100-300

EPROM

100

JFET

140-7000

CMOS

250-3000

Schottky Diodes

300-2500

Bipolar Transistors

380-7000

SCR

680-1000

Table 2: Range of device vulnerability.

Overvoltage Suppression Facts

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