AC12FSM datasheet(187/223 Pages) TECCOR

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Application Notes

AN1009

AN1009 - 3

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Thyristor Product Catalog

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Double-exponential Impulse Waveform

A double-exponential impulse waveform or waveshape of current

or voltage is designated by a combination of two numbers (tr/td or

tr x td µs). The first number is an exponential rise time (tr) or wave

front and the second number is an exponential decay time (td) or

wave tail. The rise time (tr) is the maximum rise time permitted.

The decay time (td) is the minimum time permitted. Both the tr and

the td are in the same units of time, typically microseconds, des-

ignated at the end of the waveform description as defined by

ANSI/IEEE C62.1-1989.

The rise time (tr) of a current waveform is 1.25 times the time for

the current to increase from 10% to 90% of peak value. See Fig-

ure AN1009.5.

tr = Rise Time = 1.25 • [tc – ta]

tr = 1.25 • [t(0.9 IPK) – t(0.1 IPK)] = T1 – T0

The rise time (tr) of a voltage waveform is 1.67 times the time for

the voltage to increase from 30% to 90% of peak value. (Figure

AN1009.5)

tr = Rise Time = 1.67 • [tc – tb]

tr = 1.67 • [t(0.9 VPK) – t(0.3 VPK)] = T1 – T0

The decay time (td) of a waveform is the time from virtual zero

(10% of peak for current or 30% of peak for voltage) to the time

at which one-half (50%) of the peak value is reached on the wave

tail. (Figure AN1009.5)

Current Waveform td = Decay Time

= [t(0.5 IPK) – t(0.1 IPK)] = T2 – T0

Voltage Waveform td = Decay Time

= [t(0.5 VPK) – t(0.3 VPK)] = T2 – T0

Figure AN1009.5

Double-exponential Impulse Waveform

Failure Modes of Thyristor

Thyristor failures may be broadly classified as either degrading

or catastrophic. A degrading type of failure is defined as a

change in some characteristic which may or may not cause a cat-

astrophic failure, but could show up as a latent failure. Cata-

strophic failure is when a device exhibits a sudden change in

characteristic that renders it inoperable. To minimize degrading

and catastrophic failures, devices must be operated within maxi-

mum ratings at all times.

Degradation Failures

A significant change of on-state, gate, or switching characteris-

tics is quite rare. The most vulnerable characteristic is blocking

voltage. This type of degradation increases with rising operating

voltage and temperature levels.

Catastrophic Failures

A catastrophic failure can occur whenever the thyristor is oper-

ated beyond its published ratings. The most common failure

mode is an electrical short between the main terminals, although

a triac can fail in a half-wave condition. It is possible, but not

probable, that the resulting short-circuit current could melt the

internal parts of the device which could result in an open circuit.

Failure Causes

Most thyristor failures occur due to exceeding the maximum

operating ratings of the device. Overvoltage or overcurrent oper-

ations are the most probable cause for failure. Overvoltage fail-

ures may be due to excessive voltage transients or may also

occur if inadequate cooling allows the operating temperature to

rise above the maximum allowable junction temperature. Over-

current failures are generally caused by improper fusing or circuit

protection, surge current from load initiation, load abuse, or load

failure. Another common cause of device failure is incorrect han-

dling procedures used in the manufacturing process. Mechanical

damage in the form of excessive mounting torque and/or force

applied to the terminals or leads can transmit stresses to the

internal thyristor chip and cause cracks in the chip which may not

show up until the device is thermally cycled.

Prevention of Failures

Careful selection of the correct device for the application’s oper-

ating parameters and environment will go a long way toward

extending the operating life of the thyristor. Good design practice

should also limit the maximum current through the main terminals

to 75% of the device rating. Correct mounting and forming of the

leads also help ensure against infant mortality and latent failures.

The two best ways to ensure long life of a thyristor is by proper

heat sink methods and correct voltage rating selection for worst

case conditions. Overheating, overvoltage, and surge currents

are the main killers of semiconductors.

Most Common Thyristor Failure Mode

When a thyristor is electrically or physically abused and fails either

by degradation or a catastrophic means, it will short (full-wave or

half-wave) as its normal failure mode. Rarely does it fail open

circuit. The circuit designer should add line breaks, fuses, over-

temperature interrupters or whatever is necessary to protect the

end user and property if a shorted or partially shorted thyristor

offers a safety hazard.

Virtual Start of Wavefront

(Peak Value)

100%

90%

50%

10%

30%

ta tb

tc T1

Time

Decay = e -

1.44 T2

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