Thyristor Semiconductor Products Page 82 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1009 The rise time (t r ) of a current waveform is 1.25 times the time for the current to increase from 10% to 90% of peak value. See Figure AN1009.5. t r c - t a ] t r PK ) - t(0.1 I PK )] = T 1 - T 0 The rise time (t r ) 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) t r c - t b ] t r PK ) - t(0.3 V PK )] = T 1 - T 0 The decay time (t d ) 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 t d = Decay Time = [t(0.5 I PK ) - t(0.1 I PK )] = T 2 - T 0 Voltage Waveform t d = Decay Time = [t(0.5 V PK ) - t(0.3 V PK )] = T 2 - T 0 Virtual Start of Wavefront (Peak Value) 100% 90% 50% 0% 10% 30% t a t b T 0 t c T 1 T 2 Time Percent of Current or Voltage Decay = e - t 1.44 T 2 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 catastrophic failure, but could show up as a latent failure. Catastrophic 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 maximum ratings at all times. Degradation Failures A significant change of on-state, gate, or switching characteristics 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 operated 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 operations are the most probable cause for failure. Overvoltage failures 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. Overcurrent 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 handling 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 operating 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. Miscellaneous Design Tips and Facts

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