Safe Operating Area
See also: Safe operating areaUnlike the insulated gate bipolar transistor (IGBT), the GTO thyristor requires external devices ("snubber circuits") to shape the turn on and turn off currents to prevent device destruction.
During turn on, the device has a maximum dI/dt rating limiting the rise of current. This is to allow the entire bulk of the device to reach turn on before full current is reached. If this rating is exceeded, the area of the device nearest the gate contacts will overheat and melt from over current. The rate of dI/dt is usually controlled by adding a saturable reactor (turn-on snubber), although turn-on dI/dt is a less serious constraint with GTO thyristors than it is with normal thyristors, because of the way the GTO is constructed from many small thyristor cells in parallel. Reset of the saturable reactor usually places a minimum off time requirement on GTO based circuits.
During turn off, the forward voltage of the device must be limited until the current tails off. The limit is usually around 20% of the forward blocking voltage rating. If the voltage rises too fast at turn off, not all of the device will turn off and the GTO will fail, often explosively, due to the high voltage and current focused on a small portion of the device. Substantial snubber circuits are added around the device to limit the rise of voltage at turn off. Reseting the snubber circuit usually places a minimum on time requirement on GTO based circuits.
The minimum on and off time is handled in DC motor chopper circuits by using a variable switching frequency at the lowest and highest duty cycle. This is observable in traction applications where the frequency will ramp up as the motor starts, then the frequency stays constant over most of the speed ranges, then the frequency drops back down to zero at full speed.
Read more about this topic: Gate Turn-off Thyristor
Famous quotes containing the words safe, operating and/or area:
“Franceska: I was happy in the life I built up for myself. I put a fine high wall of music around me and nothing could touch me. I was safe and secure. And then you had to come along and knock it all down and I hate you for that.
Maxwell: On the contrary, you love me.”
—Muriel Box (b. 1905)
“Many people operate under the assumption that since parenting is a natural adult function, we should instinctively know how to do it—and do it well. The truth is, effective parenting requires study and practice like any other skilled profession. Who would even consider turning an untrained surgeon loose in an operating room? Yet we “operate” on our children every day.”
—Louise Hart (20th century)
“Many women are reluctant to allow men to enter their domain. They don’t want men to acquire skills in what has traditionally been their area of competence and one of their main sources of self-esteem. So while they complain about the male’s unwillingness to share in domestic duties, they continually push the male out when he moves too confidently into what has previously been their exclusive world.”
—Bettina Arndt (20th century)